Reaction Norms in Genetical Ecology

volutionary explanations have been prominent in ecology for several decades. At first (although the idea was rarely made explicit), the rates of evolutionary processes were thought to be slow, requiring at least centuries for noticeable changes to occur. During the past decade, studies of birds-Darwin's finches (Grant 1986), snow geese (Cooke 1987), and great tits (van Noordwijk 1987)-have revealed considerable amounts of genetic variation, which indicates that traits can change rapidly in response to genetic selection. Laboratory experiments show similar genetic variation in other groups of animals and in plants. Considerable changes in traits such as body size, clutch size, or the timing of egg laying can occur in only a few generations. These rates of change, on the order of 2-7% of the mean per generation, are similar to those achieved in agricultural selection programs (Ollivier 1988). Microevolutionary processes therefore work on the same time scale as ecological processes, and so evolutionary studies demand a combination of genetics and ecology. In this article, I sketch a framework for the combined study of ecophysiology and quantitative genetics, using the example of the great tit (Parus major; see box page 455). The use of reaction norms allows combination of inforMicroevolutionary processes work on the same time scale as