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Natural selection is the differential survival and reproduction of individuals due to differences in
phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen ...

phenotype
. It is a key mechanism of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
, the change in the
heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...

heritable
traits Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
characteristic of a
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
over generations.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all sp ...

Charles Darwin
popularised the term "natural selection", contrasting it with
artificial selection This Chihuahua (dog), Chihuahua mixed-breed dog, mix and Great Dane shows the wide range of dog breed sizes created using selective breeding. Selective breeding (also called artificial selection) is the process by which humans use animal breed ...
, which in his view is intentional, whereas natural selection is not.
Variation Variation or Variations may refer to: Science and mathematics * Variation (astronomy), any perturbation of the mean motion or orbit of a planet or satellite, particularly of the moon * Genetic variation thumb File:Genetic Variation and Inhe ...
exists within all populations of
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
s. This occurs partly because random
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s arise in the
genome In the fields of molecular biology and genetics, a genome is all genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The genome includes both the genes (the coding regions) and the noncodin ...

genome
of an individual organism, and their
offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more ...

offspring
can inherit such mutations. Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations,
species In biology, a species is the basic unit of biological classification, 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 individu ...

species
, as well as the abiotic environment. Because individuals with certain variants of the trait tend to survive and reproduce more than individuals with other less successful variants, the population evolves. Other factors affecting reproductive success include
sexual selection Sexual selection is a mode of natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrat ...
(now often included in natural selection) and
fecundity selection Fecundity selection, also known as fertility selection, is the fitness advantage resulting from the preference of traits that increase the number of offspring (i.e. fecundity Fecundity is defined in two ways; in human demography, it is the pote ...
. Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, but the
genetic
genetic
(heritable) basis of any phenotype that gives that phenotype a reproductive advantage may become more common in a population. Over time, this process can result in populations that specialise for particular
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
s (
microevolution Microevolution is the change in allele frequencies that occurs over time within a population. This change is due to four different processes: mutation Image:Darwin Hybrid Tulip Mutation 2014-05-01.jpg, A tulip flower exhibiting a partially y ...
) and may eventually result in
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
(the emergence of new species,
macroevolution Macroevolution in the modern sense is evolution that is guided by selection among interspecific variation, as opposed to selection among intraspecific variation in microevolution. This modern definition differs from the original concept, which re ...
). In other words, natural selection is a key process in the evolution of a population. Natural selection is a cornerstone of modern
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
. The concept, published by Darwin and
Alfred Russel Wallace Alfred Russel Wallace (8 January 18237 November 1913) was a British naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning more toward ...
in a joint presentation of papers in 1858, was elaborated in Darwin's influential 1859 book ''
On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life ''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 Mea ...
''. He described natural selection as analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favoured for reproduction. The concept of natural selection originally developed in the absence of a valid theory of heredity; at the time of Darwin's writing, science had yet to develop modern theories of genetics. The union of traditional
Darwinian evolution Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as obser ...
with subsequent discoveries in
classical genetics Classical genetics is the branch of genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mende ...
formed the modern synthesis of the mid-20th century. The addition of
molecular genetics 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 led to
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''org ...
, which explains evolution at the molecular level. While
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
s can slowly change by random
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, M ...

genetic drift
, natural selection remains the primary explanation for
adaptive evolution In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a state reached by the popula ...

adaptive evolution
.


Historical development


Pre-Darwinian theories

Several philosophers of the classical era, including
Empedocles Empedocles (; grc-gre, Ἐμπεδοκλῆς; , 444–443 BC) was a Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in So ...

Empedocles
and his intellectual successor, the
Roman Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened to ''Romans'', a letter in the New Testament of the Christian Bible Roman ...
poet
Lucretius Titus Lucretius Carus ( , ; 99 – c. 55 BC) was a Ancient Rome, Roman Roman literature, poet and Ancient Roman philosophy, philosopher. His only known work is the philosophical poem ''De rerum natura'', a didactic work about the tenets and ...
, expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental quest ...

