Evolution is change in the
heritable characteristics of biological
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
s over successive generations. These characteristics are the
expressions of
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s, which are passed on from parent to offspring during
reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
.
Variation tends to exist within any given population as a result of genetic
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
and
recombination.
Evolution occurs when evolutionary processes such as
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
(including
sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
) and
genetic drift
Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance.
Genetic drift may cause gene variants to disappear completely and there ...
act on this variation, resulting in certain characteristics becoming more common or more rare within a population.
The
evolutionary pressures that determine whether a characteristic is common or rare within a population constantly change, resulting in a change in heritable characteristics arising over successive generations. It is this process of evolution that has given rise to
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
at every level of
biological organisation, including the levels of
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, individual
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s, and
molecules
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
.
The
theory
A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
of evolution by natural selection was conceived independently by
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
and
Alfred Russel Wallace
Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural se ...
in the mid-19th century and was set out in detail in Darwin's book ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
''. Evolution by natural selection is established by observable facts about living organisms: (1) more offspring are often produced than can possibly survive (2) traits vary among individuals with respect to their
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
*Morphology (archaeology), study of the shapes or forms of artifacts
*Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
,
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
, and behaviour (
phenotypic variation
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
); (3) different traits confer different rates of survival and reproduction (differential
fitness); and (4) traits can be passed from generation to generation (
heritability
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
of fitness).
In successive generations, members of a population are therefore more likely to be replaced by the
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
of parents with favourable characteristics. In the early 20th century, other
competing ideas of evolution such as
mutationism and
orthogenesis
Orthogenesis, also known as orthogenetic evolution, progressive evolution, evolutionary progress, or progressionism, is an obsolete biological hypothesis that organisms have an innate tendency to evolve in a definite direction towards some go ...
were
refuted as the
modern synthesis
Modern synthesis or modern evolutionary synthesis refers to several perspectives on evolutionary biology, namely:
* Modern synthesis (20th century), the term coined by Julian Huxley in 1942 to denote the synthesis between Mendelian genetics and s ...
concluded
Darwinian evolution acts on
Mendelian genetic variation.
All
life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
on Earth shares a
last universal common ancestor (LUCA),
which lived approximately 3.5–3.8 billion years ago.
The
fossil record includes a progression from early
biogenic
A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
to
microbial mat fossils
to fossilised
multicellular organism
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
s. Existing patterns of biodiversity have been shaped by repeated formations of new species (
speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
), changes within species (
anagenesis), and loss of species (
extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
) throughout the
evolutionary history of life on Earth.
Morphological and
biochemical
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
traits are more similar among species that share a more
recent common ancestor, and these traits can be used to reconstruct
phylogenetic tree
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
s.
Evolutionary biologists
Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. It is also defined as the study of ...
have continued to study various aspects of evolution by forming and testing
hypotheses
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obser ...
as well as constructing theories based on
evidence
Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field.
In epistemology, evidenc ...
from the field or laboratory and on data generated by the methods of
mathematical and theoretical biology
Mathematical and theoretical biology, or biomathematics, is a branch of biology which employs theoretical analysis, mathematical models and abstractions of the living organisms to investigate the principles that govern the structure, development a ...
. Their discoveries have influenced not just the development of
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
but numerous other scientific and industrial fields, including
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
,
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pract ...
, and
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
.
Heredity
![ADN static](https://upload.wikimedia.org/wikipedia/commons/c/c2/ADN_static.png)
Evolution in organisms occurs through changes in heritable traits—the inherited characteristics of an organism. In humans, for example,
eye colour
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conve ...
is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents. Inherited traits are controlled by genes and the complete set of genes within an organism's
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
(genetic material) is called its genotype.
The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment. As a result, many aspects of an organism's phenotype are not inherited. For example,
suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of
albinism, who do not tan at all and are very sensitive to
sunburn.
Heritable traits are passed from one generation to the next via DNA, a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
that encodes genetic information.
DNA is a long
biopolymer
Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, cl ...
composed of four types of bases. The sequence of bases along a particular DNA molecule specifies the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s. The specific location of a DNA sequence within a chromosome is known as a
locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.
However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by
quantitative trait loci (multiple interacting genes).
Some heritable changes cannot be explained by changes to the sequence of
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...
s in the DNA. These phenomena are classed as epigenetic inheritance systems.
DNA methylation
DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts t ...
marking
chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
, self-sustaining metabolic loops, gene silencing by
RNA interference and the three-dimensional
conformation of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s (such as
prion
Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...
s) are areas where epigenetic inheritance systems have been discovered at the organismic level.
Developmental biologists suggest that complex interactions in
genetic networks
A gene (or genetic) regulatory network (GRN) is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins which, in turn, determine the fu ...
and communication among cells can lead to heritable variations that may underlay some of the mechanics in
developmental plasticity
Developmental plasticity is a general term referring to changes in neural connections during development as a result of environmental interactions as well as neural changes induced by learning. Much like neuroplasticity or brain plasticity, develo ...
and
canalisation.
Heritability may also occur at even larger scales. For example, ecological inheritance through the process of
niche construction
Niche construction is the process by which an organism alters its own (or another species') local environment. These alterations can be a physical change to the organism’s environment or encompass when an organism actively moves from one habita ...
is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.
Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and
symbiogenesis.
Sources of variation
Evolution can occur if there is
genetic variation
Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, ...
within a population. Variation comes from mutations in the genome, reshuffling of genes through
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
and migration between populations (
gene flow
In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent a ...
). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species. However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.
An individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation.
The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of
fixation—when it either disappears from the population or replaces the ancestral allele entirely.
Before the discovery of Mendelian genetics, one common hypothesis was
blending inheritance. But with blending inheritance, genetic variation would be rapidly lost, making evolution by natural selection implausible. The
Hardy–Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.
Mutation
![Gene-duplication](https://upload.wikimedia.org/wikipedia/commons/5/5d/Gene-duplication.svg)
Mutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms.
When mutations occur, they may alter the
product of a gene, or prevent the gene from functioning, or have no effect. Based on studies in the fly ''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'', it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.
Mutations can involve large sections of a chromosome becoming
duplicated (usually by
genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...
), which can introduce extra copies of a gene into a genome. Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within
gene families
A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on ...
from pre-existing genes that share common ancestors. For example, the human
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
uses four genes to make structures that sense light: three for
colour vision
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of t ...
and one for
night vision
Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night vi ...
; all four are descended from a single ancestral gene.
New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the
redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed
de novo gene birth.
