most traits for which a substantial heritability has been reported, there are big differences from study to study, presumably because different populations have different amounts of genetic variation and because the different studies were carried out in different environments. Thus, breeders who want to know whether selection will be effective in changing some character in their chickens cannot count on the heritabilities found in earlier stud-ies but must estimate the heritability in the particular population and particular environment in which the se-lection program is to be carried out. Artiﬁcial selection A vast record demonstrates the effectiveness of artiﬁcial selection in changing phenotypes within a population. Animal and plant breeding has, for example, increased milk production in cows and rust resistance in wheat. Selection experiments in the laboratory have made large changes in the physiology and morphology of many or-ganisms including microorganisms, plants, and animals. No analysis of these experiments in terms of allelic fre-quencies is possible, because individual loci have not been identiﬁed and followed. Nevertheless, it is clear that genetic changes have taken place because the popu-lations maintain their characteristics even after the selec-
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