Abstract:

The evolutionary maintenance of genetic variation in natural populations is a fundamental problem in evolutionary biology. I approached this problem by examining the interactions between ecology, morphology, and behavior to understand the causes of patterns of variation in natural populations. I investigated both heritable variation in body size, a continuous, complex trait, and variation in body coloration, governed by a genetic polymorphism, in the green swordtail, Xiphophorus helleri .

With regard to body size, I examine whether (1)?alternative reproductive tactics, (2)?size-assortative mating preferences, or (3)?variation in habitat use among different-sized individuals can account for how small males achieve reproductive success despite competing with larger, superior males. I reject the first two hypotheses because small males do not employ alternative mating tactics and mating preferences are not size-assortative. My studies reveal that size-dependent differences in microhabitat use are of fundamental importance in shaping behavioral interactions among rivals and potential mates because these differences reduce competition among different-sized individuals and allow small males to pursue small females unpreferred by larger males. Furthermore, with regard to maintaining variation in body size, strong correlations between body size and habitats within a population suggest that environmental heterogeneity may play a role in the maintenance of heritable variation.

Regarding the melanophore color pattern polymorphism, my studies reveal that the polymorphic color pattern functions as a signal during mate assessment although seasonally variable turbidity may cause a breakdown in sexual selection. Furthermore, variability in pattern expression documented in natural populations influences male mating preferences suggesting that selection on the polymorphism depends on pattern expression. Lastly, I examine the hypothesis that selection on correlated traits maintains the polymorphism. Male morphs do not differ in metabolic rate---unlike in the congener X. variatus ---but female morphs differ in growth rate in a greenhouse experiment. This raises the exciting possibility that the evolutionary maintenance of the diversity of melanin-based pattern polymorphisms, present in at least 15 of 26 Xiphophorus species, may generally be related to correlated traits, in addition to any role these patterns may play in mate choice.