Aristotle
in Book II of ''
Physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of eve ...
''. He posited natural
teleology Teleology (from and )Partridge, Eric. 1977''Origins: A Short Etymological Dictionary of Modern English'' London: Routledge, p. 4187. or finalityDubray, Charles. 2020 912 Year 912 ( CMXII) was a leap year starting on Wednesday (link will disp ...
in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof. Nevertheless, he accepted in his biology that new types of animals, monstrosities (τερας), can occur in very rare instances (''
Generation of Animals The ''Generation of Animals'' (or ''On the Generation of Animals''; Greek Περὶ ζῴων γενέσεως; Latin ''De Generatione Animalium'') is one of Aristotle's major Aristotle's biology, texts on biology. It describes the means by which a ...
'', Book IV). As quoted in Darwin's 1872 edition of ''
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 Mea ...
'', Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived: But Aristotle rejected this possibility in the next paragraph, making clear that he is talking about the development of animals as embryos with the phrase "either invariably or normally come about", not the origin of species: The
struggle for existence The concept of the struggle for existence concerns the competition or battle for resources needed to live. It can refer to human society, or to organisms in nature. The concept is ancient, and the term ''struggle for existence'' was in use by the e ...
was later described by the
Islam Islam (; ar, اَلْإِسْلَامُ, al-’Islām, "submission
o God Oh God may refer to: * An exclamation; similar to "oh no", "oh yes", "oh my", "aw goodness", "ah gosh", "ah gawd"; see interjection An interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling ...
) is an Abrahamic religions, Abrahamic monotheistic religion teaching that Muhammad is a Muhammad in Islam, messenger of God.Peters, F. E. 2009. "Allāh." In , ed ...
ic writer
Al-Jahiz Abū ʿUthman ʿAmr ibn Baḥr al-Kinānī al-Baṣrī ( ar, أبو عثمان عمرو بن بحر الكناني البصري), commonly known as al-Jāḥiẓ ( ar, links=no, الجاحظ, ''The Bug Eyed'', born 776; died December 868/Januar ...

Al-Jahiz
in the 9th century. The classical arguments were reintroduced in the 18th century by
Pierre Louis Maupertuis Pierre Louis Moreau de Maupertuis (; ; 1698 – 27 July 1759) was a French mathematician, philosopher and man of letters An intellectual is a person who engages in critical thinking, research, and Human self-reflection, reflection about t ...
and others, including Darwin's grandfather,
Erasmus Darwin Erasmus Robert Darwin (12 December 173118 April 1802) was an English physician. One of the key thinkers of the Midlands Enlightenment, he was also a natural philosophy, natural philosopher, physiology, physiologist, Society for Effecting the A ...
. Until the early 19th century, the prevailing view in
Western societies The Western world, also known as the West, refers to various regions, nation A nation is a community of people formed on the basis of a common language, history, ethnicity, or a common culture, and, in many cases, a shared territory. ...

Western societies
was that differences between individuals of a species were uninteresting departures from their Platonic ideals (or typus) of created kinds. However, the theory of
uniformitarianism at Jedburgh.Above: John Clerk of Eldin's 1787 illustration.Below: 2003 photograph. Uniformitarianism, also known as the Doctrine of Uniformity or the Uniformitarian Principle, is the assumption that the same natural laws and processes that operat ...
in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
's landscape. The success of this theory raised awareness of the vast scale of
geological time The geologic time scale (GTS) is a system of chronological datingChronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously establish ...

geological time
and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species. The early 19th-century zoologist
Jean-Baptiste Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist Natural history is a domain of inquiry involving organisms, including animals, fu ...

Jean-Baptiste Lamarck
suggested the
inheritance of acquired characteristics Inheritance is the practice of passing on private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causing
transmutation of species Transmutation of species and transformism are 18th and 19th-century evolutionary ideas about the change of one species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism ...
. This theory,
Lamarckism Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies ...

Lamarckism
, was an influence on the Soviet biologist
Trofim Lysenko Trofím Denísovich Lysénko (russian: Трофи́м Дени́сович Лысе́нко, uk, Трохи́м Дени́сович Лисе́нко, Trokhym Denysovych Lysenko; 20 November 1976) was a Soviet agronomist and biologist France ...
's antagonism to mainstream genetic theory as late as the mid 20th century. Between 1835 and 1837, the zoologist
Edward Blyth Edward Blyth (23 December 1810 – 27 December 1873) was an English zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology Biology is the natural scien ...
worked on the area of variation, artificial selection, and how a similar process occurs in nature. Darwin acknowledged Blyth's ideas in the first chapter on variation of ''On the Origin of Species''.


