Natural selection on seed-beetle egg size in nature and the laboratory: Variation among environments

The size of eggs that evolves within a population will in theory reflect a balance between selection for large eggs and for increased fecundity. Theory also predicts that the intensity of selection for large eggs (via effects of egg size on progeny survival and growth) increases as environmental quality decreases. I took advantage of variation in seed quality among individuals within one population of a single tree species, Cercidium floridum, to (1) quantify variation in the magnitude of selection on seed beetle (Stator limbatus) egg size, and (2) test the hypothesis that the intensity of selection on egg size increases with decreasing host plant quality. The proportion of larvae that failed to penetrate the seed coat varied substantially among individual trees and was used as a measure of seed-coat resistance. The intensities of selection (i) on egg size also varied among trees. This measure i was positively correlated (among trees) with the resistance of seed coats to penetration by larvae of S. limbatus, indicating that that selection is greatest on trees upon which larval survivorship was lowest. This study thus demonstrates that the intensity of selection on egg size increases as larval environmental quality decreases.