Evolution of Lake Malawi cichlid fishes (Perciformes: Teleostei)
The cichlid fish species flocks of East Africa provide a compelling model system in which to study the process of speciation. In Lake Malawi, greater than 1000 species of cichlids have emerged since the filling of the lake basin about 1 million years ago. Over 99% of the Lake Malawi haplochromine cichlids are endemic, suggesting that most of this diversification has taken place within the temporal and spatial boundaries set by the Lake's shores. Moreover, many of these species are endemic to small areas within the lake, indicating that speciation has occurred very recently or perhaps is in progress in some instances. Examination of the genetic structure of recently divergent populations or species is relevant to our understanding of the process of speciation in general.
Here I present work on several populations, insipient [sic] species and species of Lake
Malawi haplochromine cichlids. I have examined the genetic diversity of microsatellite DNA
markers and applied these data to specific hypotheses relating to speciation. First I review
the literature relevant to the latter chapters. Chapter two is an examination of several
populations with two distinct color patterns and lake-wide distributions. This chapter
contributes genetic data to a persistent biogeographical problem among these fishes - monophyly
of color pattern versus independent derivation of similar patterns in disparate portions of
the lake. This work showed that many like-colored populations share genetic similarity, but
one population likely represents parallel evolution of coloration. In chapter 3, I present
genetic data that suggests that hybridization has occurred among closely related species
differing in coloration. These data contribute to a growing list of confirmed and suspected
cases of hybridization among African cichlids, and is the first genetic data supporting
hybridization among congeners in Lake Malawi. Finally, I present a microsatellite phylogeny
of several populations and newly derived species. This examination confirmed conclusions
from chapter two, that color convergence is a common phenomenon among Lake Malawi cichlids.
Indeed several characters of coloration evolved multiple times among the taxa examined.
This dissertation contributes several new findings to the field of speciation among
African cichlids, though many questions remain. While it is unlikely that a single mechanism
is responsible for generating this remarkable species diversity, revealing the conditions
that have allowed the cooperation of multiple mechanisms will be a fascinating advance in
the study of evolution.