Darwin's theory

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for
adaptation In , adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits s to their environment, enhancing their . Secondly, it is a state reached by the population during that process. Thirdly, it is a or adapti ...

adaptation
and speciation. He defined natural selection as the "principle by which each slight variation f a trait if useful, is preserved". The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them and that variation is
heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...

heritable
, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to the evolution of particular
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
s of a species, and populations that evolve to be sufficiently different eventually become different species. Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS ''Beagle'' (1831–1836), and by the work of a political economist,
Thomas Robert Malthus Thomas Robert Malthus (; 13/14 February 1766 – 23 December 1834) was an English cleric, scholar and influential economist in the fields of political economy and demography. In his 1798 book ''An Essay on the Principle of Population The ...

Thomas Robert Malthus
, who, in ''
An Essay on the Principle of Population The book ''An Essay on the Principle of Population'' was first published anonymously in 1798, but the author was soon identified as Thomas Robert Malthus Thomas Robert Malthus (; 13/14 February 1766 – 23 December 1834) was an English cler ...

An Essay on the Principle of Population
'' (1798), noted that population (if unchecked)
increases exponentially
increases exponentially
, whereas the food supply grows only
arithmetically
arithmetically
; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence". When Darwin read Malthus in 1838 he was already primed by his work as a
naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classif ...

naturalist
to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species." Darwin wrote: Once he had his theory, Darwin was meticulous about gathering and refining evidence before making his idea public. He was in the process of writing his "big book" to present his research when the naturalist
Alfred Russel Wallace Alfred Russel Wallace (8 January 18237 November 1913) was a British naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning more toward ...
independently conceived of the principle and described it in an essay he sent to Darwin to forward to
Charles Lyell Sir Charles Lyell, 1st Baronet, (14 November 1797 – 22 February 1875) was a Scottish geologist who demonstrated the power of known natural causes in explaining the earth Earth is the third planet from the Sun and the only astro ...

Charles Lyell
. Lyell and
Joseph Dalton Hooker Sir Joseph Dalton Hooker (30 June 1817 – 10 December 1911) was a United Kingdom of Great Britain and Ireland, British botanist and explorer in the 19th century. He was a founder of geographical botany and Charles Darwin's closest friend ...

Joseph Dalton Hooker
decided to present his essay together with unpublished writings that Darwin had sent to fellow naturalists, and ''
On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection" is the title of a journal article, comprising and resulting from the joint presentation of two scientific papers to the ...
'' was read to the
Linnean Society of London The Linnean Society of London is a learned society dedicated to the study and dissemination of information concerning natural history, evolution, and Taxonomy (biology), taxonomy. It possesses several important biological specimen, manuscript an ...
announcing co-discovery of the principle in July 1858. Darwin published a detailed account of his evidence and conclusions in ''
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 ...
'' in 1859. In the 3rd edition of 1861 Darwin acknowledged that others—like
William Charles WellsDr William Charles Wells FRS FRSE FRCP (24 May 1757–18 September 1817) was a Scottish-American physician and printer. He lived a life of extraordinary variety, did some notable medical research, and made the first clear statement about natu ...
in 1813, and
Patrick Matthew Patrick Matthew (20 October 1790 – 8 June 1874) was a Scottish grain merchant, fruit farmer, forester, and landowner, who contributed to the understanding of horticulture, silviculture, and agriculture in general, with a focus on maintaining the ...
in 1831—had proposed similar ideas, but had neither developed them nor presented them in notable scientific publications. Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "
artificial selection This Chihuahua (dog), Chihuahua mixed-breed dog, mix and Great Dane shows the wide range of dog breed sizes created using selective breeding. Selective breeding (also called artificial selection) is the process by which humans use animal breed ...
"; in his early manuscripts he referred to a "Nature" which would do the selection. At the time, other mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, and Darwin believed that selection was likely only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification." In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural Preservation". For Darwin and his contemporaries, natural selection was in essence synonymous with evolution by natural selection. After the publication of ''On the Origin of Species'', educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a mechanism, partly because it was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its "unguided" and non-
progressive Progressive may refer to: Politics * Progressivism is a political philosophy in support of social reform Political organizations * Congressional Progressive Caucus, members within the Democratic Party in the United States Congress dedicated to th ...
nature, a response that has been characterised as the single most significant impediment to the idea's acceptance. However, some thinkers enthusiastically embraced natural selection; after reading Darwin,
Herbert Spencer Herbert Spencer (27 April 1820 – 8 December 1903) was an English philosopher, biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has speciali ...

