Evolution is change in the heritable
A characteristic is a distinguishing feature of a person or thing. It may refer to:
* Characteristic (biased exponent), an ambiguous term formerly used by some authors to specify some type of exponent of a floating point number
* Chara ...
of biological population
s over successive generations. These characteristics are the expressions
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
tends to exist within any given population as a result of genetic mutation
Evolution occurs when evolutionary processes such as
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. C ...
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (i ...
) and genetic drift
act on this variation, resulting in certain characteristics becoming more common or more rare within a population.
The evolutionary pressure
s 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 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'') ...
at every level of
Biological organisation is the hierarchy of complex biological structures and systems that define life using a reductionistic approach. The traditional hierarchy, as detailed below, extends from atoms to biospheres. The higher levels of this s ...
, including the levels of
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 ...
, individual organism
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 ...
of evolution by natural selection was conceived independently by Charles Darwin
and Alfred Russel Wallace
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
, and behaviour (
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 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 heri ...
In successive generations, members of a population are therefore more likely to be replaced by the offspring
of parents with favourable characteristics. In the early 20th century, other competing ideas of evolution
Mutationism is one of several alternatives to evolution by natural selection that have existed both before and after the publication of Charles Darwin's 1859 book ''On the Origin of Species''. In the theory, mutation was the source of novelty, cr ...
as the modern synthesis
Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that ...
acts on Mendelian
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, energy t ...
on Earth shares a last universal common ancestor
which lived approximately 3.5–3.8 billion years ago.
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
includes a progression from early biogenic graphite
to microbial mat
to fossilised multicellular organism
s. Existing patterns of biodiversity have been shaped by repeated formations of new species (
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 is the gradual evolution of a species that continues to exist as an interbreeding population. This contrasts with cladogenesis, which occurs when there is branching or splitting, leading to two or more lineages and resulting in separa ...
), and loss of species ( extinction
) throughout the
evolutionary history of life
The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''gigaannum'') and evide ...
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
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 ...
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life for ...
have continued to study various aspects of evolution by forming and testing hypotheses
as well as constructing theories based on evidence
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 ...
. Their discoveries have influenced not just the development of biology
but numerous other scientific and industrial fields, including
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 t ...
, and computer science
Evolution in organisms occurs through changes in heritable traits—the inherited characteristics of an organism. In humans, for example, eye colour
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
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 ...
(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 is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino.
Varied use and interpretation of the term ...
, who do not tan at all and are very sensitive to
Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and animals include: red or reddish skin that is ho ...
Heritable traits are passed from one generation to the next via DNA, a molecule
that encodes genetic information.
DNA is a long
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, c ...
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
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 (plural loci) is Latin for "place". It may refer to:
* Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front
* ''Locus'' (magazine), science fiction and fantasy magazine
** ''Locus Award ...
. 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
A quantitative trait locus (QTL) is a locus (section of DNA) that correlates with variation of a quantitative trait in the phenotype of a population of organisms. QTLs are mapped by identifying which molecular markers (such as SNPs or AFLPs) c ...
(multiple interacting genes).
Some heritable changes cannot be explained by changes to the sequence of
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 ...
s in the DNA. These phenomena are classed as epigenetic inheritance systems.
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 ...
, self-sustaining metabolic loops, gene silencing by RNA interference
and the three-dimensional conformation
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, resp ...
s (such as
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 i ...
s) are areas where epigenetic inheritance systems have been discovered at the organismic level.
Developmental biologists suggest that complex interactions in genetic networks
and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity
Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction
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 (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possib ...
Sources of variation
Evolution can occur if there is
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, b ...
within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction
and migration between populations (
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 may refer to:
* The process of mixing in process engineering
* Mixing paints to achieve a greater range of colors
* Blending (alcohol production), a technique to produce alcoholic beverages by mixing different brews
* Blending (linguis ...
. But with blending inheritance, genetic variation would be rapidly lost, making evolution by natural selection implausible. The
In population genetics, the Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in t ...