The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions (
exon shuffling
Exon shuffling is a molecular mechanism for the formation of new genes. It is a process through which two or more exons from different genes can be brought together ectopic recombination, ectopically, or the same exon can be duplicated, to create ...
). When new genes are assembled from shuffling pre-existing parts,
domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions. For example,
polyketide synthases are large
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s that make
antibiotics
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention o ...
; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line.
One example of mutation is
wild boar
The wild boar (''Sus scrofa''), also known as the wild swine, common wild pig, Eurasian wild pig, or simply wild pig, is a suid native to much of Eurasia and North Africa, and has been introduced to the Americas and Oceania. The species is ...
piglets. They are camouflage colored and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in ''
melanocortin 1 receptor'' (''MC1R'') disrupt the pattern. The majority of pig breeds carry ''MC1R'' mutations disrupting wild-type color and different mutations causing dominant black color of the pigs.
Sex and recombination
In
asexual organisms, genes are inherited together, or ''linked'', as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of
sex
Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones (ova, oft ...
ual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
, sexual organisms exchange DNA between two matching chromosomes. Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.
Sex usually increases genetic variation and may increase the rate of evolution.
![Evolsex-dia1a](https://upload.wikimedia.org/wikipedia/commons/f/fc/Evolsex-dia1a.png)
The two-fold cost of sex was first described by
John Maynard Smith.
The first cost is that in sexually dimorphic species only one of the two sexes can bear young. This cost does not apply to hermaphroditic species, like most plants and many
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s. The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes.
Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. The
Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to
coevolution
In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...
with other species in an ever-changing environment.
Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.
Gene flow
Gene flow is the exchange of genes between populations and between species.
It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of
pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.
Gene transfer between species includes the formation of
hybrid
Hybrid may refer to:
Science
* Hybrid (biology), an offspring resulting from cross-breeding
** Hybrid grape, grape varieties produced by cross-breeding two ''Vitis'' species
** Hybridity, the property of a hybrid plant which is a union of two dif ...
organisms and
horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
. In medicine, this contributes to the spread of
antibiotic resistance
Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...
, as when one bacteria acquires resistance genes it can rapidly transfer them to other species.
Horizontal transfer of genes from bacteria to eukaryotes such as the yeast ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...
'' and the adzuki bean weevil ''
Callosobruchus chinensis
''Callosobruchus chinensis'' is a common species of beetle found in the bean weevil subfamily, and is known to be a pest to many stored legumes. Although it is commonly known as the adzuki bean weevil it is in fact not a true weevil, belonging ins ...
'' has occurred. An example of larger-scale transfers are the eukaryotic
bdelloid rotifers
Bdelloidea (Greek ''βδέλλα'', ''bdella'', "leech") is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainl ...
, which have received a range of genes from bacteria,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and plants.
Virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
es can also carry DNA between organisms, allowing transfer of genes even across
biological domains.
Large-scale gene transfer has also occurred between the ancestors of
eukaryotic cells
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
and bacteria, during the acquisition of
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s and
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
. It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
.
Evolutionary processes
![Mutation and selection diagram](https://upload.wikimedia.org/wikipedia/commons/f/f3/Mutation_and_selection_diagram.svg)
From a
neo-Darwinian
Neo-Darwinism is generally used to describe any integration of Charles Darwin's theory of evolution by natural selection with Gregor Mendel's theory of genetics. It mostly refers to evolutionary theory from either 1895 (for the combinations of Dar ...
perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms,
for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, gene flow and mutation bias.
Natural selection
Evolution by natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:
* Variation exists within populations of organisms with respect to morphology, physiology and behaviour (phenotypic variation).
* Different traits confer different rates of survival and reproduction (differential fitness).
* These traits can be passed from generation to generation (heritability of fitness).
More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage.
This
teleonomy
Teleonomy is the quality of apparent purposefulness and of goal-directedness of structures and functions in living organisms brought about by natural processes like natural selection. The term derives from the Greek "τελεονομία", compound ...
is the quality whereby the process of natural selection creates and preserves traits that are
seemingly fitted for the
functional
Functional may refer to:
* Movements in architecture:
** Functionalism (architecture)
** Form follows function
* Functional group, combination of atoms within molecules
* Medical conditions without currently visible organic basis:
** Functional sy ...
roles they perform. Consequences of selection include
nonrandom mating and
genetic hitchhiking
Genetic may refer to:
*Genetics, in biology, the science of genes, heredity, and the variation of organisms
**Genetic, used as an adjective, refers to genes
***Genetic disorder, any disorder caused by a genetic mutation, whether inherited or de nov ...
.
The central concept of natural selection is the
evolutionary fitness
Fitness (often denoted w or ω in population genetics models) is the quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individua ...
of an organism.
Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.
However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes.
For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.
If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected ''for''." Examples of traits that can increase fitness are enhanced survival and increased
fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer—they are "selected ''against''."
Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.
However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see
Dollo's law
Dollo's law of irreversibility (also known as Dollo's law and Dollo's principle), proposed in 1893 by Belgian paleontologist Louis Dollo states that, "an organism never returns exactly to a former state, even if it finds itself placed in condition ...
). However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in
dolphin
A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the ...
s, teeth in
chicken
The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adult m ...
s, wings in wingless stick
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s, tails and additional nipples in humans etc. "Throwbacks" such as these are known as
atavisms.
![Genetic Distribution](https://upload.wikimedia.org/wikipedia/commons/6/62/Genetic_Distribution.svg)
Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is
directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly,
disruptive selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population ...
is selection for extreme trait values and often results in
two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in
stabilising selection
Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of a ...
there is selection against extreme trait values on both ends, which causes a decrease in
variance
In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbers ...
around the average value and less diversity.
This would, for example, cause organisms to eventually have a similar height.
Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
, that is, a system in which organisms interact with every other element,
physical
Physical may refer to:
*Physical examination
In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally co ...
as well as
biological
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
, in their local environment.
Eugene Odum, a founder of
ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and nonliving parts) within the system...."
Each population within an ecosystem occupies a distinct
niche
Niche may refer to:
Science
*Developmental niche, a concept for understanding the cultural context of child development
*Ecological niche, a term describing the relational position of an organism's species
*Niche differentiation, in ecology, the ...
, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the
food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.
Natural selection can act at
different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species.
Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called
transposons
A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transpo ...
, which can replicate and spread throughout a genome. Selection at a level above the individual, such as
group selection, may allow the evolution of cooperation.
Genetic hitchhiking
Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as
linkage
Linkage may refer to:
* ''Linkage'' (album), by J-pop singer Mami Kawada, released in 2010
*Linkage (graph theory), the maximum min-degree of any of its subgraphs
*Linkage (horse), an American Thoroughbred racehorse
* Linkage (hierarchical cluster ...