Herbert Spencer
introduced the phrase ''
survival of the fittest "Survival of the fittest" is a phrase that originated from Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemp ...
'', which became a popular summary of the theory. The fifth edition of ''On the Origin of Species'' published in 1869 included Spencer's phrase as an alternative to natural selection, with credit given: "But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient." Although the phrase is still often used by non-biologists, modern biologists avoid it because it is tautological if "fittest" is read to mean "functionally superior" and is applied to individuals rather than considered as an averaged quantity over populations.


The modern synthesis

Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
. Although the
Moravia Moravia ( , also , ; cs, Morava ; german: link=no, Mähren ; pl, Morawy ; szl, Morawa; la, Moravia) is a in the east of the and one of three historical , with and . The medieval and early modern was a of the from 1348 to 1918, an ...

Moravia
n monk
Gregor Mendel Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders tha ...

Gregor Mendel
, the father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900. With the early 20th century integration of evolution with
Mendel's laws Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 and popularized by William Bateson. These princi ...
of inheritance, the so-called modern synthesis, scientists generally came to accept natural selection. The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wrote ''
The Genetical Theory of Natural Selection ''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British statistician, geneticist, and academic. For his work in statistics, he has been describ ...
'' (1930). introduced the concept of the "cost" of natural selection.
Sewall Wright Sewall Green Wright FRS(For) H FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory Evolutionary thought, the recognition that species change over time and the perceived unde ...

Sewall Wright
elucidated the nature of selection and adaptation. In his book ''
Genetics and the Origin of Species ''Genetics and the Origin of Species'' is a 1937 book by the Ukrainian-American evolutionary biologist Theodosius Dobzhansky. It is regarded as one of the most important works of the modern synthesis, and was one of the earliest. The book popular ...
'' (1937),
Theodosius Dobzhansky Theodosius Grygorovych Dobzhansky ( uk, Теодо́сій Григо́рович Добжа́нський; russian: Феодо́сий Григо́рьевич Добржа́нский; January 25, 1900 – December 18, 1975) was a prominent Ukra ...
established the idea that mutation, once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity.


A second synthesis

Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
recognised the key importance of
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
for speciation in his ''
Systematics and the Origin of Species ''Systematics and the Origin of Species from the Viewpoint of a Zoologist'' is a book written by zoologist and evolutionary biologist Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolu ...
'' (1942). W. D. Hamilton conceived of
kin selection Kin selection is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that ...
in 1964. This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. A second synthesis was brought about at the end of the 20th century by advances in
molecular genetics 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 ...
, creating the field of
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''org ...
("evo-devo"), which seeks to explain the evolution of
form Form is the shape A shape or figure is the form of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other properties such as color, Surface texture, texture, or material type. A plane shape, ...
in terms of the which control the development of the embryo at molecular level. Natural selection is here understood to act on embryonic development to change the morphology of the adult body.


Terminology

The term ''natural selection'' is most often defined to operate on heritable traits, because these directly participate in evolution. However, natural selection is "blind" in the sense that changes in phenotype can give a reproductive advantage regardless of whether or not the trait is heritable. Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "(phenotypic) natural selection" specifically as "those mechanisms that contribute to the selection of individuals that reproduce", without regard to whether the basis of the selection is heritable. Traits that cause greater reproductive success of an organism are said to be ''selected for'', while those that reduce success are ''selected against''.


Mechanism


Heritable variation, differential reproduction

Natural variation occurs among the individuals of any population of organisms. Some differences may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring. If the traits that give these individuals a reproductive advantage are also
heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...
, that is, passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way the
natural environment The natural environment or natural world encompasses all life, living and non-living things occurring nature, naturally, meaning in this case not Artificiality, artificial. The term is most often applied to the Earth or some parts of Earth. Th ...

natural environment
of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described. This gives the appearance of purpose, but in natural selection there is no intentional choice. Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it. The
peppered moth The peppered moth (''Biston betularia'') is a temperate species of night-flying moth Moths are a paraphyletic group of insects that includes all members of the Order (biology), order Lepidoptera that are not Butterfly, butterflies, with mot ...
exists in both light and dark colours in Great Britain, but during the
industrial revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Continental Europe or mainland Europe is the contiguous continent A continent is any of several large landmasse ...
, many of the trees on which the moths rested became blackened by
soot Soot ( ) is a mass of impure Carbonaceous, carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the resi ...
, giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial
Manchester Manchester () is the most-populous city and metropolitan borough A metropolitan borough is a type of local government district The districts of England (also known as local authority districts or local government districts to distinguis ...

