Identifying the different factors that influence female preference is essential to our understanding of the evolution of female preferences. Plastic mate preferences can be favored by natural selection in varying environments, including social environments. In this dissertation, I addressed the following questions in Xiphophorus birchmanni, X. cortezi and X. malinche: 1) What factors affect variation in female preference for vertical bars; 2) Does experience with different male phenotypes affect female preferences for vertical bars; 3) Does female preference contribute to the evolution of the exaggerated dorsal fin; 4) Did the exaggerated dorsal fin evolve to enhance courtship displays; 5) What are the phylogenetic relationships within and across these species?
Female preferences for vertical bars in X. cortezi varied with female bar state, female size, and population in wild-caught females. By further examining differences using lab-reared fish, I found that preferences varied by social experience with barred and barless males, and this relationship varied across populations. These results indicated a combination of both genetic (population, bar state) and environmental (female size) on female preferences. I also found that barless males were more aggressive toward females, and suggest that these behavioral differences could explain how experience influences female preferences.
Xiphophorus birchmanni females preferred dorsal fins that were larger than expected given the male’s size, and during male–female interactions, males raised their dorsal fins as part of their courtship display directed towards females. I suggest that female preference selected for enlarged dorsal fins in male X. birchmanni, and that female preferences are potentially disruptive for dorsal fin size. I also found that raising the dorsal fin is a signal directed at females in several species of Xiphophorus. However, sexual dimorphism in dorsal fin size evolved prior to increased use of the dorsal fin during courtship. It is possible that either female preferences or male competition led to initial exaggeration of the dorsal fin.
In the three species phylogeography, haplotypes of X. malinche and X. cortezi were paraphyletic with respect to each other, while X. birchmanni haplotypes formed a monophyletic group. However, microsatellite analyses uncovered three genetic groups corresponding to the three species. Populations of X. malinche and X. cortezi clustered within their respective species, suggesting that these are separate species.