. This tendency is measured by finding how often two alleles occur together on a single chromosome compared to
expectations, which is called their
linkage disequilibrium. A set of alleles that is usually inherited in a group is called a
haplotype
A haplotype ( haploid genotype) is a group of alleles in an organism that are inherited together from a single parent.
Many organisms contain genetic material ( DNA) which is inherited from two parents. Normally these organisms have their DNA or ...
. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a
selective sweep
In genetics, a selective sweep is the process through which a new beneficial mutation that increases its frequency and becomes fixed (i.e., reaches a frequency of 1) in the population leads to the reduction or elimination of genetic variation among ...
that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.
Sexual selection
![Rana arvalis2](https://upload.wikimedia.org/wikipedia/commons/8/8e/Rana_arvalis2.jpg)
A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates. Traits that evolved through sexual selection are particularly prominent among males of several animal species. Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises the survival of individual males.
This survival disadvantage is balanced by higher reproductive success in males that show these
hard-to-fake, sexually selected traits.
Genetic drift
![Allele-frequency](https://upload.wikimedia.org/wikipedia/commons/4/46/Allele-frequency.png)
Genetic drift is the random fluctuation of
allele frequencies within a population from one generation to the next.
When selective forces are absent or relatively weak, allele frequencies are equally likely to ''drift'' upward or downward in each successive generation because the alleles are subject to
sampling error.
This drift halts when an allele eventually becomes fixed, either by disappearing from the population or by replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that begin with the same genetic structure to drift apart into two divergent populations with different sets of alleles.
According to the now largely abandoned
neutral theory of molecular evolution
The neutral theory of molecular evolution holds that most evolutionary changes occur at the molecular level, and most of the variation within and between species are due to random genetic drift of mutant alleles that are selectively neutral. The ...
most evolutionary changes are the result of the fixation of
neutral mutations by genetic drift.
In this model, most genetic changes in a population are thus the result of constant mutation pressure and genetic drift. This form of the neutral theory is now largely abandoned since it does not seem to fit the genetic variation seen in nature. A better-supported version of this model is the
nearly neutral theory, according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.
Other theories propose that genetic drift is dwarfed by other
stochastic
Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselv ...
forces in evolution, such as genetic hitchhiking, also known as genetic draft.
Another concept is
constructive neutral evolution Constructive neutral evolution (CNE) is a theory that seeks to explain how complex systems can evolve through neutral transitions and spread through a population by chance fixation (genetic drift). Constructive neutral evolution is a competitor for ...
(CNE), which explains that complex systems can emerge and spread into a population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting, and it has been applied in areas ranging from the origins of the
spliceosome
A spliceosome is a large ribonucleoprotein (RNP) complex found primarily within the nucleus of eukaryotic cells. The spliceosome is assembled from small nuclear RNAs (snRNA) and numerous proteins. Small nuclear RNA (snRNA) molecules bind to specifi ...
to the complex interdependence of
microbial communities Microbial population biology is the application of the principles of population biology to microorganisms.
Distinguishing from other biological disciplines
Microbial population biology, in practice, is the application of population ecology and popu ...
.
The time it takes a neutral allele to become fixed by genetic drift depends on population size; fixation is more rapid in smaller populations. The number of individuals in a population is not critical, but instead a measure known as the effective population size.
The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.
The effective population size may not be the same for every gene in the same population.
It is usually difficult to measure the relative importance of selection and neutral processes, including drift. The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of
current research
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (s ...
.
Gene flow
Gene flow involves the exchange of genes between populations and between species.
The presence or absence of gene flow fundamentally changes the course of evolution. Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the
Bateson-Dobzhansky-Muller model, even if both populations remain essentially identical in terms of their adaptation to the environment.
If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Thus, exchange of genetic information between individuals is fundamentally important for the development of the
''Biological Species Concept''.
During the development of the modern synthesis, Sewall Wright developed his
shifting balance theory, which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution. However, recently there has been substantial criticism of the importance of the shifting balance theory.
Mutation bias
Mutation bias Mutation bias refers to a pattern in which some type of mutation occurs more often than expected under uniformity. The types are most often defined by the molecular nature of the mutational change (see examples below), but sometimes they are based ...
is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of
developmental bias
Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosi ...
. Haldane
and Fisher
argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. This opposing-pressures argument was long used to dismiss the possibility of internal tendencies in evolution,
until the molecular era prompted renewed interest in neutral evolution.
Noboru Sueoka
and
Ernst Freese
Dr. Ernst Freese (September 27, 1925 - March 30, 1990) was a molecular biologist who worked on the mechanism of mutations in DNA. From 1962 until his death he was Chief of the National Institute of Neurological Disorders and Stroke (NINDS) Labora ...
proposed that systematic biases in mutation might be responsible for systematic differences in genomic GC composition between species. The identification of a GC-biased ''E. coli'' mutator strain in 1967,
along with the proposal of the
neutral theory Neutral theory may refer to one of these two related theories:
* Neutral theory of molecular evolution
* Unified neutral theory of biodiversity
The unified neutral theory of biodiversity and biogeography (here "Unified Theory" or "UNTB") is a t ...
, established the plausibility of mutational explanations for molecular patterns, which are now common in the molecular evolution literature.
For instance, mutation biases are frequently invoked in models of codon usage.
Such models also include effects of selection, following the mutation-selection-drift model,
which allows both for mutation biases and differential selection based on effects on translation. Hypotheses of mutation bias have played an important role in the development of thinking about the evolution of genome composition, including isochores.
Different insertion vs. deletion biases in different
taxa can lead to the evolution of different genome sizes. The hypothesis of Lynch regarding genome size relies on mutational biases toward increase or decrease in genome size.
However, mutational hypotheses for the evolution of composition suffered a reduction in scope when it was discovered that (1) GC-biased gene conversion makes an important contribution to composition in diploid organisms such as mammals
and (2) bacterial genomes frequently have AT-biased mutation.
Contemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work
showed that the original "pressures" theory assumes that evolution is based on standing variation: when evolution depends on the introduction of new alleles, mutational and developmental biases in the introduction can impose biases on evolution ''without requiring neutral evolution or high mutation rates''.
Several studies report that the mutations implicated in adaptation reflect common mutation biases
though others dispute this interpretation.
Applications
Concepts and models used in evolutionary biology, such as natural selection, have many applications.
Artificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the
domestication of plants and animals. More recently, such selection has become a vital part of
genetic engineering
Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including t ...
