The effects of a heterogeneous environment on the genetics of natural populations.

Naturalists have always been struck by the variety of organisms and their manifestly diverse modes of life. Equally obvious yet nonetheless en thralling is the observation that or ganisms are highly adapted to the various conditions in which they live. It was precisely these facts that led Darwin, when writing his introduc tion to the Origin of Species in 1858, to enquire "how the innumerable species inhabiting this world have been modified, so as to acquire that per fection of structure and coadaptation which justly excites our admiration." Darwin realized that the variety of species was the result of the multitude of environments in which organisms live, environments that could be both of physical and biological origin. Darwin, unlike some of his prede cessors, concluded that these various habitats did not evoke adaptation directly but tended to preserve the inherited variations that were favor able (and to eliminate those that were unfavorable) in a particular habitat. In this way natural selection has perfected the organism's adaptation to its environment.