Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not. Variation exists within all populations of organisms; this occurs because random mutations arise in the genome of an individual organism, 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, 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 fecundity selection. Natural selection acts on the phenotype, the characteristics of the organism which interact with the environment, but the genetic 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 niches and may result in speciation. In other words, natural selection is a key process in the evolution of a population. Natural selection is a cornerstone of modern biology; 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. 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 developed in the absence of a valid theory of heredity. The union of traditional Darwinian evolution with subsequent discoveries in classical genetics formed the modern synthesis of the mid-20th century; the addition of molecular genetics has led to evolutionary developmental biology, which explains evolution at the molecular level. While genotypes can change by random genetic drift, natural selection remains the primary explanation for adaptive evolution. 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 and that only those creatures that manage to provide for themselves and reproduce persist. Empedocles' idea that organisms arose by the incidental workings of causes such as heat and cold was criticised by Aristotle in Book II of Physics, he posited natural teleology in its place, believed that form was achieved for a purpose, citing the regularity of heredity in species as proof.
He accepted in his biology that new types of animals, can occur in rare instances. As quoted in Darwin's 1872 edition of The Origin of Species, Aristotle considered whether different forms might have appeared accidentally, but only the useful forms survived: So what hinders the different parts from having this accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, the grinders flat, serviceable for masticating the food. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, all things together happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, whatsoever things were not thus constituted and still perish, 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 come about", not the origin of species:...
Yet it is impossible. For teeth and all other natural things either invariably or come about in a given way. We do not ascribe to chance or mere coincidence the frequency of rain in winter, but frequent rain in summer we do. If it is agreed that things are either the result of coincidence or for an end, these cannot be the result of coincidence or spontaneity, it follows that they must be for an end; therefore action for an end is present in things which are by nature. The struggle for existence was described by the Islamic writer Al-Jahiz in the 9th century; the classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis and others, including Darwin's grandfather, Erasmus Darwin. 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 i
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation and population structure. Population genetics was a vital ingredient in the emergence of the modern evolutionary synthesis, its primary founders were Sewall Wright, J. B. S. Haldane and Ronald Fisher, who laid the foundations for the related discipline of quantitative genetics. Traditionally a mathematical discipline, modern population genetics encompasses theoretical and field work. Population genetic models are used both for statistical inference from DNA sequence data and for proof/disproof of concept. What sets population genetics apart today from newer, more phenotypic approaches to modelling evolution, such as evolutionary game theory and adaptive dynamics, is its emphasis on genetic phenomena as dominance and the degree to which genetic recombination breaks up linkage disequilibrium.
This makes it appropriate for comparison to population genomics data. Population genetics began as a reconciliation of Mendelian biostatistics models. Natural selection will only cause evolution. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance, but with blending inheritance, genetic variance would be lost, making evolution by natural or sexual selection implausible. The Hardy–Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. According to this principle, the frequencies of alleles will remain constant in the absence of selection, mutation and genetic drift; the next key step was the work of statistician Ronald Fisher. In a series of papers starting in 1918 and culminating in his 1930 book The Genetical Theory of Natural Selection, Fisher showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, that natural selection could change allele frequencies in a population, resulting in evolution.
In a series of papers beginning in 1924, another British geneticist, J. B. S. Haldane, worked out the mathematics of allele frequency change at a single gene locus under a broad range of conditions. Haldane applied statistical analysis to real-world examples of natural selection, such as peppered moth evolution and industrial melanism, showed that selection coefficients could be larger than Fisher assumed, leading to more rapid adaptive evolution as a camouflage strategy following increased pollution; the American biologist Sewall Wright, who had a background in animal breeding experiments, focused on combinations of interacting genes, the effects of inbreeding on small isolated populations that exhibited genetic drift. In 1932 Wright introduced the concept of an adaptive landscape and argued that genetic drift and inbreeding could drive a small, isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks; the work of Fisher and Wright founded the discipline of population genetics.
This integrated natural selection with Mendelian genetics, the critical first step in developing a unified theory of how evolution worked. John Maynard Smith was Haldane's pupil, whilst W. D. Hamilton was influenced by the writings of Fisher; the American George R. Price worked with both Maynard Smith. American Richard Lewontin and Japanese Motoo Kimura were influenced by Wright; the mathematics of population genetics were developed as the beginning of the modern synthesis. Authors such as Beatty have asserted that population genetics defines the core of the modern synthesis. For the first few decades of the 20th century, most field naturalists continued to believe that Lamarckism and orthogenesis provided the best explanation for the complexity they observed in the living world. During the modern synthesis, these ideas were purged, only evolutionary causes that could be expressed in the mathematical framework of population genetics were retained. Consensus was reached as to which evolutionary factors might influence evolution, but not as to the relative importance of the various factors.
Theodosius Dobzhansky, a postdoctoral worker in T. H. Morgan's lab, had been influenced by the work on genetic diversity by Russian geneticists such as Sergei Chetverikov, he helped to bridge the divide between the foundations of microevolution developed by the population geneticists and the patterns of macroevolution observed by field biologists, with his 1937 book Genetics and the Origin of Species. Dobzhansky examined the genetic diversity of wild populations and showed that, contrary to the assumptions of the population geneticists, these populations had large amounts of genetic diversity, with marked differences between sub-populations; the book took the mathematical work of the population geneticists and put it into a more accessible form. Many more biologists were influenced by population genetics via Dobzhansky than were able to read the mathematical works in the original. In Great Britain E. B. Ford, the pioneer of ecological genetics, continued throughout the 1930s and 1940s to empirically demonstrate the power of selection due to ecological factors including the ability to maintain genetic diversity through genetic polymorphisms such as human blood types.
Ford's work, in collaboration with Fisher, contributed to a shift in emphasis during the course of the modern synthesis towards natural selection as the dominant force. The original, modern synthesis view of population genetics assumes that mutations provi