Sexual selection and speciation in Lake Malawi cichlids

Knight, Mairi Elspeth (1999) Sexual selection and speciation in Lake Malawi cichlids. University of Southampton, Doctoral Thesis.

Abstract

The presented thesis focuses on three members of the Pseudotropheus (Maylandia) complex: shallow-dwelling rock-frequenting cichlids of the mbuna group. These species differ primarily in male breeding coloration. Experiments were conducted on these taxa to address one of the key questions in relation to evolution via sexual selection: what maintains reproductive isolation between closely related sympatric dwelling forms? It was demonstrated that males cannot visually distinguish conspecific females but a study determining parentage using microsatellite loci found that reproductive isolation between the three taxa is nonetheless maintained without exception in the artificial confines of aquaria. This provides indirect evidence for reproductive isolation by direct mate choice and suggests that females may play a more important role in mate recognition than males. An analysis of morphometric data also identified potential male secondarily sexually selected traits. This information is likely to be of use of future studies investigating the cues females may be using to choose spawning partners.

Within-population dispersal, another important parameter on which no data have hitherto been available, was addressed here using microsatellite molecular markers to statistically estimate pairwise relatedness, which was in turn correlated with pairwise geographic distance. The results of this study were highly suggestive of male-biased dispersal. Male-biased dispersal is more conductive to speciation under recently proposed sexual selection models than is female-biased dispersal. Further, knowledge of small-scale dispersal takes the field closer to a comprehensive understanding of the population dynamics of these species.

The three Pseudotropheus species studied show colour polymorphism. Although males are almost all of one morph, two or three morphs are common in females. A field survey recording morph frequencies along with a study of the genetical inheritance of these morphs was undertaken in an attempt to clarify previously unresolved issues concerning the possible adaptive and/or evolutionary significance of these morphs.

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