Natural selection

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For other uses, see Natural Selection (disambiguation). Natural selection_sentence_0

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. Natural selection_sentence_1

It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Natural selection_sentence_2

Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not. Natural selection_sentence_3

Variation exists within all populations of organisms. Natural selection_sentence_4

This occurs partly because random mutations arise in the genome of an individual organism, and their offspring can inherit such mutations. Natural selection_sentence_5

Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. Natural selection_sentence_6

The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment. Natural selection_sentence_7

Because individuals with certain variants of the trait tend to survive and reproduce more than individuals with other less successful variants, the population evolves. Natural selection_sentence_8

Other factors affecting reproductive success include sexual selection (now often included in natural selection) and fecundity selection. Natural selection_sentence_9

Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, but the genetic (heritable) basis of any phenotype that gives that phenotype a reproductive advantage may become more common in a population. Natural selection_sentence_10

Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species, macroevolution). Natural selection_sentence_11

In other words, natural selection is a key process in the evolution of a population. Natural selection_sentence_12

Natural selection is a cornerstone of modern biology. Natural selection_sentence_13

The concept, published by Darwin and Alfred Russel Wallace 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. Natural selection_sentence_14

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. Natural selection_sentence_15

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. Natural selection_sentence_16

The union of traditional Darwinian evolution with subsequent discoveries in classical genetics formed the modern synthesis of the mid-20th century. Natural selection_sentence_17

The addition of molecular genetics has led to evolutionary developmental biology, which explains evolution at the molecular level. Natural selection_sentence_18

While genotypes can slowly change by random genetic drift, natural selection remains the primary explanation for adaptive evolution. Natural selection_sentence_19

Historical development Natural selection_section_0

Main article: History of evolutionary thought Natural selection_sentence_20

Pre-Darwinian theories Natural selection_section_1

Several philosophers of the classical era, including Empedocles and his intellectual successor, the Roman poet Lucretius, 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. Natural selection_sentence_21

Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by Aristotle in Book II of Physics. Natural selection_sentence_22

He posited natural teleology in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof. Natural selection_sentence_23

Nevertheless, he accepted in his biology that new types of animals, monstrosities (τερας), can occur in very rare instances (Generation of Animals, Book IV). Natural selection_sentence_24

As quoted in Darwin's 1872 edition of The Origin of Species, Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived: Natural selection_sentence_25

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: Natural selection_sentence_26

The struggle for existence was later described by the Islamic writer Al-Jahiz in the 9th century. Natural selection_sentence_27

The classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis and others, including Darwin's grandfather, Erasmus Darwin. Natural selection_sentence_28

Until the early 19th century, the prevailing view in Western societies was that differences between individuals of a species were uninteresting departures from their Platonic ideals (or ) of created kinds. Natural selection_sentence_29

However, the theory of uniformitarianism in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. Natural selection_sentence_30

The success of this theory raised awareness of the vast scale of 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. Natural selection_sentence_31

The early 19th-century zoologist Jean-Baptiste Lamarck suggested the inheritance of acquired characteristics 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. Natural selection_sentence_32

This theory, Lamarckism, was an influence on the Soviet biologist Trofim Lysenko's antagonism to mainstream genetic theory as late as the mid 20th century. Natural selection_sentence_33

Between 1835 and 1837, the zoologist Edward Blyth worked on the area of variation, artificial selection, and how a similar process occurs in nature. Natural selection_sentence_34

Darwin acknowledged Blyth's ideas in the first chapter on variation of On the Origin of Species. Natural selection_sentence_35

Darwin's theory Natural selection_section_2

Main articles: Inception of Darwin's theory and Development of Darwin's theory Natural selection_sentence_36

Further information: Coloration evidence for natural selection Natural selection_sentence_37

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. Natural selection_sentence_38

He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved". Natural selection_sentence_39

The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. Natural selection_sentence_40

As long as there is some variation between them and that variation is heritable, there will be an inevitable selection of individuals with the most advantageous variations. Natural selection_sentence_41