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.
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
'', 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
), 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
from pre-existing genes that share common ancestors. For example, the human eye
uses four genes to make structures that sense light: three for
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 ...
and one for night vision
; 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
). 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,
Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, discovery, biosynth ...
s are large
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 product ...
s that make antibiotics
; 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
piglets. They are camouflage colored and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in ''
melanocortin 1 receptor
The melanocortin 1 receptor (MC1R), also known as melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor, is a G protein–coupled receptor that binds to a class of pituitary peptide hormones ...
'' (''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
organisms, genes are inherited together, or ''linked'', as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sex
ual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called
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 be ...
, 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.
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
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
The Red Queen hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended t ...
has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to
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 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
between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.
Gene transfer between species includes the formation of
Hybrid may refer to:
* 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 di ...
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HG ...
. 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 (; 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 amon ...
. In medicine, this contributes to the spread of antibiotic 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'' () (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 ...
'' and the adzuki bean weevil '' Callosobruchus chinensis
'' has occurred. An example of larger-scale transfers are the eukaryotic bdelloid rotifers
, which have received a range of genes from bacteria, fungi
and plants. Virus
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
and bacteria, during the acquisition of chloroplast
A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
. It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea
From a neo-Darwinian
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.
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.
is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted
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
The central concept of natural selection is the evolutionary fitness
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 is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
. 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
). 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
s, teeth in chicken
s, wings in wingless stick insect
s, tails and additional nipples in humans etc. "Throwbacks" such as these are known as
In biology, an atavism is a modification of a biological structure whereby an ancestral genetic trait reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in several ways; one of which is when ...
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
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
there is selection against extreme trait values on both ends, which causes a decrease in
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 numbe ...
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
, that is, a system in which organisms interact with every other element, physical
as well as
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 ...
, in their local environment. Eugene Odum
, a founder of ecology
, 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
, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the
A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), det ...
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
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. Trans ...
, 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.
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
. 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
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
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.
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 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
most evolutionary changes are the result of the fixation of neutral mutation
s 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 (, ) 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 themselve ...
forces in evolution, such as genetic hitchhiking, also known as genetic draft.
Another concept is constructive neutral evolution
(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
to the complex interdependence of microbial communities
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
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.
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
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.
and Ernst Freese
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
, 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
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
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.
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
, with selectable marker
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
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
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
s. For example, the Mexican tetra
Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino.
Varied use and interpretation of the term ...
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
s evolve to be resistant to host immune defences, as well as to pharmaceutical drug
s. These same problems occur in agriculture with pesticide and herbicide
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
, simulations of evolution using
In computational intelligence (CI), an evolutionary algorithm (EA) is a subset of evolutionary computation, a generic population-based metaheuristic optimization algorithm. An EA uses mechanisms inspired by biological evolution, such as reprod ...
s 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
in the 1960s. He used evolution strategies
to solve complex engineering problems. Genetic algorithm
s in particular became popular through the writing of
John Henry Holland
John Henry Holland (February 2, 1929 – August 9, 2015) was an American scientist and Professor of psychology and Professor of electrical engineering and computer science at the University of Michigan, Ann Arbor. He was a pioneer in what became ...
. 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.
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
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 th ...
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 (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 parasi ...
. 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
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
A unit of selection is a biological entity within the hierarchy of biological organization (for example, an entity such as: a self-replicating molecule, a gene, a cell, an organism, a group, or a species) that is subject to natural 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
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 & Conn ...
s, which form about half the world's biomass
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 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
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'' (),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 '' Esc ...
'' evolving the ability to use
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'' 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
manufacturing, and the soil bacterium '' Sphingobium
'' evolving an entirely new metabolic pathway
that degrades the synthetic pesticide pentachlorophenol
. 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).
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
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
feet and primate
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
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
During evolution, some structures may lose their original function and become vestigial structures
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
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 ( : coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and othe ...