, with
selectable marker A selectable marker is a gene introduced into a cell, especially a bacterium or to cells in culture, that confers a trait suitable for artificial selection. They are a type of reporter gene used in laboratory microbiology, molecular biology, and gen ...
s such as antibiotic resistance genes being used to manipulate DNA. Proteins with valuable properties have evolved by repeated rounds of mutation and selection (for example modified enzymes and new
antibodies
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
) in a process called
directed evolution
Directed evolution (DE) is a method used in protein engineering that mimics the process of natural selection to steer proteins or nucleic acids toward a user-defined goal. It consists of subjecting a gene to iterative rounds of mutagenesis (cre ...
.
Understanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
s. For example, the
Mexican tetra is an
albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. This helped identify genes required for vision and pigmentation.
Evolutionary theory has many
applications in medicine. Many human diseases are not static phenomena, but capable of evolution. Viruses, bacteria, fungi and
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
s evolve to be resistant to host immune defences, as well as to
pharmaceutical drug
A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and re ...
s. These same problems occur in agriculture with pesticide and
herbicide
Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
resistance. It is possible that we are facing the end of the effective life of most of available antibiotics and predicting the evolution and evolvability of our pathogens and devising strategies to slow or circumvent it is requiring deeper knowledge of the complex forces driving evolution at the molecular level.
In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, simulations of evolution using
evolutionary algorithms and
artificial life started in the 1960s and were extended with simulation of artificial selection. Artificial evolution became a widely recognised optimisation method as a result of the work of
Ingo Rechenberg
Ingo Rechenberg (November 20, 1934 - September 25, 2021) was a German researcher and professor in the field of bionics. Rechenberg was a pioneer of the fields of evolutionary computation and artificial evolution. In the 1960s and 1970s he inven ...
in the 1960s. He used
evolution strategies
In computer science, an evolution strategy (ES) is an optimization technique based on ideas of evolution. It belongs to the general class of evolutionary computation or artificial evolution methodologies.
History
The 'evolution strategy' optimizat ...
to solve complex engineering problems.
Genetic algorithm
In computer science and operations research, a genetic algorithm (GA) is a metaheuristic inspired by the process of natural selection that belongs to the larger class of evolutionary algorithms (EA). Genetic algorithms are commonly used to gene ...
s in particular became popular through the writing of
John Henry Holland. Practical applications also include
automatic evolution of computer programmes. Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers and also to optimise the design of systems.
Natural outcomes
Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding
predators
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
or attracting mates. Organisms can also respond to selection by
cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial
symbiosis
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are distinguished based on time scale as
macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in allele frequency and adaptation.
Macroevolution the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new
habitat
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
s, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as
species selection acting on entire species and affecting their rates of speciation and extinction.
A common misconception is that evolution has goals, long-term plans, or an innate tendency for "progress", as expressed in beliefs such as orthogenesis and evolutionism; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity.
Although
complex species have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the biosphere.
For example, the overwhelming majority of species are microscopic
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s, which form about half the world's
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
despite their small size, and constitute the vast majority of Earth's biodiversity.
Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is
more noticeable. Indeed, the evolution of microorganisms is particularly important to evolutionary research, since their rapid reproduction allows the study of
experimental evolution and the observation of evolution and adaptation in real time.
Adaptation
![Homology vertebrates-en](https://upload.wikimedia.org/wikipedia/commons/5/5e/Homology_vertebrates-en.svg)
Adaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
s' teeth to the grinding of grass. By using the term ''adaptation'' for the evolutionary process and ''adaptive trait'' for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky:
# ''Adaptation'' is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.
# ''Adaptedness'' is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.
# An ''adaptive trait'' is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.
Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' evolving the ability to use
citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
as a nutrient in a
long-term laboratory experiment, ''
Flavobacterium
''Flavobacterium'' is a genus of Gram-negative, nonmotile and motile, rod-shaped bacteria that consists of 130 recognized species. Flavobacteria are found in soil and fresh water in a variety of environments. Several species are known to cause ...
'' evolving a novel enzyme that allows these bacteria to grow on the by-products of
nylon
Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic.
Nylon is a silk-like thermoplastic, generally made from petro ...
manufacturing, and the soil bacterium ''
Sphingobium
Sphingobium species are different from other sphingomonads in that they are commonly isolated from soil; however, ''Sphingobium yanoikuyae'' was isolated from a clinical specimen. They can degrade a variety of chemicals in the environment such ...
'' evolving an entirely new
metabolic pathway
In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...
that degrades the synthetic
pesticide
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
pentachlorophenol
Pentachlorophenol (PCP) is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latter of which diss ...
. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).
![Whale skeleton](https://upload.wikimedia.org/wikipedia/commons/1/16/Whale_skeleton.png)
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
wings, for example, are very similar to those in
mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
feet and
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
hands, due to the descent of all these structures from a common mammalian ancestor. However, since all living organisms are related to some extent,
even organs that appear to have little or no structural similarity, such as
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
,
squid
True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...
and
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called
deep homology
In evolutionary developmental biology, the concept of deep homology is used to describe cases where growth and differentiation processes are governed by genetic mechanisms that are homologous and deeply conserved across a wide range of specie ...
.
During evolution, some structures may lose their original function and become
vestigial structures
Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
.
Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include
pseudogene
Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by Reverse transcriptase, reverse transcription of an mRNA trans ...
s, the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds, the presence of hip bones in whales and snakes,
and sexual traits in organisms that reproduce via asexual reproduction. Examples of
vestigial structures in humans include
wisdom teeth, the
coccyx,
the
vermiform appendix,
and other behavioural vestiges such as
goose bumps
Goose bumps, goosebumps or goose-pimples are the bumps on a person's skin at the base of body hairs which may involuntarily develop when a person is tickled, cold or experiencing strong emotions such as fear, euphoria or sexual arousal.
The fo ...
and
primitive reflexes.
However, many traits that appear to be simple adaptations are in fact
exaptation
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...
s: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard ''Holaspis guentheri'', which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells,
molecular machine
A molecular machine, nanite, or nanomachine is a molecular component that produces quasi-mechanical movements (output) in response to specific stimuli (input). In cellular biology, macromolecular machines frequently perform tasks essential for l ...
s such as the bacterial
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
and
protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously had different functions.
Another example is the recruitment of enzymes from
glycolysis
Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...
and
xenobiotic metabolism
Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set of ...
to serve as structural proteins called
crystallins within the lenses of organisms' eyes.
An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the
middle ear in mammals. It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of
crocodile
Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...
s. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.