Manchester
were dark. The balance was reversed by the effect of the
Clean Air Act 1956 The Clean Air Act 1956 was an Act of the Parliament of the United Kingdom The Parliament of the United Kingdom is the of the , the and the . It alone possesses and thereby ultimate power over all other political bodies in the UK and t ...
, and the dark moths became rare again, demonstrating the influence of natural selection on
peppered moth evolution The evolution of the peppered moth is an evolutionary instance of directional colour change in the moth population as a consequence of air pollution during the Industrial Revolution The Industrial Revolution was the transition to new man ...
. A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies the
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard The leopard (''Pan ...

camouflage
of moths to
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

predation
risk.


Fitness

The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "
survival of the fittest "Survival of the fittest" is a phrase that originated from Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemp ...
", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection". Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman. An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment. In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting.
Richard Lenski Richard Eimer Lenski (born August 13, 1956) is an American evolutionary biology, evolutionary biologist, a MacArthur "genius" fellow, a Hannah Distinguished Professor of Microbial ecology, Microbial Ecology at Michigan State University, and a memb ...

Richard Lenski
's classic ''E. coli'' long-term evolution experiment is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest"). The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.


Competition

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on a limited supply of a resource such as food, water, or
territory A territory is an administrative division Administrative division, administrative unitArticle 3(1). , country subdivision, administrative region, subnational entity, first-level subdivision, as well as many similar terms, are generic names ...
. Competition may be within or between species, and may be direct or indirect. Species less suited to compete should in theory either adapt or die out, since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among larger
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
s. Competition is modelled by ''r/K'' selection theory, which is based on
Robert MacArthur Robert Helmer MacArthur (April 7, 1930 – November 1, 1972) was a Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadi ...
and
E. O. Wilson Edward Osborne Wilson (born June 10, 1929), usually cited as E. O. Wilson, is an American biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has ...
's work on
island biogeography Insular biogeography or island biogeography is a field within biogeography Biogeography is the study of the distribution of species and ecosystems in geography, geographic space and through evolutionary history of life, geological time. Organis ...
. In this theory, selective pressures drive evolution in one of two stereotyped directions: ''r''- or ''K''-selection. These terms, ''r'' and ''K'', can be illustrated in a logistic model of
population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of populations as dynamical systems. History Population dynamics has traditionally been the dominant branch of mathematical biology, which has a ...
: : \frac=rN\left(1 - \frac\right) \qquad \! where ''r'' is the growth rate of the population (''N''), and ''K'' is the
carrying capacity The carrying capacity of an environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical ...
of its local environmental setting. Typically, ''r''-selected species exploit empty , and produce many offspring, each with a relatively low
probability Probability is the branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained ...

probability
of surviving to adulthood. In contrast, ''K''-selected species are strong competitors in crowded niches, and
invest Investment is the dedication of an asset to attain an increase in value over a period of time. Investment requires a sacrifice of some present asset, such as time, money, or effort. In finance Finance is the study of financial institution ...
more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood.


Classification

Natural selection can act on any heritable
phenotypic trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. Th ...
, and selective pressure can be produced by any aspect of the environment, including sexual selection and
competition Competition is a rivalry A rivalry is the state of two people or groups engaging in a lasting competitive relationship. Rivalry is the "against each other" spirit between two competing sides. The relationship itself may also be called "a riv ...
with members of the same or other species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants. Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.


By effect on a trait

Selection has different effects on traits.
Stabilizing selection Stabilizing selection (not to be confused with Negative selection (natural selection), negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait Trait may refer to: ...

Stabilizing selection
acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous.
Directional selection upThree types of selection In population genetics, directional selection, or positive selection is a mode of natural selection Natural selection is the differential survival and reproduction of individuals due to differences in pheno ...

Directional selection
favours extreme values of a trait. The uncommon
disruptive selection 225px, A chart showing three types of selection Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a phenotype, trait are favored over intermediate values. In this cas ...

disruptive selection
also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and
univariate In mathematics, a univariate object is an expression, equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geom ...
then both higher and lower trait levels are favoured. Disruptive selection can be a precursor to
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
.


