The Strength of Selection: Intraspecific Variation in Host-Plant Quality and the Fitness of Herbivores

Abstract

To date, the body of evidence that supports fine-scale local adaptation by herbivores to individual host-plant phenotypes has been found in insects that are relatively sedentary and specialized. At the extreme, such species can consist of genetically divergent populations that are each adapted to neighboring individual plant phenotypes. While such cases have been documented, there are also other instances in which local adaptation has not been shown to occur (Cobb and Whitham 1993, Strauss 1997). Local adaptation is a result of differing selective regimes imposed by different host-plant individuals. Conditions favoring local adaptation can be offset, however, by factors that tend to homogenize subpopulations genetically (e.g., gene flow among populations of herbivores on these plants). A more general issue is: How likely is it to find fine-scale local adaptation to individual plants in herbivorous insects? In this chapter, we explore the strength of selection imposed on insect populations by intraspecific variation in the host plant. We try to relate this value to the strength of other effects, such as nongenetic parental effects that could influence insect performance. Finally, we ask how much gene flow would be required to homogenize these populations genetically, thus preventing local adaptation.