Coevolution
![Thamnophis sirtalis sirtalis Wooster](https://upload.wikimedia.org/wikipedia/commons/f/f7/Thamnophis_sirtalis_sirtalis_Wooster.jpg)
Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
and a
host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
* Michel Host ...
, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. An example is the production of
tetrodotoxin
Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an order that includes pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Although tetrodotoxin was discovered ...
in the
rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the
common garter snake. In this predator-prey pair, an
evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.
Cooperation
Not all co-evolved interactions between species involve conflict. Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the
mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil. This is a
reciprocal
Reciprocal may refer to:
In mathematics
* Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal''
* Reciprocal polynomial, a polynomial obtained from another pol ...
relationship as the plants provide the fungi with sugars from
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending
signals
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
that suppress the plant
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
.
Coalitions between organisms of the same species have also evolved. An extreme case is the
eusociality found in social insects, such as
bee
Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyly, monophyletic lineage within the ...
s,
termite
Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
s and
ants, where sterile insects feed and guard the small number of organisms in a
colony
In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the ''metropole, metropolit ...
that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth
causes cancer.
Such cooperation within species may have evolved through the process of
kin selection
Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like Altruism in animals, altruistic behavio ...
, which is where one organism acts to help raise a relative's offspring. This activity is selected for because if the ''helping'' individual contains alleles which promote the helping activity, it is likely that its kin will ''also'' contain these alleles and thus those alleles will be passed on. Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.
Speciation
![Speciation modes edit](https://upload.wikimedia.org/wikipedia/commons/d/d3/Speciation_modes_edit.svg)
Speciation is the process where a species diverges into two or more descendant species.
There are multiple ways to define the concept of "species." The choice of definition is dependent on the particularities of the species concerned.
For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic.
The ''Biological Species Concept'' is a classic example of the interbreeding approach. Defined by evolutionary biologist
Ernst Mayr
Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists. He was also a renowned Taxonomy (biology), taxonomist, tropical explorer, ornithologist, Philosophy of biology, philosopher o ...
in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes, and this is called the
species problem
The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for se ...
.
Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.
Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their
most recent common ancestor, it may still be possible for them to produce offspring, as with horses and
donkey
The domestic donkey is a hoofed mammal in the family Equidae, the same family as the horse. It derives from the African wild ass, ''Equus africanus'', and may be classified either as a subspecies thereof, ''Equus africanus asinus'', or as a ...
s mating to produce
mule
The mule is a domestic equine hybrid between a donkey and a horse. It is the offspring of a male donkey (a jack) and a female horse (a mare). The horse and the donkey are different species, with different numbers of chromosomes; of the two pos ...
s. Such hybrids are generally
infertile
Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing
new species
A species description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species that have be ...
of animals is unclear, although cases have been seen in many types of animals, with the
gray tree frog
The gray treefrog (''Dryophytes versicolor'') is a species of small arboreal holarctic tree frog native to much of the eastern United States and southeastern Canada.
It is sometimes referred to as the eastern gray treefrog, northern gray tree ...
being a particularly well-studied example.
Speciation has been observed multiple times under both controlled laboratory conditions (see
laboratory experiments of speciation
Laboratory experiments of speciation have been conducted for all four modes of speciation: allopatric, peripatric, parapatric, and sympatric; and various other processes involving speciation: hybridization, reinforcement, founder effects, among ...
) and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is
allopatric speciation
Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
, which occurs in populations initially isolated geographically, such as by
habitat fragmentation
Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological processes ...
or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.
As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.
The second mode of speciation is
peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the
founder effect
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using ...
causes rapid speciation after an increase in
inbreeding
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...
increases selection on homozygotes, leading to rapid genetic change.
The third mode is
parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.
Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass ''
Anthoxanthum odoratum
''Anthoxanthum odoratum'', known as sweet vernal grass, is a short-lived perennial grass that is native to acidic grassland in Eurasia and northern Africa. It is grown as a lawn grass and a house plant, due to its sweet scent, and can also be f ...
'', which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause
reinforcement, which is the evolution of traits that promote mating within a species, as well as
character displacement
Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not o ...
, which is when two species become more distinct in appearance.
![Darwin's finches](https://upload.wikimedia.org/wikipedia/commons/9/97/Darwin%27s_finches.jpeg)
Finally, in
sympatric speciation
Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organi ...
species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both
genetic differences and nonrandom mating, to allow reproductive isolation to evolve.
One type of sympatric speciation involves
crossbreed
A crossbreed is an organism with purebred parents of two different breeds, varieties, or populations. ''Crossbreeding'', sometimes called "designer crossbreeding", is the process of breeding such an organism, While crossbreeding is used to main ...
ing of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
the
homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form
polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already. An example of such a speciation event is when the plant species ''
Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land.
A winter a ...
'' and ''
Arabidopsis arenosa'' crossbred to give the new species ''Arabidopsis suecica''. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.
Speciation events are important in the theory of
punctuated equilibrium
In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a Scientific theory, theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolution, evol ...
, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged. In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.
Extinction
![Palais de la Decouverte Tyrannosaurus rex p1050042](https://upload.wikimedia.org/wikipedia/commons/a/ab/Palais_de_la_Decouverte_Tyrannosaurus_rex_p1050042.jpg)
Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction. Nearly all animal and plant species that have lived on Earth are now extinct, and extinction appears to be the ultimate fate of all species. These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass
extinction event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...
s.
The
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
, during which the non-avian dinosaurs became extinct, is the most well-known, but the earlier
Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
was even more severe, with approximately 96% of all marine species driven to extinction.
The
Holocene extinction event
The Holocene extinction, or Anthropocene extinction, is the ongoing extinction event during the Holocene epoch. The extinctions span numerous families of bacteria, fungi, plants, and animals, including mammals, birds, reptiles, amphibians, fish, ...
is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century. Human activities are now the primary cause of the ongoing extinction event;
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
may further accelerate it in the future. Despite the estimated extinction of more than 99 percent of all species that ever lived on Earth,
about 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.
The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.
The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (the
competitive exclusion principle
In ecology, the competitive exclusion principle, sometimes referred to as Gause's law, is a proposition that two species which compete for the same limited resource cannot coexist at constant population values. When one species has even the sligh ...
).
If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction.
The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.
Evolutionary history of life
Origin of life
The
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is about 4.54 billion years old.
The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,
during the
Eoarchean
The Eoarchean (; also spelled Eoarchaean) is the first era of the Archean Eon of the geologic record. It spans 400 million years, from the end of the Hadean Eon 4 billion years ago (4000 Mya) to the start of the Paleoarchean Era 3600 Mya. The ...