The appendix (or vermiform appendix; also cecal r caecalappendix; vermix; or vermiform process) is a finger-like, blind-ended tube connected to the cecum, from which it develops in the embryo. The cecum is a pouch-like structure of the large i ...
and other behavioural vestiges such as goose bumps
and primitive reflexes
However, many traits that appear to be simple adaptations are in fact
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 commo ...
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,
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 ...
s such as the bacterial flagella
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 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
to serve as structural proteins called crystallin
s 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
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
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.
Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a
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 ger ...
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
* Host, Pennsylvania, a village in Berks County
* Jim Host (born 1937), American businessman
* Michel Hos ...
, 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
The rough-skinned newt or roughskin newt (''Taricha granulosa'') is a North American newt known for the strong toxin exuded from its skin.
A stocky newt with rounded snout, it ranges from light brown to olive or brownish-black on t ...
and the evolution of tetrodotoxin resistance in its predator, the common garter snake
. In this predator-prey pair, an
evolutionary arms race
In evolutionary biology, an evolutionary arms race is an ongoing struggle between competing sets of co-evolving genes, phenotypic and behavioral traits that develop escalating adaptations and counter-adaptations against each other, resembling an ...
has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.
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
relationship as the plants provide the fungi with sugars from photosynthesis
. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals
that suppress the plant immune system
Coalitions between organisms of the same species have also evolved. An extreme case is the
Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping genera ...
found in social insects, such as
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 monophyletic lineage within the superfami ...
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 Blatto ...
Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of 22 ...
s, where sterile insects feed and guard the small number of organisms in a colony
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 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 altruistic behaviour whose evolution i ...
, 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 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
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
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
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
, it may still be possible for them to produce offspring, as with horses and donkey
s mating to produce mule
s. Such hybrids are generally
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 sta ...
. 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
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 tre ...
being a particularly well-studied example.
Speciation has been observed multiple times under both controlled laboratory conditions (see laboratory experiments of speciation
) 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 () – 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 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 proces ...
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 is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging g ...
, 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
causes rapid speciation after an increase in
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 ...
increases selection on homozygotes, leading to rapid genetic change.
The third mode is
In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes. This mode of speciation has three distinguishing characteristics: 1) mating occurs non-randomly, 2) gen ...
. 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
'', 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
In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher fr ...
, which is the evolution of traits that promote mating within a species, as well as
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 ...
, which is when two species become more distinct in appearance.
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 organism ...
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
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
the homologous chromosome
s 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'', 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
In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of i ...
, 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 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
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.
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;
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 sli ...
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
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
Era after a geological crust
started to solidify following the earlier molten
The Hadean ( ) is a geologic eon of Earth history preceding the Archean. On Earth, the Hadean began with the planet's formation about 4.54 billion years ago (although the start of the Hadean is defined as the age of the oldest solid materi ...
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
" 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
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
and the assembly of simple cells.
All organisms on Earth are descended from a common ancestor or ancestral gene pool
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
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.
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
" 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 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 ...
s. The development of
Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
has revealed the record of evolution left in organisms' genomes: dating when species diverged through the
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
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosome
s. Another engulfment of cyanobacteria
l-like organisms led to the formation of chloroplasts in algae and plants.
The history of life was that of the
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 an ...
eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran
The evolution of multicellularity
occurred in multiple independent events, in organisms as diverse as
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 ...
Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and p ...
, cyanobacteria, slime moulds
The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
. 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
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 reco ...
. 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
s first appeared around 364 million years ago, followed by early amniote
s and birds around 155 million years ago (both from " reptile
"-like lineages), mammal
s around 129 million years ago,
Homininae (), also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini (with the genus ''Homo'' including modern humans and numer ...
around 10 million years ago and modern humans
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
The proposal that one type of organism could descend from another type goes back to some of the first
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 ...
s, such as Anaximander
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 ...
. Such proposals survived into Roman times. The poet
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 som ...
'' (''On the Nature of Things'').
