THE QUANTITATIVE GENETICS OF POLYPHAGY IN AN INSECT HERBIVORE. II. GENETIC CORRELATIONS IN LARVAL PERFORMANCE WITHIN AND AMONG HOST PLANTS

Because the overall evolution of the phenotype is a composite of direct and correlated responses to selection, the evolutionary trajectories of genetically correlated characters are fundamentally interdependent (Hazel, 1943; Falconer, 1981 Ch. 19). The effects of genetic correlations on phenotypic evolution are twofold. First, genetic covariance can influence the rate of response to selection. If characters simultaneously selected to increase are influenced by genes with positively correlated effects, the response to selection will be more rapid for the pair than for either character selected separately until the underlying genes are fixed. Conversely, negative genetic correlations among characters selected to increase can slow their rate of simultaneous evolution from that predicted by the amount of genetic variation which exists for each character separately (Dickerson, 1955; Antonovics, 1976; Lande, 1982b). Secondly, genetic correlations can affect the direction of evolution in suites of cliaracters such that traits no longer evolve directly toward their individual optima. In some cases, traits may even be drawn away from their optima for many generations due to selection on genetically correlated characters (Lande, 1980; Via and Lande, unpubl.). The manifold evolutionary effects of genetic correlations dictate a multivariate approach to the study of the quantitative genetic basis of evolution (Lande, 1979, 1980, 1982a, 1982b). Inmost non-

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