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  Charles Darwin (1809 - 1882) proposed a theoretical explanation for evolution, based on his studies of similar species and their biogeographical relationships with one another. The concept of evolution was first mooted in the 18th century, by, among others, Darwin\'s grandfather Erasmus Darwin, but no satisfactory cause was suggested. Darwin observed great similarities between certain organisms and explained them by proposing common ancestry. Where similar organisms differed, he suggested that adaptation had occurred in response to pressure from the environment, leading to the establishment of a new but related species. The inheritance of characteristics ensured that offspring were very similar to their parents—thus Darwin recognized heredity as improving stability on the process of reproduction. This stability was modified by variation, a liberalizing force which permits limited change to arise in the population. Although Darwin was unable to explain the mechanism behind these two forces (he was a proponent of pangenesis), he realized that they were important to the evolution of species through the process of natural selection in which variants of a species compete for survival. The outcome of this competition is determined by a selective death rate, dependent on the fitness of each individual. Those individuals best adapted to the environment would thus survive to pass their advantageous characteristics to their offspring.

Darwinism, the chief ideas of which were set out in Darwin\'s book On the Origin of the Species in 1859, stimulated a major re-evaluation of biology despite intense criticism from contemporary scientists and theologians, who protested that he was unable adequately to explain the mechanisms behind heredity and variation, and could not deliver proof that evolution had happened. Experimental support for his ideas grew through the remainder of the 19th century, but it became apparent, particularly as genetics began to be understood, that the theory was over-simplified. Today, conventional explanations for evolution are termed Neo-Darwinism, in which the principles of Darwinism are retained. Thus Neo-Darwinism does not accept that acquired characteristics can be inherited, an idea which was prevalent at the time of Darwin. The concept of natural selection is more precisely defined in terms of reproductive success or fitness. RB

See also creationism; homology; Lamarckism; Mendelism; morphology; palaeontology; speciation.Further reading Douglas Futuyma, Evolutionary Biology.



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