If the variations are heritable, then differential reproductive success leads to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species. Natural selection_sentence_42

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, who, in An Essay on the Principle of Population (1798), noted that population (if unchecked) increases exponentially, whereas the food supply grows only arithmetically; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence". Natural selection_sentence_43

When Darwin read Malthus in 1838 he was already primed by his work as a naturalist to appreciate the "struggle for existence" in nature. Natural selection_sentence_44

It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. Natural selection_sentence_45

The result of this would be the formation of new species." Natural selection_sentence_46

Darwin wrote: Natural selection_sentence_47

Once he had his theory, Darwin was meticulous about gathering and refining evidence before making his idea public. Natural selection_sentence_48

He was in the process of writing his "big book" to present his research when the naturalist Alfred Russel Wallace independently conceived of the principle and described it in an essay he sent to Darwin to forward to Charles Lyell. Natural selection_sentence_49

Lyell and 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 was read to the Linnean Society of London announcing co-discovery of the principle in July 1858. Natural selection_sentence_50

Darwin published a detailed account of his evidence and conclusions in On the Origin of Species in 1859. Natural selection_sentence_51

In the 3rd edition of 1861 Darwin acknowledged that others—like William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar ideas, but had neither developed them nor presented them in notable scientific publications. Natural selection_sentence_52

Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "artificial selection"; in his early manuscripts he referred to a "Nature" which would do the selection. Natural selection_sentence_53

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." Natural selection_sentence_54

In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural Preservation". Natural selection_sentence_55

For Darwin and his contemporaries, natural selection was in essence synonymous with evolution by natural selection. Natural selection_sentence_56

After the publication of On the Origin of Species, educated people generally accepted that evolution had occurred in some form. Natural selection_sentence_57

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 nature, a response that has been characterised as the single most significant impediment to the idea's acceptance. Natural selection_sentence_58

However, some thinkers enthusiastically embraced natural selection; after reading Darwin, Herbert Spencer introduced the phrase survival of the fittest, which became a popular summary of the theory. Natural selection_sentence_59

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." Natural selection_sentence_60

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. Natural selection_sentence_61

The modern synthesis Natural selection_section_3

Main article: Modern synthesis (20th century) Natural selection_sentence_62

Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of genetics. Natural selection_sentence_63

Although the Moravian monk Gregor Mendel, the father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900. Natural selection_sentence_64

With the early 20th century integration of evolution with Mendel's laws of inheritance, the so-called modern synthesis, scientists generally came to accept natural selection. Natural selection_sentence_65

The synthesis grew from advances in different fields. Natural selection_sentence_66

Ronald Fisher developed the required mathematical language and wrote The Genetical Theory of Natural Selection (1930). Natural selection_sentence_67

J. Natural selection_sentence_68 B. S. Haldane introduced the concept of the "cost" of natural selection. Natural selection_sentence_69

Sewall Wright elucidated the nature of selection and adaptation. Natural selection_sentence_70

In his book Genetics and the Origin of Species (1937), Theodosius Dobzhansky established the idea that mutation, once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity. Natural selection_sentence_71

A second synthesis Natural selection_section_4

Main article: Evolutionary developmental biology § History Natural selection_sentence_72

Ernst Mayr recognised the key importance of reproductive isolation for speciation in his Systematics and the Origin of Species (1942). Natural selection_sentence_73

W. Natural selection_sentence_74 D. Hamilton conceived of kin selection in 1964. Natural selection_sentence_75

This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. Natural selection_sentence_76

A second synthesis was brought about at the end of the 20th century by advances in molecular genetics, creating the field of evolutionary developmental biology ("evo-devo"), which seeks to explain the evolution of form in terms of the genetic regulatory programs which control the development of the embryo at molecular level. Natural selection_sentence_77

Natural selection is here understood to act on embryonic development to change the morphology of the adult body. Natural selection_sentence_78