Era after a geological
crust started to solidify following the earlier molten
Hadean
The Hadean ( ) is a Eon (geology), geologic eon of History of Earth, Earth history preceding the Archean. On Earth, the Hadean began with the Formation of the Earth, planet's formation about 4.54 billion years ago (although the start of the H ...
Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia.
Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old
metasedimentary rocks discovered in Western Greenland
as well as "remains of
biotic life
Biotic material or biological derived material is any material that originates from living organisms. Most such materials contain carbon and are capable of decay.
The earliest life on Earth arose at least 3.5 billion years ago.Schopf, JW, Kudrya ...
" found in 4.1 billion-year-old rocks in Western Australia.
Commenting on the Australian findings,
Stephen Blair Hedges wrote, "If life arose relatively quickly on Earth, then it could be common in the universe."
In July 2016, scientists reported identifying a set of 355
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s from the
last universal common ancestor (LUCA) of all organisms living on Earth.
More than 99 percent of all species, amounting to over five billion species,
that ever lived on Earth are estimated to be extinct.
Estimates on the number of Earth's current species range from 10 million to 14 million,
of which about 1.9 million are estimated to have been named
and 1.6 million documented in a central database to date,
leaving at least 80 percent not yet described.
Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.
The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.
The beginning of life may have included self-replicating molecules such as
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
and the assembly of simple cells.
Common descent
All organisms on Earth are descended from a common ancestor or ancestral
gene pool
The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species.
Description
A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
.
Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events. The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third,
vestigial trait
Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
s with no clear purpose resemble functional ancestral traits. Fourth, organisms can be classified using these similarities into a hierarchy of nested groups, similar to a family tree.
![Ape skeletons](https://upload.wikimedia.org/wikipedia/commons/4/49/Ape_skeletons.png)
Due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree, since some genes have spread independently between distantly related species. To solve this problem and others, some authors prefer to use the "
Coral of life The coral of life is a metaphor or a mathematical model useful to illustrate evolution of life or phylogeny at various levels of resolution, including individual organisms, populations, species and large taxonomic groups. Its use in biology resolves ...
" as a metaphor or a mathematical model to illustrate the evolution of life. This view dates back to an idea briefly mentioned by Darwin but later abandoned.
Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.
By comparing the anatomies of both modern and extinct species, palaeontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.
More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s. The development of
molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the
molecular clock
The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleoti ...
produced by mutations. For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.
Evolution of life
Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.
No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years. The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called
endosymbiosis.
The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or
hydrogenosomes. Another engulfment of
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
l-like organisms led to the formation of chloroplasts in algae and plants.
The history of life was that of the
unicellular
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the
Ediacaran
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
period.
The
evolution of multicellularity
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
occurred in multiple independent events, in organisms as diverse as
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
s,
brown algae, cyanobacteria,
slime moulds
Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mu ...
and
myxobacteria. In January 2016, scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.
Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the
Cambrian explosion
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
. Here, the majority of
types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.
Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.
About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals. Insects were particularly successful and even today make up the majority of animal species.
Amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s first appeared around 364 million years ago, followed by early
amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
s and birds around 155 million years ago (both from "
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
"-like lineages),
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s around 129 million years ago,
Homininae around 10 million years ago and
modern humans
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, an ...
around 250,000 years ago. However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.
History of evolutionary thought
Classical antiquity
The proposal that one type of organism could descend from another type goes back to some of the first
pre-Socratic
Pre-Socratic philosophy, also known as early Greek philosophy, is ancient Greek philosophy before Socrates. Pre-Socratic philosophers were mostly interested in cosmology, the beginning and the substance of the universe, but the inquiries of thes ...
Greek
philosopher
A philosopher is a person who practices or investigates philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit=philosophos, meaning 'lover of wisdom'. The coining of the term has been attributed to the Greek th ...
s, such as
Anaximander
Anaximander (; grc-gre, Ἀναξίμανδρος ''Anaximandros''; ) was a pre-Socratic Greek philosopher who lived in Miletus,"Anaximander" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 1, p. 403. a city of Ionia (in moder ...
and
Empedocles
Empedocles (; grc-gre, Ἐμπεδοκλῆς; , 444–443 BC) was a Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily. Empedocles' philosophy is best known for originating the cosmogonic theory of the fo ...
. Such proposals survived into Roman times. The
poet
A poet is a person who studies and creates poetry. Poets may describe themselves as such or be described as such by others. A poet may simply be the creator ( thinker, songwriter, writer, or author) who creates (composes) poems (oral or writte ...
and philosopher
Lucretius
Titus Lucretius Carus ( , ; – ) was a Roman poet and philosopher. His only known work is the philosophical poem ''De rerum natura'', a didactic work about the tenets and philosophy of Epicureanism, and which usually is translated into E ...
followed Empedocles in his masterwork ''
De rerum natura
''De rerum natura'' (; ''On the Nature of Things'') is a first-century BC didactic poem by the Roman poet and philosopher Lucretius ( – c. 55 BC) with the goal of explaining Epicurean philosophy to a Roman audience. The poem, written in some 7 ...
'' (''On the Nature of Things'').
Middle Ages
In contrast to these
materialistic Materialism is the view that the universe consists only of organized matter and energy.
Materialism or materialist may also refer to:
* Economic materialism, the desire to accumulate material goods
* Christian materialism, the combination of Chris ...
views,
Aristotelianism had considered all natural things as
actualisations of fixed natural possibilities, known as
forms.
This became part of a medieval
teleological understanding of
nature
Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
in which all things have an intended role to play in a
divine
Divinity or the divine are things that are either related to, devoted to, or proceeding from a deity.[divine ...](_blank)
cosmic
Cosmic commonly refers to:
* The cosmos, a concept of the universe
Cosmic may also refer to:
Media
* ''Cosmic'' (album), an album by Bazzi
* Afro/Cosmic music
* "Cosmic", a song by Kylie Minogue from the album '' X''
* CosM.i.C, a member of ...
order. Variations of this idea became the standard understanding of the
Middle Ages
In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire a ...
and were integrated into
Christian
Christians () are people who follow or adhere to Christianity, a monotheistic Abrahamic religion based on the life and teachings of Jesus Christ. The words ''Christ'' and ''Christian'' derive from the Koine Greek title ''Christós'' (Χρι ...
learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be.
A number of Arab Muslim scholars wrote about evolution, most notably
Ibn Khaldun
Ibn Khaldun (; ar, أبو زيد عبد الرحمن بن محمد بن خلدون الحضرمي, ; 27 May 1332 – 17 March 1406, 732-808 AH) was an Arab
The Historical Muhammad', Irving M. Zeitlin, (Polity Press, 2007), p. 21; "It is, of ...
