Natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in the relative fitness endowed on them by their own particular complement of observable characteristics. It is a key law or mechanism of evolution which changes the heritable traits characteristic of a population or species over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.
For Darwin natural selection was a law or principle which resulted from three different kinds of process: inheritance, including the transmission of heritable material from parent to offspring and its development in the offspring; variation, which partly resulted from an organism's own agency ; and the struggle for existence, which included both competition between organisms and cooperation or 'mutual aid'.
Variation of traits, both genotypic and phenotypic, exists within all populations of organisms. However, some traits are more likely to facilitate survival and reproductive success. Thus, these traits are more likely to be passed the next generation. These traits can also become more common within a population if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specific niche, microevolution occurs. If new traits become more favoured due to changes in the broader environment, macroevolution occurs. Sometimes, new species can arise especially if these new traits are radically different from the traits possessed by their predecessors.
The likelihood of these traits being 'selected' and passed down are determined by many factors. Some are likely to be passed down because they adapt well to their environments. Others are passed down because these traits are actively preferred by mating partners, which is known as sexual selection. Female bodies also prefer traits that confer the lowest cost to their reproductive health, which is known as fecundity selection.
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 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 with subsequent discoveries in classical genetics formed the modern synthesis of the mid-20th century.
New evidence has prompted 21st century evolutionary biologists to challenge the 20th century's gene-centred view of evolution, producing several extended evolutionary syntheses which bring organisms back to the heart of the theory of natural selection. Convergently, the growth of molecular genetics has led to evolutionary developmental biology, which compares the developmental processes of different organisms to infer how developmental processes evolved. While it is now recognised that genotypes can slowly change by random genetic drift, natural selection remains the primary explanation for adaptive evolution.
Historical development
Pre-Darwinian theories
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. 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. 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. Nevertheless, he accepted in his biology that new types of animals, monstrosities, can occur in very 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: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 was later described by the Islamic writer Al-Jahiz in the 9th century, particularly in the context of top-down population regulation, but not in reference to individual variation or natural selection.
At the turn of the 16th century Leonardo da Vinci collected a set of fossils of ammonites as well as other biological material. He extensively reasoned in his writings that the shapes of animals are not given once and forever by the "upper power" but instead are generated in different forms naturally and then selected for reproduction by their compatibility with the environment.
The more recent 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 ideals of created kinds. 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. 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.
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. This theory, Lamarckism, was an influence on the Soviet biologist Trofim Lysenko's ill-fated antagonism to mainstream genetic theory as late as the mid-20th century.
Between 1835 and 1837, the zoologist Edward Blyth 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 and speciation. He defined natural selection as the "principle by which each slight variation , 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, 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 populations of a species, and populations that evolve to be sufficiently different eventually become different species.File:Malthus 1826 vol 1 page 435 top Table England Population Growth 1780-1810.jpg|thumb|Part of Thomas Malthus's table of population growth in England 1780–1810, from his Essay on the Principle of Population, 6th edition, 1826
Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS Beagle, and by the work of a political economist, Thomas Robert Malthus, who, in An Essay on the Principle of Population, noted that population increases exponentially, whereas the food supply grows only 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 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 this hypothesis, Darwin was meticulous about gathering and refining evidence of consilience to meet standards of methodology before making his scientific theory public. 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. 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. Darwin published a detailed account of his evidence and conclusions in On the Origin of Species in 1859. In later editions Darwin acknowledged that earlier writers—like William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar basic ideas. However, they had not developed their ideas, or presented evidence to persuade others that the concept was useful.
File:LA2-NSRW-3-0536 cropped.jpg|thumb|upright|Charles Darwin noted that pigeon fanciers had created many kinds of pigeon, such as Tumblers, Fantails, and Pouters by selective breeding.
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. At the time, mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, but, even in 1859, Darwin clearly stated that selection was only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification". The final edition of The Origin of Species documented several other contributors to evolutionary modification: sexual selection; the inherited effects of the use and disuse of parts ; "the direct action of external conditions" ; and "variations which seem to us in our ignorance to arise spontaneously". 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, evolution 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 law or 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. 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. 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.