Terminology Natural selection_section_5

The term natural selection is most often defined to operate on heritable traits, because these directly participate in evolution. Natural selection_sentence_79

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. Natural selection_sentence_80

Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. Natural selection_sentence_81

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. Natural selection_sentence_82

Traits that cause greater reproductive success of an organism are said to be selected for, while those that reduce success are selected against. Natural selection_sentence_83

Mechanism Natural selection_section_6

Heritable variation, differential reproduction Natural selection_section_7

Main article: Genetic variation Natural selection_sentence_84

Natural variation occurs among the individuals of any population of organisms. Natural selection_sentence_85

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. Natural selection_sentence_86

If the traits that give these individuals a reproductive advantage are also heritable, 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. Natural selection_sentence_87

Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. Natural selection_sentence_88

In this way the natural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described. Natural selection_sentence_89

This gives the appearance of purpose, but in natural selection there is no intentional choice. Natural selection_sentence_90

Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it. Natural selection_sentence_91

The peppered moth exists in both light and dark colours in Great Britain, but during the industrial revolution, many of the trees on which the moths rested became blackened by soot, giving the dark-coloured moths an advantage in hiding from predators. Natural selection_sentence_92

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 were dark. Natural selection_sentence_93

The balance was reversed by the effect of the Clean Air Act 1956, and the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution. Natural selection_sentence_94

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 of moths to predation risk. Natural selection_sentence_95

Fitness Natural selection_section_8

Main article: Fitness (biology) Natural selection_sentence_96

The concept of fitness is central to natural selection. Natural selection_sentence_97

In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. Natural selection_sentence_98

Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. Natural selection_sentence_99

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. Natural selection_sentence_100

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. Natural selection_sentence_101

The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. Natural selection_sentence_102

A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". Natural selection_sentence_103

"Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. Natural selection_sentence_104

A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection". Natural selection_sentence_105

Haldane called this process "substitution" or more commonly in biology, this is called "fixation". Natural selection_sentence_106

This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. Natural selection_sentence_107

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. Natural selection_sentence_108

The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. Natural selection_sentence_109

The mathematics of "improvement in fitness was described by Kleinman. Natural selection_sentence_110

An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment. Natural selection_sentence_111

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. Natural selection_sentence_112

Fixation or substitution is not required for this "improvement in fitness". Natural selection_sentence_113

On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. Natural selection_sentence_114

Richard Lenski's classic E. Natural selection_sentence_115 coli long-term evolution experiment is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest"). Natural selection_sentence_116

The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. Natural selection_sentence_117

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. Natural selection_sentence_118

Competition Natural selection_section_9

Main article: Competition (biology) Natural selection_sentence_119

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. Natural selection_sentence_120

This may be because both rely on a limited supply of a resource such as food, water, or territory. Natural selection_sentence_121

Competition may be within or between species, and may be direct or indirect. Natural selection_sentence_122

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 clades. Natural selection_sentence_123

Competition is modelled by r/K selection theory, which is based on Robert MacArthur and E. Natural selection_sentence_124 O. Wilson's work on island biogeography. Natural selection_sentence_125

In this theory, selective pressures drive evolution in one of two stereotyped directions: r- or K-selection. Natural selection_sentence_126

These terms, r and K, can be illustrated in a logistic model of population dynamics: Natural selection_sentence_127

where r is the growth rate of the population (N), and K is the carrying capacity of its local environmental setting. Natural selection_sentence_128

Typically, r-selected species exploit empty niches, and produce many offspring, each with a relatively low probability of surviving to adulthood. Natural selection_sentence_129

In contrast, K-selected species are strong competitors in crowded niches, and invest more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood. Natural selection_sentence_130

Classification Natural selection_section_10

Natural selection can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species. Natural selection_sentence_131

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. Natural selection_sentence_132

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. Natural selection_sentence_133

