The theory, put forward by Charles Darwin in 1858, that species evolve by * natural selection. Darwin noted that although organisms produce more than enough offspring to replace themselves, the numbers of a species tend to remain constant. He concluded that there is a struggle for existence and organisms compete with one another for food, space, etc. He also noted the considerable variation exhibited by members of the same species and concluded that only those best adapted to the environment survive. As many of the variations are hereditary, those that survive because of favourable variations will transmit these to their offspring. Those with unsuitable characteristics will not survive and such characteristics will be lost. This process of selective birth and selective death or 'survival of the fittest' as it came to be called Darwin termed natural selection. Finally, because the environment changes, natural selection continuously acting on a large number of variations could result in the formation of new species, the origin of species. Darwin could not explain how variation arose and made no distinction between quantitative and qualitative variation. He was later driven to accept Lamarck's theory of the inheritance of acquired characteristics (see pangenesis). The origin and maintenance of variation was later explained by work in genetics (see Neo-Darwinism). See also Weismannism. |