, who wrote the book ''
Muqaddimah'' in 1377 AD, in which he asserted that humans developed from "the world of the monkeys", in a process by which "species become more numerous".
[Kiros, Teodros. ''Explorations in African Political Thought''. 2001, page 55]
Pre-Darwinian
The
"New Science" of the 17th century rejected the Aristotelian approach. It sought to explain natural phenomena in terms of
physical law
Scientific laws or laws of science are statements, based on repeated experiments or observations, that describe or predict a range of natural phenomena. The term ''law'' has diverse usage in many cases (approximate, accurate, broad, or narrow) a ...
s that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences: the last bastion of the concept of fixed natural types.
John Ray
John Ray FRS (29 November 1627 – 17 January 1705) was a Christian English naturalist widely regarded as one of the earliest of the English parson-naturalists. Until 1670, he wrote his name as John Wray. From then on, he used 'Ray', after ...
applied one of the previously more general terms for fixed natural types, "species", to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The
biological classification
In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
introduced by
Carl Linnaeus
Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan.
Other
naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751,
Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species.
Georges-Louis Leclerc, Comte de Buffon, suggested that species could degenerate into different organisms, and
Erasmus Darwin
Erasmus Robert Darwin (12 December 173118 April 1802) was an English physician. One of the key thinkers of the Midlands Enlightenment, he was also a natural philosopher, physiologist, slave-trade abolitionist, inventor, and poet.
His poems ...
proposed that all warm-blooded animals could have descended from a single microorganism (or "filament"). The first full-fledged evolutionary scheme was
Jean-Baptiste Lamarck
Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biologi ...
's "transmutation" theory of 1809, which envisaged
spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.
(The latter process was later called
Lamarckism
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
.)
These ideas were condemned by established naturalists as speculation lacking empirical support. In particular,
Georges Cuvier
Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuvier ...
insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, Ray's ideas of benevolent design had been developed by
William Paley
William Paley (July 174325 May 1805) was an English clergyman, Christian apologist, philosopher, and utilitarian. He is best known for his natural theology exposition of the teleological argument for the existence of God in his work ''Natural T ...
into the ''
'' (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.
[
* Letter 2532, 22 November 1859.]
Darwinian revolution
The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
and
Alfred Wallace in terms of variable populations. Darwin used the expression "descent with modification" rather than "evolution". Partly influenced by ''
An Essay on the Principle of Population
An, AN, aN, or an may refer to:
Businesses and organizations
* Airlinair (IATA airline code AN)
* Alleanza Nazionale, a former political party in Italy
* AnimeNEXT, an annual anime convention located in New Jersey
* Anime North, a Canadian an ...
'' (1798) by
Thomas Robert Malthus
Thomas Robert Malthus (; 13/14 February 1766 – 29 December 1834) was an English cleric, scholar and influential economist in the fields of political economy and demography.
In his 1798 book '' An Essay on the Principle of Population'', Mal ...
, Darwin noted that population growth would lead to a "struggle for existence" in which favourable variations prevailed as others perished. In each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism.
Darwin developed his theory of "natural selection" from 1838 onwards and was writing up his "big book" on the subject when
Alfred Russel Wallace
Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural se ...
sent him a version of virtually the same theory in 1858. Their
separate papers were presented together at an 1858 meeting of the
Linnean Society of London
The Linnean Society of London is a learned society dedicated to the study and dissemination of information concerning natural history, evolution, and taxonomy. It possesses several important biological specimen, manuscript and literature colle ...
. At the end of 1859, Darwin's publication of his "abstract" as ''On the Origin of Species'' explained natural selection in detail and in a way that led to an increasingly wide acceptance of
Darwin's concepts of evolution at the expense of
alternative theories.
Thomas Henry Huxley
Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.
The storie ...
applied Darwin's ideas to humans, using
paleontology
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
and
comparative anatomy
Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species).
The science began in the classical era, continuing in t ...
to provide strong evidence that humans and
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its siste ...
s shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the
universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
.
Pangenesis and heredity
The mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of
pangenesis.
In 1865,
Gregor Mendel reported that traits were inherited in a predictable manner through the
independent assortment
Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
and segregation of elements (later known as genes). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.
August Weismann made the important distinction between
germ cell
Germ or germs may refer to:
Science
* Germ (microorganism), an informal word for a pathogen
* Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually
* Germ layer, a primary layer of cells that forms during embry ...
s that give rise to
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s (such as
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
and
egg cell
The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...
s) and the
somatic cell
A somatic cell (from Ancient Greek σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells compo ...
s of the body, demonstrating that heredity passes through the germ line only.
Hugo de Vries connected Darwin's pangenesis theory to Weismann's germ/soma cell distinction and proposed that Darwin's pangenes were concentrated in the
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
and when expressed they could move into the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
to change the
cell's structure. De Vries was also one of the researchers who made Mendel's work well known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline.
To explain how new variants originate, de Vries developed
a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. In the 1930s, pioneers in the field of
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and pop ...
, such as
Ronald Fisher,
Sewall Wright
Sewall Green Wright FRS(For) Honorary FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. He was a founder of population genetics alongsi ...
and
J. B. S. Haldane
John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolog ...
set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and
Mendelian inheritance was thus reconciled.
The 'modern synthesis'
In the 1920s and 1930s, the so-called modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that applied generally to any branch of biology. It explained patterns observed across species in populations, through
fossil transitions in palaeontology.
Further syntheses
Since then, further syntheses have extended evolution's explanatory power in the light of numerous discoveries, to cover biological phenomena across the whole of the
biological hierarchy from genes to populations.
The publication of the structure of
DNA by
James Watson
James Dewey Watson (born April 6, 1928) is an American molecular biologist, geneticist, and zoologist. In 1953, he co-authored with Francis Crick the academic paper proposing the double helix structure of the DNA molecule. Watson, Crick and ...
and
Francis Crick
Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was an English molecular biologist, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in deciphering the helical struc ...
with contribution of
Rosalind Franklin
Rosalind Elsie Franklin (25 July 192016 April 1958) was a British chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, co ...
in 1953 demonstrated a physical mechanism for inheritance.
Molecular biology
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
improved understanding of the relationship between
genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
and
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
. Advances were also made in phylogenetic
systematics
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tre ...
, mapping the transition of traits into a comparative and testable framework through the publication and use of
evolutionary trees.