By effect on a trait Natural selection_section_11

Selection has different effects on traits. Natural selection_sentence_134

Stabilizing selection acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous. Natural selection_sentence_135

Directional selection favours extreme values of a trait. Natural selection_sentence_136

The uncommon disruptive selection also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. Natural selection_sentence_137

In particular, if the trait is quantitative and univariate then both higher and lower trait levels are favoured. Natural selection_sentence_138

Disruptive selection can be a precursor to speciation. Natural selection_sentence_139

By effect on genetic diversity Natural selection_section_12

Alternatively, selection can be divided according to its effect on genetic diversity. Natural selection_sentence_140

Purifying or negative selection acts to remove genetic variation from the population (and is opposed by de novo mutation, which introduces new variation. Natural selection_sentence_141

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. Natural selection_sentence_142

One mechanism for this is heterozygote advantage, where individuals with two different alleles have a selective advantage over individuals with just one allele. Natural selection_sentence_143

The polymorphism at the human ABO blood group locus has been explained in this way. Natural selection_sentence_144

By life cycle stage Natural selection_section_13

Another option is to classify selection by the life cycle stage at which it acts. Natural selection_sentence_145

Some biologists recognise just two types: 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. Natural selection_sentence_146

Others split the life cycle into further components of selection. Natural selection_sentence_147

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. Natural selection_sentence_148

By unit of selection Natural selection_section_14

Selection can also be classified by the level or unit of selection. Natural selection_sentence_149

Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. Natural selection_sentence_150

Gene selection acts directly at the level of the gene. Natural selection_sentence_151

In kin selection and intragenomic conflict, gene-level selection provides a more apt explanation of the underlying process. Natural selection_sentence_152

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. Natural selection_sentence_153

There is an ongoing debate over the degree to which group selection occurs in nature. Natural selection_sentence_154

By resource being competed for Natural selection_section_15

Further information: Sexual selection Natural selection_sentence_155

Finally, selection can be classified according to the resource being competed for. Natural selection_sentence_156

Sexual selection results from competition for mates. Natural selection_sentence_157

Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. Natural selection_sentence_158

Ecological selection is natural selection via any means other than sexual selection, such as kin selection, competition, and infanticide. Natural selection_sentence_159

Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism. Natural selection_sentence_160

Sexual selection as first articulated by Darwin (using the example of the 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 chooses mates, most often with males displaying and females choosing. Natural selection_sentence_161

However, in some species, mate choice is primarily by males, as in some fishes of the family Syngnathidae. Natural selection_sentence_162

Phenotypic traits can be 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. Natural selection_sentence_163

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. Natural selection_sentence_164

Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of stags, which are used in combat with other stags. Natural selection_sentence_165

More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species. Natural selection_sentence_166

Arms races Natural selection_section_16

Further information: Antimicrobial resistance Natural selection_sentence_167

Natural selection is seen in action in the development of antibiotic resistance in microorganisms. Natural selection_sentence_168

Since the discovery of penicillin in 1928, antibiotics have been used to fight bacterial diseases. Natural selection_sentence_169

The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to the point that the 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. Natural selection_sentence_170

Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. Natural selection_sentence_171

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. Natural selection_sentence_172

A similar situation occurs with pesticide resistance in plants and insects. Natural selection_sentence_173

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 Natural selection_sentence_174

Evolution by means of natural selection Natural selection_section_17

Main articles: Evolution and Darwinism Natural selection_sentence_175

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. Natural selection_sentence_176

Genetic variation is the result of mutations, genetic recombinations and alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes). Natural selection_sentence_177

Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are rare. Natural selection_sentence_178

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. Natural selection_sentence_179

However, many mutations in non-coding DNA have deleterious effects. Natural selection_sentence_180

Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious. Natural selection_sentence_181

Some mutations occur in "toolkit" or regulatory genes. Natural selection_sentence_182

Changes in these often have large effects on the phenotype of the individual because they regulate the function of many other genes. Natural selection_sentence_183

