Ancient lakes are often unusually species rich, mostly as a result of radiation and species-flock formation having taken place in only one or a few of many taxa present. Understanding why some taxa radiate and others do not is at the heart of understanding biodiversity. In this chapter I discuss possible explanations for disproportionally large species numbers in some cichlid fish lineages in East African Great Lakes: the haplochromine cichlid fishes in Lakes Victoria and Malawi. I show that speciation rates in this group are higher than in any other lacustrine fish radiation.
Against this background, I review hypotheses put forward to explain diversity in cichlid species flocks. The evolution of species diversity requires three processes: speciation, ecological radiation and anatomical diversification, and it is wrong to consider hypotheses that are relevant to different processes as alternatives to each other. The African cichlid species flocks show unusually high ecological species packing in several phylogenetic groups and unusually high speciation rates in haplochromines. Therefore, it maybe concluded that at least two evolutionary models are required to explain the difference between cichlid diversity and other fish diversity in East African Lakes: one for speciation in haplochromines and one for coexistence.
Subsequently I review work on speciation in haplochromines, and in particular studies aimed at testing the hypothesis of speciation by sexual selection. Haplochromines have a polygynous mating system, conducive to sexual selection, but other polygynous cichlids are not particularly species rich. This suggests that more than just strong sexual selection is required to explain haplochromine species richness. Recent palaeoecological evidence undermines the previously popular hypotheses that explained the species richness of Lake Victoria in terms of speciation under varying natural or sexual selection regimes in satellite lakes or in isolated lake basins. I summarize experimental and comparative studies, which provide evidence for two mechanisms of sympatric speciation by disruptive sexual selection on polymorphic coloration. Such modes of speciation may explain (i) the high speciation rates in colour polymorphic lineages of haplochromine cichlids under conditions where colour variation is visible in clear water, and (ii) in combination with factors that affect population survival, the unusual species richness in haplochromine species flocks.
I argue that sexual selection, if disruptive, can accelerate the pace of adaptive radiation
because the resultant genetic population fragmentation allows a much increased rate of
differential response to disruptive natural selection. Hence, the ecological pattern of
diversity resembles that produced by disruptive natural selection, with the difference that
disruptive sexual selection continues to cause (gross) speciation even after niche space
is saturated. This may explain the unusually high numbers of very closely related and
ecologically similar species in haplochromine species flocks. The role of disruptive
sexual selection is twofold: it not only causes speciation, but also maintains reproductive
isolation in sympatry between species that have evolved in sympatry or allopatry. Therefore,
the maintenance of diversity in species flocks that originated through sexual selection
depends on the persistence of the selection regime within the environmental signal space
under which that diversity evolved.