By effect on genetic diversity

Alternatively, selection can be divided according to its effect on
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species In biology, a species is the basic unit of biological classificati ...
. Purifying or negative selection acts to remove genetic variation from the population (and is opposed by ''de novo'' mutation, which introduces new variation. In contrast, balancing selection acts to maintain genetic variation in a population, even in the absence of ''de novo'' mutation, by negative frequency-dependent selection. One mechanism for this is heterozygote advantage, where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human ABO blood group locus has been explained in this way.


By life cycle stage

Another option is to classify selection by the Biological life cycle, life cycle stage at which it acts. Some biologists recognise just two types: Natural selection#Types of selection, viability (or survival) selection, which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase the rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on gamete survival, and compatibility selection, acting on zygote formation.


By unit of selection

Selection can also be classified by the level or unit of selection. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. Gene selection acts directly at the level of the gene. In
kin selection Kin selection is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that ...
and intragenomic conflict, gene-level selection provides a more apt explanation of the underlying process. Group selection, if it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.


By resource being competed for

Finally, selection can be classified according to the Resource (biology), resource being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. Ecological selection is natural selection via any means other than sexual selection, such as kin selection, competition, and Infanticide (zoology), infanticide. Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism. Sexual selection as first articulated by Darwin (using the example of the Peafowl, peacock's tail) refers specifically to competition for mates, which can be ''intrasexual'', between individuals of the same sex, that is male–male competition, or ''intersexual'', where one gender mate choice, chooses mates, most often with males displaying and females choosing. However, in some species, mate choice is primarily by males, as in some fishes of the family Syngnathidae. Phenotypic traits can be signalling theory, displayed in one sex and desired in the other sex, causing a positive feedback loop called a Fisherian runaway, for example, the extravagant plumage of some male birds such as the peacock. An alternate theory proposed by the same Ronald Fisher in 1930 is the sexy son hypothesis, that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of Deer, stags, which are used in combat with other stags. More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.


Arms races

Natural selection is seen in action in the development of Antimicrobial resistance, antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928, antibiotics have been used to fight bacterial diseases. The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to the point that the Methicillin-resistant Staphylococcus aureus, methicillin-resistant ''Staphylococcus aureus'' (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs. Response strategies typically include the use of different, stronger antibiotics; however, new strain (biology), strains of MRSA have recently emerged that are resistant even to these drugs. This is an evolutionary arms race, in which bacteria develop strains less susceptible to antibiotics, while medical researchers attempt to develop new antibiotics that can kill them. A similar situation occurs with pesticide resistance in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the spread of a gene in the butterfly ''Hypolimnas bolina'' suppressing male-killing activity by ''Wolbachia'' bacteria parasites on the island of Samoa, where the spread of the gene is known to have occurred over a period of just five years


Evolution by means of natural selection

A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that results in fitness differences. Genetic variation is the result of mutations, genetic recombinations and alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in noncoding DNA or resulted in a synonymous substitution. However, many mutations in non-coding DNA have deleterious effects. Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious. Some mutations occur in evo-devo gene toolkit, "toolkit" or regulatory genes. Changes in these often have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable embryos. Some nonlethal regulatory mutations occur in Homeobox#Hox genes, HOX genes in humans, which can result in a cervical rib or polydactyly, an increase in the number of fingers or toes. When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in the population. Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in a Vestigiality, vestigial manifestation of the trait, also called evolutionary baggage. In many circumstances, the apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known as Exaptation, preadaptation. A famous example of a vestigial structure, the eye of the Spalax, blind mole-rat, is believed to retain function in Photoperiodism, photoperiod perception.


Speciation

Speciation requires a degree of
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
—that is, a reduction in gene flow. However, it is intrinsic to the concept of a
species In biology, a species is the basic unit of biological classification, 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 individu ...

species
that hybrid (biology), hybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur in allopatric speciation with geographically separated populations, which can diverge with different sets of mutations. E. B. Poulton realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene. Selection against the heterozygote would then directly create reproductive isolation, leading to the Bateson–Dobzhansky–Muller model, further elaborated by H. Allen Orr and Sergey Gavrilets. With Reinforcement (speciation), reinforcement, however, natural selection can favor an increase in pre-zygotic isolation, influencing the process of speciation directly.


Genetic basis


Genotype and phenotype

Natural selection acts on an organism's phenotype, or physical characteristics. Phenotype is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. When different organisms in a population possess different versions of a gene for a certain trait, each of these versions is known as an allele. It is this genetic variation that underlies differences in phenotype. An example is the ABO blood type antigens in humans, where three alleles govern the phenotype. Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can produce a continuum of possible phenotypic values.