Most, but not all, mutations in regulatory genes result in non-viable embryos. Natural selection_sentence_184

Some nonlethal regulatory mutations occur in HOX genes in humans, which can result in a cervical rib or polydactyly, an increase in the number of fingers or toes. Natural selection_sentence_185

When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in the population. Natural selection_sentence_186

Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. Natural selection_sentence_187

In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in a vestigial manifestation of the trait, also called evolutionary baggage. Natural selection_sentence_188

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 preadaptation. Natural selection_sentence_189

A famous example of a vestigial structure, the eye of the blind mole-rat, is believed to retain function in photoperiod perception. Natural selection_sentence_190

Speciation Natural selection_section_18

Main article: Speciation Natural selection_sentence_191

Speciation requires a degree of reproductive isolation—that is, a reduction in gene flow. Natural selection_sentence_192

However, it is intrinsic to the concept of a species that hybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. Natural selection_sentence_193

The problem does not occur in allopatric speciation with geographically separated populations, which can diverge with different sets of mutations. Natural selection_sentence_194

E. Natural selection_sentence_195 B. Poulton realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene. Natural selection_sentence_196

Selection against the heterozygote would then directly create reproductive isolation, leading to the Bateson–Dobzhansky–Muller model, further elaborated by H. Natural selection_sentence_197 Allen Orr and Sergey Gavrilets. Natural selection_sentence_198

With reinforcement, however, natural selection can favor an increase in pre-zygotic isolation, influencing the process of speciation directly. Natural selection_sentence_199

Genetic basis Natural selection_section_19

Genotype and phenotype Natural selection_section_20

Main article: Genotype–phenotype distinction Natural selection_sentence_200

Natural selection acts on an organism's phenotype, or physical characteristics. Natural selection_sentence_201

Phenotype is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. Natural selection_sentence_202

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. Natural selection_sentence_203

It is this genetic variation that underlies differences in phenotype. Natural selection_sentence_204

An example is the ABO blood type antigens in humans, where three alleles govern the phenotype. Natural selection_sentence_205

Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. Natural selection_sentence_206

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. Natural selection_sentence_207

Directionality of selection Natural selection_section_21

Main article: Directional selection Natural selection_sentence_208

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. Natural selection_sentence_209

Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional, stabilizing, and disruptive selection. Natural selection_sentence_210

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. Natural selection_sentence_211

This process can continue until the allele is fixed and the entire population shares the fitter phenotype. Natural selection_sentence_212

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. Natural selection_sentence_213

This process can continue until the allele is eliminated from the population. Natural selection_sentence_214

Stabilizing selection conserves functional genetic features, such as protein-coding genes or regulatory sequences, over time by selective pressure against deleterious variants. Natural selection_sentence_215

Disruptive (or diversifying) selection is selection favoring extreme trait values over intermediate trait values. Natural selection_sentence_216

Disruptive selection may cause sympatric speciation through niche partitioning. Natural selection_sentence_217

Some forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. Natural selection_sentence_218

This can occur in diploid species (with pairs of chromosomes) when heterozygous individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). Natural selection_sentence_219

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. Natural selection_sentence_220

Maintenance of allelic variation can also occur through 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 bimodal distribution of trait values. Natural selection_sentence_221

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. Natural selection_sentence_222

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. Natural selection_sentence_223

Selection, genetic variation, and drift Natural selection_section_22

Main articles: Genetic variation and Genetic drift Natural selection_sentence_224

A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness. Natural selection_sentence_225

Motoo Kimura's neutral theory of molecular evolution by genetic drift proposes that this variation accounts for a large fraction of observed genetic diversity. Natural selection_sentence_226

Neutral events can radically reduce genetic variation through population bottlenecks. Natural selection_sentence_227

which among other things can cause the founder effect in initially small new populations. Natural selection_sentence_228

When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. Natural selection_sentence_229

As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness. Natural selection_sentence_230

However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection_sentence_231

Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently the mutations that caused the maladaptation. Natural selection_sentence_232