In contrast to these materialistic
Aristotelianism ( ) is a philosophical tradition inspired by the work of Aristotle, usually characterized by deductive logic and an analytic inductive method in the study of natural philosophy and metaphysics. It covers the treatment of the soci ...
had considered all natural things as actualisations
of fixed natural possibilities, known as
Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens.
Form also refers to:
*Form (document), a document (printed or electronic) with spaces in which to write or enter data
This became part of a medieval teleology, teleological understanding of Nature (philosophy), nature in which all things have an intended role to play in a divinity, divine cosmos, cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christianity, Christian 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, 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]
The Scientific revolution, "New Science" of the 17th century rejected the Aristotelian approach. It sought to explain natural phenomena in terms of physical laws 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 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 introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan.
Other Natural history, 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 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'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.)
These ideas were condemned by established naturalists as speculation lacking empirical support. In particular, Georges 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 into the ''Natural Theology or Evidences of the Existence and Attributes of the Deity'' (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.
* Letter 2532, 22 November 1859.]
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
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'' (1798) by Thomas Robert Malthus, 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
sent him a version of virtually the same theory in 1858. Their On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection, separate papers were presented together at an 1858 meeting of the Linnean Society of London. 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 Darwinism, Darwin's concepts of evolution at the expense of Alternatives to evolution by natural selection, alternative theories. Thomas Henry Huxley applied Darwin's ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe.
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 Mendelian inheritance#Law of Independent Assortment, independent assortment 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 cells that give rise to gametes (such as sperm and egg cells) and the somatic cells 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 and when expressed they could move into the cytoplasm to change the Cell (biology), 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 Mutationism, 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, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane 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 Transitional fossil, fossil transitions in palaeontology.
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 organisation, biological hierarchy from genes to populations.
The publication of the structure of DNA by James Watson and Francis Crick with contribution of Rosalind Franklin in 1953 demonstrated a physical mechanism for inheritance.
Molecular biology improved understanding of the relationship between genotype and phenotype. Advances were also made in phylogenetic systematics, mapping the transition of traits into a comparative and testable framework through the publication and use of Phylogenetic tree, evolutionary trees.
In 1973, evolutionary biologist Theodosius Dobzhansky penned that "Nothing in Biology Makes Sense Except in the Light of Evolution, nothing in biology makes sense except in the light of evolution," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent Explanation, explanatory body of knowledge that describes and predicts many observable facts about life on this planet.
One extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations (evolution) act on patterns of change within individual organisms (Developmental biology, development).
Since the beginning of the 21st century, some biologists have argued for an extended evolutionary synthesis, which would account for the effects of non-genetic inheritance modes, such as epigenetics, Maternal effect, parental effects, ecological inheritance and Dual inheritance theory, cultural inheritance, and evolvability.
Social and cultural responses
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 Theism, theists.
While Level of support for evolution#Religious, various religions and denominations have reconciled their beliefs with evolution through concepts such as theistic evolution, there are creationism, creationists who believe that evolution is contradicted by the creation myths found in their religions and who raise various objections to evolution.
As had been demonstrated by responses to the publication of ''Vestiges of the Natural History of Creation'' in 1844, the most controversial aspect of evolutionary biology is the implication of human evolution that humans share common ancestry with apes and that the mental and Evolution of morality, 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 and creation and evolution in public education, public education.
While other scientific fields such as physical cosmology, cosmology
and Earth science
also conflict with literal interpretations of many religious texts, 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'' 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 Pseudoscience, pseudoscientific form as intelligent design (ID), to be excluded once again in the 2005 ''Kitzmiller v. Dover Area School District'' case.
The debate over Darwin's ideas did not generate significant controversy in China.
* The notebook is available froThe Complete Work of Charles Darwin Online
. Retrieved 2019-10-09.
* The book is available froThe 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"
** American version.
* Adobe Flash required.
*History of Evolution in the United States
. Salon.com, Salon (Retrieved 2021-08-24)
Evolutionary biology, *