Directionality of selection

When some component of a trait is heritable, selection alters the frequencies of the different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional selection, directional, stabilizing selection, stabilizing, and
disruptive selection 225px, A chart showing three types of selection Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a phenotype, trait are favored over intermediate values. In this cas ...

disruptive selection
. Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until the allele is fixation (population genetics), fixed and the entire population shares the fitter phenotype. Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selection Conserved sequence, conserves functional genetic features, such as protein biosynthesis, protein-coding genes or regulatory sequences, over time by selective pressure against deleterious variants. Disruptive (or diversifying) selection is selection favoring extreme trait values over intermediate trait values. Disruptive selection may cause sympatric speciation through niche partitioning. Some forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in diploid species (with pairs of chromosomes) when Zygosity#Heterozygous, heterozygous individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous for sickle-cell anaemia. Maintenance of allelic variation can also occur through disruptive selection, disruptive or diversifying selection, which favours genotypes that depart from the average in either direction (that is, the opposite of over-dominance), and can result in a Multimodal distribution, bimodal distribution of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of game theory have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution of reciprocal altruism.


Selection, genetic variation, and drift

A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness. Motoo Kimura's neutral theory of molecular evolution by
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, M ...

genetic drift
proposes that this variation accounts for a large fraction of observed genetic diversity. Neutral events can radically reduce genetic variation through population bottlenecks. which among other things can cause the founder effect in initially small new populations. When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness. However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a mutation–selection balance. The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavourable the mutation proves to be. Genetic linkage occurs when the locus (genetics), loci of two alleles are in close proximity on a chromosome. During the formation of gametes, recombination reshuffles the alleles. The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them. Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, closely linked alleles can also become more common by "genetic hitchhiking", whether they are neutral or even slightly deleterious. A strong selective sweep results in a region of the genome where the positively selected haplotype (the allele and its neighbours) are in essence the only ones that exist in the population. Selective sweeps can be detected by measuring linkage disequilibrium, or whether a given haplotype is overrepresented in the population. Since a selective sweep also results in selection of neighbouring alleles, the presence of a block of strong linkage disequilibrium might indicate a 'recent' selective sweep near the centre of the block. Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation tends to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it does not produce clear blocks of linkage disequilibrium, although with low recombination it can still lead to slightly negative linkage disequilibrium overall.


Impact

Darwin's ideas, along with those of Adam Smith and Karl Marx, had a profound influence on 19th century thought, including his radical claim that "elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner" evolved from the simplest forms of life by a few simple principles. This inspired some of Darwin's most ardent supporters—and provoked the strongest opposition. Natural selection had the power, according to Stephen Jay Gould, to "dethrone some of the deepest and most traditional comforts of Western thought", such as the belief that humans have a special place in the world. In the words of the philosopher Daniel Dennett, "Darwin's dangerous idea" of evolution by natural selection is a "universal acid," which cannot be kept restricted to any vessel or container, as it soon leaks out, working its way into ever-wider surroundings. Thus, in the last decades, the concept of natural selection has spread from evolutionary biology to other disciplines, including evolutionary computation, quantum Darwinism, evolutionary economics, evolutionary epistemology, evolutionary psychology, and Lee Smolin#Cosmological natural selection, cosmological natural selection. This unlimited applicability has been called universal Darwinism.


Origin of life

How life originated from inorganic matter remains an unresolved problem in biology. One prominent hypothesis is that life first appeared RNA world, in the form of short self-replicating RNA polymers. On this view, life may have come into existence when RNA chains first experienced the basic conditions, as conceived by Charles Darwin, for natural selection to operate. These conditions are: heritability, Genetic variability, variation of type, and competition for limited resources. The fitness of an early Abiogenesis#RNA synthesis and replication, RNA replicator would likely have been a function of adaptive capacities that were intrinsic (i.e., determined by the Nucleic acid sequence, nucleotide sequence) and the availability of resources. The three primary adaptive capacities could logically have been: (1) the capacity to replicate with moderate fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and process resources. These capacities would have been determined initially by the folded configurations (including those configurations with ribozyme activity) of the RNA replicators that, in turn, would have been encoded in their individual nucleotide sequences.