In 1973, evolutionary biologist
Theodosius Dobzhansky
Theodosius Grigorievich Dobzhansky (russian: Феодо́сий Григо́рьевич Добржа́нский; uk, Теодо́сій Григо́рович Добржа́нський; January 25, 1900 – December 18, 1975) was a prominent ...
penned that "
," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent
explanatory
An explanation is a set of Statement (logic), statements usually constructed to description, describe a set of facts which clarifies the causality, causes, wiktionary:context, context, and Logical consequence, consequences of those facts. It may ...
body of knowledge that describes and predicts many observable facts about life on this planet.
One extension, known as
evolutionary developmental biology
Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.
The field grew from 19th-century beginni ...
and informally called "evo-devo," emphasises how changes between generations (evolution) act on patterns of change within individual organisms (
development).
Since the beginning of the 21st century, some biologists have argued for an
extended evolutionary synthesis
The extended evolutionary synthesis consists of a set of theoretical concepts argued to be more comprehensive than the earlier modern synthesis of evolutionary biology that took place between 1918 and 1942. The extended evolutionary synthesis was ...
, which would account for the effects of non-genetic inheritance modes, such as
epigenetics
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
,
parental effects, ecological inheritance and
cultural inheritance, and
evolvability.
Social and cultural responses
![Editorial cartoon depicting Charles Darwin as an ape (1871)](https://upload.wikimedia.org/wikipedia/commons/6/6f/Editorial_cartoon_depicting_Charles_Darwin_as_an_ape_%281871%29.jpg)
In the 19th century, particularly after the publication of ''On the Origin of Species'' in 1859, the idea that life had evolved was an active source of academic debate centred on the philosophical, social and religious implications of evolution. Today, the modern evolutionary synthesis is accepted by a vast majority of scientists.
However, evolution remains a contentious concept for some
theists.
While
various religions and denominations have reconciled their beliefs with evolution through concepts such as
theistic evolution
Theistic evolution (also known as theistic evolutionism or God-guided evolution) is a theological view that God creates through laws of nature. Its religious teachings are fully compatible with the findings of modern science, including biological ...
, there are
creationists who believe that evolution is contradicted by the
creation myth
A creation myth (or cosmogonic myth) is a symbolic narrative of how the world began and how people first came to inhabit it., "Creation myths are symbolic stories describing how the universe and its inhabitants came to be. Creation myths develop ...
s found in their
religion
Religion is usually defined as a social- cultural system of designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that generally relates humanity to supernatural, ...
s and who raise various
objections to evolution
Objections to evolution have been raised since evolutionary ideas came to prominence in the 19th century. When Charles Darwin published his 1859 book ''On the Origin of Species'', his theory of evolution (the idea that species arose through desc ...
.
As had been demonstrated by responses to the publication of ''
Vestiges of the Natural History of Creation
''Vestiges of the Natural History of Creation'' is an 1844 work of speculative natural history and philosophy by Robert Chambers. Published anonymously in England, it brought together various ideas of stellar evolution with the progressive tra ...
'' in 1844, the most controversial aspect of evolutionary biology is the implication of
human evolution
Human evolution is the evolutionary process within the history of primates that led to the emergence of ''Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development of ...
that humans share common ancestry with apes and that the mental and
moral faculties of humanity have the same types of natural causes as other inherited traits in animals. In some countries, notably the United States, these tensions between science and religion have fuelled the current creation–evolution controversy, a religious conflict focusing on
politics
Politics (from , ) is the set of activities that are associated with making decisions in groups, or other forms of power relations among individuals, such as the distribution of resources or status. The branch of social science that studies ...
and
public education
State schools (in England, Wales, Australia and New Zealand) or public schools (Scottish English and North American English) are generally primary or secondary schools that educate all students without charge. They are funded in whole or in pa ...
.
While other scientific fields such as
cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...
and
Earth science
Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...
also conflict with literal interpretations of many
religious text
Religious texts, including scripture, are texts which various religions consider to be of central importance to their religious tradition. They differ from literature by being a compilation or discussion of beliefs, mythologies, ritual prac ...
s, evolutionary biology experiences significantly more opposition from religious literalists.
The teaching of evolution in American secondary school biology classes was uncommon in most of the first half of the 20th century. The
Scopes Trial decision of 1925 caused the subject to become very rare in American secondary biology textbooks for a generation, but it was gradually re-introduced later and became legally protected with the 1968 ''
Epperson v. Arkansas
''Epperson v. Arkansas'', 393 U.S. 97 (1968), was a landmark United States Supreme Court case that invalidated an Arkansas statute prohibiting the teaching of human evolution in the public schools. The Court held that the First Amendment to the Un ...
'' decision. Since then, the competing religious belief of creationism was legally disallowed in secondary school curricula in various decisions in the 1970s and 1980s, but it returned in
pseudoscientific
Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...
form as
intelligent design
Intelligent design (ID) is a pseudoscientific argument for the existence of God, presented by its proponents as "an evidence-based scientific theory about life's origins". Numbers 2006, p. 373; " Dcaptured headlines for its bold attempt to ...
(ID), to be excluded once again in the 2005 ''
Kitzmiller v. Dover Area School District
''Kitzmiller v. Dover Area School District'', 400 F. Supp. 2d 707 (M.D. Pa. 2005) was the first direct challenge brought in the United States federal courts testing a public school district policy that required the teaching of intelligent design ...
'' case.
The debate over Darwin's ideas did not generate significant controversy in China.
References
Bibliography
*
*
*
*
*
*
*
*
*
*
* The notebook is available fro
The Complete Work of Charles Darwin Online. Retrieved 2019-10-09.
* The book is available fro
The Complete Work of Charles Darwin Online. Retrieved 2014-11-21.
*
*
*
*
*
*
*
*
*
* "Proceedings of a symposium held at the American Museum of Natural History in New York, 2002."
*
*
*
*
*
*
*
*
*
*
* . Retrieved 2014-11-29.
*
*
*
*
*
* "Papers from the Symposium on the Limits of Reductionism in Biology, held at the Novartis Foundation, London, May 13–15, 1997."
*
*
*
*
*
* "Based on a conference held in Bellagio, Italy, June 25–30, 1989"
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Further reading
Introductory reading
*
*
*
*
*
** American version.
*
*
*
Advanced reading
*
*
*
*
*
*
*
*
External links
General information
*
*
*
*
* Adobe Flash required.
*
History of Evolution in the United States.
Salon
Salon may refer to:
Common meanings
* Beauty salon, a venue for cosmetic treatments
* French term for a drawing room, an architectural space in a home
* Salon (gathering), a meeting for learning or enjoyment
Arts and entertainment
* Salon (P ...
(Retrieved 2021-08-24)
*
Experiments
*
*
Online lectures
*
*
{{Authority control
Biology theories
*