Evolutionary forces driving population differentiation in Lake Malawi rock-dwelling cichlids (Pisces: Cichlidae)

Abstract

The East African cichlids with more than 2000 species represent the most diverse vertebrate radiation known. Lake Malawi harbors the most species rich flock with more than 700 endemic cichlids. In this work I use a population based approach to study the forces driving the divergence of populations and the factors contributing to the maintenance of species diversity. I examine the effect of genetic drift on population divergence through time and space using an analysis of effective population sizes. This study indicates that populations of the widespread Maylandia zebra are over 550 individuals in size and are at an equilibrium state. The microendemic Maylandia benetos has a relatively small population size (~500 individuals) and evidence for drift is found. I also examine the phenotypic divergence in ecological and sexual characters in M. zebra. This study reveals that ecological selection plays an important role in the divergence of body shape and length in M. zebra. Divergence in male color pattern shows weaker evidence for selection. Furthermore, the divergence estimates for body shape and melanophore count are slightly correlated. This correlation between an ecological and a sexual trait may indicate that both phenotypes might be under correlational selection. Correlational selection on ecological and sexual traits is further supported by the correlation of body shape and male coloration in replicated sympatric species pairs of Maylandia. A common garden experiment using the sympatric M. zebra and M. benetos, representing a sympatric species pair with divergent coloration, shows that body shape had a strong genetic component but also exhibited phenotypic plasticity. Hybrid crosses of the two species reveal a complicated mode of inheritance for body shape differences and demonstrated high degrees of transgressive segregation. Overall, selection appears to be the driving force of phenotypic evolution in rock-dwelling cichlids. However, small population sizes of microendemics expose them to the effects of drift. Evidence for correlational selection suggests that specific combinations of reproductive and ecological traits might be favored. Finally, phenotypic plasticity and transgressive segregation are two mechanisms generating new phenotypic diversity contributing to the diversification of cichlids.