Cell and molecular biology

In 1881, the embryologist Wilhelm Roux published ''Der Kampf der Theile im Organismus'' (''The Struggle of Parts in the Organism'') in which he suggested that the development of an organism results from a Darwinian competition between the parts of the embryo, occurring at all levels, from molecules to organs. In recent years, a modern version of this theory has been proposed by :fr:Jean-Jacques Kupiec, Jean-Jacques Kupiec. According to this cellular Darwinism, stochastic, random variation at the molecular level generates diversity in cell types whereas cell interactions impose a characteristic order on the developing embryo.


Social and psychological theory

The social implications of the theory of evolution by natural selection also became the source of continuing controversy. Friedrich Engels, a German Political philosophy, political philosopher and co-originator of the ideology of communism, wrote in 1872 that "Darwin did not know what a bitter satire he wrote on mankind, and especially on his countrymen, when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the ''animal kingdom''." Herbert Spencer and the eugenics advocate Francis Galton's interpretation of natural selection as necessarily progressive, leading to supposed advances in intelligence and civilisation, became a justification for colonialism, eugenics, and social Darwinism. For example, in 1940, Konrad Lorenz, in writings that he subsequently disowned, used the theory as a justification for policies of the Nazi state. He wrote "... selection for toughness, heroism, and social utility ... must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect." Others have developed ideas that human societies and culture Sociocultural evolution, evolve by mechanisms analogous to those that apply to evolution of species. More recently, work among anthropologists and psychologists has led to the development of sociobiology and later of evolutionary psychology, a field that attempts to explain features of Psychology, human psychology in terms of adaptation to the ancestral environment. The most prominent example of evolutionary psychology, notably advanced in the early work of Noam Chomsky and later by Steven Pinker, is the hypothesis that the human brain has adapted to language acquisition, acquire the grammar, grammatical rules of natural language. Other aspects of human behaviour and social structures, from specific cultural norms such as Westermarck effect#Westermarck effect, incest avoidance to broader patterns such as gender roles, have been hypothesised to have similar origins as adaptations to the early environment in which modern humans evolved. By analogy to the action of natural selection on genes, the concept of memes—"units of cultural transmission," or culture's equivalents of genes undergoing selection and recombination—has arisen, first described in this form by Richard Dawkins in 1976 and subsequently expanded upon by philosophers such as Daniel Dennett as explanations for complex cultural activities, including human consciousness.


Information and systems theory

In 1922, Alfred J. Lotka proposed that natural selection might be understood as a physical principle that could be described in terms of the use of energy by a system, a concept later developed by Howard T. Odum as the maximum power principle in thermodynamics, whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation. The principles of natural selection have inspired a variety of computational techniques, such as "soft" artificial life, that simulate selective processes and can be highly efficient in 'adapting' entities to an environment defined by a specified fitness function. For example, a class of heuristic Mathematical optimization, optimisation algorithms known as genetic algorithms, pioneered by John Henry Holland in the 1970s and expanded upon by David E. Goldberg, identify optimal solutions by simulated reproduction and mutation of a population of solutions defined by an initial probability distribution. Such algorithms are particularly useful when applied to problems whose energy landscape is very rough or has many local minima.


In fiction

Darwinian evolution by natural selection is pervasive in literature, whether taken optimistically in terms of how humanity may evolve towards perfection, or pessimistically in terms of the dire consequences of the interaction of human nature and the struggle for survival. Among major responses is Samuel Butler (novelist), Samuel Butler's 1872 pessimistic ''Erewhon'' ("nowhere", written mostly backwards). In 1893 H. G. Wells imagined "The Man of the Year Million", transformed by natural selection into a being with a huge head and eyes, and shrunken body.


Notes


References


Sources

* * * * * * Modified from Christiansen by adding survival selection in the reproductive phase. * The book is available fro
The Complete Work of Charles Darwin Online
Retrieved 2015-07-23. * * * * * * * ** * * . * * The book is available from th

* * * * * "This book is based on a series of lectures delivered in January 1931 at the Prifysgol Cymru, Aberystwyth, and entitled 'A re-examination of Darwinism'." * * * . * * * * * * The book is availabl

from Frank Elwell, Rogers State University. * * * * * * * * Retrieved 2015-08-11. * * * * *


Further reading

* For technical audiences ** ** ** ** ** ** ** ** * For general audiences ** ** ** ** ** ** * Historical ** **


External links

*  – Chapter 4, Natural Selection {{Authority control Natural selection, Biological interactions Charles Darwin Competition Ecological processes Ethology Evolution Evolutionary biology Selection Sexual selection