Abstract of Doctoral Dissertation

The haplochromine cichlid fishes of Lake Malawi, Africa, have formed an immense species flock that represents the greatest monophyletic assemblage of vertebrate animals in the world. The causes of this evolutionary explosion have been attributed to the effects of both natural and sexual selection. In this dissertation project, I explore four mechanisms by which natural and/or sexual selection may have achieved this incredible diversity using the two species of the widespread Malawian genus Labeotropheus. In Chapter 1, I examine naturally- and sexually-selected constraints on male nuptial coloration and find that both depth and geography constrain this coloration; red-colored individuals of both species are found most prevalently at shallower depths and in the northwestern portion of Lake Malawi. Furthermore, there is an effect of the presence of Labeotropheus fuelleborni on the nuptial coloration of L. trewavasae. When found allopatrically from L. fuelleborni, L. trewavasae tends to be blue, while in sympatry it tends to be red. This suggests a possible role for nuptial coloration as a reproductive isolation/mate recognition signal. Chapter 2 is a description of my investigation of inter- and intraspecific morphological divergence in the Labeotropheus using geometric morphometric methods. The results further confirm the well-established morphological distinctions between these species, and go on to demonstrate morphological differences among populations within species. In Chapter 3, I examine whether or not males of L. fuelleborni can distinguish between sympatric and allopatric conspecific opponents in agonistic encounters. While the results suggest that they cannot, there may be extraneous factors that my experiment did not consider or examine. Finally, in Chapter 4, I present the first empirical evidence demonstrating that male nuptial coloration is indeed a sexually-selected characteristic in haplochromine cichlids. My experiments investigating the effects of differences in both chroma and within-pattern color contrast show that females do prefer males with more saturated colors, which demonstrates directional selection on an aspect of nuptial color in the Labeotropheus.