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RESPONSES TO SELECTION ON FLIGHT BEHAVIOR IN A MIGRATORY POPULATION OF MILKWEED BUG (ONCOPELTUS FASCIATUS)

The nature of genetic variation and covariation influencing life-history traits has been the subject of an increasing number of theoretical and empirical studies (Steams, 1983; Arnold, 1987). Knowledge of the socalled "genetic structure" among traits is essential to understanding both the potential and constraints for phenotypic evolution in response to a particular selective regimen, be it natural or artificial (Loeschcke, 1987). Indeed, artificial selection is widely used to detect genetic associations between traits, as revealed by correlated changes in phenotype when selection is applied to a single trait (Falconer, 1981 pp. 286-287). We have used artificial-selection experiments to compare the genetic structure of migratory and nonmigratory populations of the large milkweed bug, Oncopeltusfasciatus (Palmer and Dingle, 1986; Dingle et al., 1988). In these studies, we chose wing length as the target of directional selection because previous work indicated that it should respond rapidly and that other traits might show correlated responses (Hegmann and Dingle, 1982). Migratory (Iowa) and nonmigratory (Puerto Rico) populations showed similar rapid direct responses to selection on wing length but differed in their pattern of correlated responses. Of particular interest was the result that the migratory population showed positive correlated responses in the propensity for long-duration (presumed migratory) flight behavior (Palmer, 1985) and early reproductive output (Palmer and Dingle, 1986) while the nonmigratory population showed no significant correlated responses in these same traits (Dingle and Evans, 1987; Dingle et al., 1988). In both populations, adult age at first reproduction failed to show any correlated response to selection on wing length. Thus, for the migratory Iowa population the positive genetic correlations among body size, flight potential, and fecundity provide the potential for these traits to evolve together in response to selection pressures that favor a colonizing life history. However, in nature, wing length may not be the direct, or only, target of selection. For example, the flight potential of migratory milkweed bugs, which allows them to reach and colonize more northern habitats such as those occurring in Iowa, may be a more direct target of natural selection. The purpose of this study was to see whether selection on flight propensity in this migratory popu-