Modern summaries (and this colloquium) arrange the drivers of “descent with modification” into natural, sexual, and artificial selection, but Darwin’s conceptual organization was somewhat different from our own. He saw sexual selection as a part of natural selection, and artificial selection as a coin with 2 sides, one he called Methodical and the other Unconscious (Darwin, 1890). Unconscious selection supposes no conscious wish or expectation to permanently alter a breed, whereas Methodical selection is guided by some predetermined standard as to what is best; intention therefore is the substantial difference (Darwin, 1890). This distinction has largely lapsed in today’s debate, although Darwin thought it worth discussing.
We perceive today, as did Darwin, that natural selection is the environmentally driven mechanistic process by which more advantageous traits are, on the whole, passed on to succeeding generations more often than less advantageous traits because of differential reproduction of the individuals possessing them. Sexual selection is a natural process of intraspecific competition for mating rights. Artificial selection, generally the motive force behind domestication, is often equated with selective breeding. This often amounts to prezygotic selection (where mates are chosen by humans) versus postzygotic selection (where the most fit progeny reproduce differentially) as in natural selection. Although natural selection plays a considerable role in the evolution of many traits (e.g., disease resistance) during the animal domestication process, sexual selection is effectively trumped by the human-imposed arrangements of matings and often by the human desire for particular secondary sexual characters. Artificial selection is a conscious, if unintentional, process, and therefore is generally considered to be effected only by humans [but see Schultz et al. (2005)].
We suggest that artificial selection has both a “weak” and a “strong” form. In weak artificial selection, selection pressure is applied postzygotically (selectively culling a herd of deer, for example) and natural selection proceeds from this modified genetic baseline. In strong artificial selection, selection is prezygotic, as well as postzygotic (e.g., mating male offspring of high-yielding dairy cows to high-yielding cows). This will result in a dramatic acceleration of evolutionary processes and entail a much greater level of control over the selected organism.
Darwin’s The Variation of Animals and Plants Under Domestication (Darwin, 1890) offers a litany of facts and examples of artificial selection in action at the hands of plant and animal breeders. Darwin felt that an understanding and appreciation of the depth of artificial selection was fundamental to the acceptance of natural selection. In Variation, Darwin