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  Evolution (from Latin evolutio, ‘unrolling’) is a theoretical explanation for the mechanism by which species change and have changed since the origin of life on Earth. The overall evolutionary trend has been diversification of species, coupled with increases in complexity of structure and function; the theory of evolution explains these observations. Aristotle (4th century  BCE) noted that variations on a living theme do not often appear to be generated independently, and he argued that this suggested that gradual changes of form had occurred. Creationist explanations of the origins of the species were dominant throughout the Middle Ages and Renaissance. It was generally held that the species had been created as immutable and could not change or interbreed. The idea of evolution was not seriously considered until the 18th century, though it was accepted that extinctions could occur because the fossil evidence for this was overwhelming.

In the 18th century, the French philosopher Charles-Louis Montesquieu examined the issue of the species and declared that in the beginning there were very few species, and that these had multiplied since. The French naturalist Georges-Louis Buffon suggested that the horse was related to the ass because they could interbreed, and he also noted similarities between man and the apes. Erasmus Darwin (Charles Darwin\'s grandfather) summed up the views at the end of the 18th century in his poetical work Zoonomia (1796), concluding that evolution had occurred but proposing no serious mechanism.

In the early 19th century the idea that species might change as a result of pressures from their environment began to be considered. In 1809, Jean-Baptiste Lamarck proposed an evolutionary tree, indicating common ancestry, and stated that life tended to increase in complexity. The driving force behind this was the benefit that the organism could accrue if it became better suited to its lifestyle—Lamarck, along with most other naturalists at the time, thought that characteristics acquired by individuals during life could be inherited by their offspring. Although Lamarck\'s mechanism of evolution was in error (and this was realized by many of his contemporaries) he was the first to present a systematic idea of the course of evolution.

The modern concept of the mechanism of evolution was pioneered by Charles Darwin (1809 - 1882), who had observed the relationships between similar species which were geographically adjacent and attempted to relate these differences to the lifestyles of each species. He also studied fossil remains in the light of his knowledge of geology and began to draw conclusions concerning the transmutation of species. Over a similar period the Scottish naturalist Alfred Wallace was engaged in a similar study. In 1858, the two read a paper to the Linnaean Society and the following year Darwin\'s The Origin of the Species was published, to a storm of controversy. However, by the end of the 19th century Darwin\'s ideas had been accepted and they have stood the test of time, becoming clarified by the advent of genetics and reinforced by observations in all fields of biological study.

The evidence for evolution comes from the fossils of extinct species and the comparative study of existing species. When rocks are arranged in order of age, the fossil records show that simple organisms preceded complex ones; for example, the first fish appeared well before reptiles, which appeared before birds and mammals. When Darwin and Wallace were constructing their theories, comparative anatomy and embryology provided important evidence on the relationships between different species, while the biogeographical distribution of species was important as supporting evidence. Modern techniques of biochemistry and molecular biology allow evolutionary relationships to be quantified because metabolic pathways and similarities between the genes themselves can be compared.

Darwin\'s principal ideas on evolution were based on his observation that living organisms have the capacity to multiply, yet their populations tend to be fairly stable so a proportion of each generation must fail to reproduce. Furthermore, all individuals vary from one to another. He concluded that those offspring which are best able to compete for available resources will be more likely to survive and reproduce, passing the characteristics which made them successful to their own offspring. This is the theory of natural selection and is central to the conventional Darwinian theory of evolution. The process of natural selection is entirely automatic, driven and regulated by the environment. It leads to diversification and increases in the overall number of species because it tends to produce and favour specialized adaptations among groups of organisms; the many species of mammal which exist today evolved from a few ancestral mammals by adapting to live in more diverse habitats, such as trees, burrows, water, etc. The theory accepted that variation occurred and explained it by the theory of pangenesis, which was disproved and replaced first by the germ plasm theory and later by Mendelian genetics. At first it seemed that the idea of random mutations occurring within genes was incompatible with natural selection because of the influence of chance. However, experimental work and mathematical studies showed that the possibilities for variation which Mendelian genetics permitted were just what was required for Darwin\'s theories, and served only to reinforce the theory of natural selection.

Modern biology is based upon evolutionary theory and it is a common thread which unites diverse areas though the theory is continually re-examined in the light of fresh evidence, and there is dispute over the relative importance of natural selection, mutation and sexual recombination. Evolution resulting in the formation of new species is too slow to be observed in action, but the genetic drift of existing species can be observed; for example, the famous case of the butterflies which became darker in colour so that they were still camouflaged in polluted, smutcovered industrial areas. RB

See also adaptation; adaptive radiation; analogy; creationism; Darwinism; emergence; extinction; group selection; hybridization; homology; kin selection; Lamarckism; morphology; niche; palaeontology; sexual selection; speciation; taxonomy; uniformitarianism.Further reading P.L. Forey, The Evolving Biosphere; , Salvador Luria, A View of Life; , John Maynard-Smith, The Theory of Evolution.



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