At the same time, new mutations occur, resulting in a mutation–selection balance. Natural selection_sentence_233

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. Natural selection_sentence_234

Genetic linkage occurs when the loci of two alleles are in close proximity on a chromosome. Natural selection_sentence_235

During the formation of gametes, recombination reshuffles the alleles. Natural selection_sentence_236

The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them. Natural selection_sentence_237

Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. Natural selection_sentence_238

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. Natural selection_sentence_239

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. Natural selection_sentence_240

Selective sweeps can be detected by measuring linkage disequilibrium, or whether a given haplotype is overrepresented in the population. Natural selection_sentence_241

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. Natural selection_sentence_242

Background selection is the opposite of a selective sweep. Natural selection_sentence_243

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. Natural selection_sentence_244

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. Natural selection_sentence_245

Impact Natural selection_section_23

Main article: Universal Darwinism Natural selection_sentence_246

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. Natural selection_sentence_247

This inspired some of Darwin's most ardent supporters—and provoked the strongest opposition. Natural selection_sentence_248

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. Natural selection_sentence_249

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. Natural selection_sentence_250

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 cosmological natural selection. Natural selection_sentence_251

This unlimited applicability has been called universal Darwinism. Natural selection_sentence_252

Origin of life Natural selection_section_24

Main article: Abiogenesis Natural selection_sentence_253

How life originated from inorganic matter remains an unresolved problem in biology. Natural selection_sentence_254

One prominent hypothesis is that life first appeared in the form of short self-replicating RNA polymers. Natural selection_sentence_255

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. Natural selection_sentence_256

These conditions are: heritability, variation of type, and competition for limited resources. Natural selection_sentence_257

The fitness of an early RNA replicator would likely have been a function of adaptive capacities that were intrinsic (i.e., determined by the nucleotide sequence) and the availability of resources. Natural selection_sentence_258

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. Natural selection_sentence_259

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. Natural selection_sentence_260

Cell and molecular biology Natural selection_section_25

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. Natural selection_sentence_261

In recent years, a modern version of this theory has been proposed by . Natural selection_sentence_262

According to this cellular Darwinism, random variation at the molecular level generates diversity in cell types whereas cell interactions impose a characteristic order on the developing embryo. Natural selection_sentence_263

Social and psychological theory Natural selection_section_26

The social implications of the theory of evolution by natural selection also became the source of continuing controversy. Natural selection_sentence_264

Friedrich Engels, a German 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." Natural selection_sentence_265

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. Natural selection_sentence_266

For example, in 1940, Konrad Lorenz, in writings that he subsequently disowned, used the theory as a justification for policies of the Nazi state. Natural selection_sentence_267

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. Natural selection_sentence_268

The racial idea as the basis of our state has already accomplished much in this respect." Natural selection_sentence_269

Others have developed ideas that human societies and culture evolve by mechanisms analogous to those that apply to evolution of species. Natural selection_sentence_270

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 human psychology in terms of adaptation to the ancestral environment. Natural selection_sentence_271

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 acquire the grammatical rules of natural language. Natural selection_sentence_272

Other aspects of human behaviour and social structures, from specific cultural norms such as 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. Natural selection_sentence_273

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. Natural selection_sentence_274

Information and systems theory Natural selection_section_27

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. Natural selection_sentence_275

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. Natural selection_sentence_276

For example, a class of heuristic 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. Natural selection_sentence_277

Such algorithms are particularly useful when applied to problems whose energy landscape is very rough or has many local minima. Natural selection_sentence_278

In fiction Natural selection_section_28

Main article: Evolution in fiction Natural selection_sentence_279

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. Natural selection_sentence_280

Among major responses is Samuel Butler's 1872 pessimistic Erewhon ("nowhere", written mostly backwards). Natural selection_sentence_281

In 1893 H. Natural selection_sentence_282 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. Natural selection_sentence_283

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: selection.