Abstract

Traits are often assumed to be optimal, but evolution may be constrained by genetics, the adaptive landscape, development, and functional limitations. Optimal body size is determined by the net effect of selection acting throughout the life history of the organism. Body size is correlated between the sexes, and sexual size dimorphism (SSD) provides natural experiments in the evolution of traits constrained by genetic correlations. In this thesis I relate sex-specific fitness functions, opportunity for selection, and genetic correlations among body size components to the adaptive significance of SSD in the waterstrider Aquarius remigis . Laboratory studies have revealed significant genetic correlations among body size components but Lynch's (1999) method of "estimation in the absence of pedigree information" does not provide useful field estimates in this study, and the use of polymorphic genetic markers to improve these estimates remains untested. I use multiple regression to estimate fitness functions for total length and components of body size, and compare these with observed trait distributions. My analysis shows that female-biased SSD in total length is favoured in A. remigis , but evolution of total length is better understood through analysis of selection on body size components. Partitioning opportunity for selection, I , indicates that although the strongest selection reported in A. remigis occurs during the reproductive season, the greatest determinant of fitness through the adult stage is survival ability from eclosion to first reproduction. I suggest that while I is a useful adjunct to selection gradient analysis in studies of the adaptive significance of trait values, its most important contribution may lie in studies of the evolution and adaptive significance of life history components, which are themselves components of fitness. I find evidence of spatial and temporal heterogeneity in fitness functions, and conclude that variance in the pattern and intensity of selection on total length observed in this species may be due to interactions between phenotypic distributions of components of body size and locally variable fitness functions. This suggests that evolutionary equilibrium may be dynamic, and estimating selection in one population or generation may fail to detect long-term or broad-scale fitness functions actually driving trait evolution.