Sexual selection, speciation and taxonomy of Lake Malawi cichlid fishes
The cichlid fishes inhabiting Lakes Malawi, Tanganyika and Victoria have undergone explosive speciation, and are now unparalleled as a system in which to study large scale rapid speciation in vertebrates. Current theories on the origin and diversification of these cichlid faunas were critically reviewed. Sexual selection by female choice was concluded to be the most likely mechanism behind the phenomenal explosion in species numbers. Sexual selection by female choice was investigated in Copadichromis thinos, a bower-building cichlid. A significant relationship was found between the asymmetry of an extended phenotypic trait (sand bowers), and male parasite load (not previously demonstrated in any taxa). Males which spawned were found to have a significantly lower total parasite load than those which did not spawn, never before demonstrated in a lekking fish. Further, males which spawned were found to have significantly higher eviscerated gonadosomatic indices than unsuccessful males. Significant correlations were also found between hepatosomatic index and some measures of reproductive success. This may indicate females are choosing to mate with males in better condition. A morphological study of the sand-dwelling species of the genus Aulonocara showed that differently coloured males represented distinct morphospecies, rather than colour 'morphs' of a single or few species. Twelve new species were subsequently described, along with preliminary descriptions of three further species. Preliminary laboratory studies of three sympatric species of the Pseudotropheus zebra species complex from Nkhata Bay demonstrated that females were able to distinguish between conspecific and heterospecific males using visual cues only. Further, no significant differences were found in female choice between males using visual cues only and visual and olfactory cues.