Assortative mating among rock-dwelling cichlid fishes supports high estimates of species richness from Lake Malawi

It has been estimated that Lake Malawi, Africa, contains 500-650 endemic species of cichlid fishes, the largest number of vertebrate species endemic to any comparable sized area on the planet. As many of these putative species cannot be distinguished anatomically, these estimates of species richness depend to a great extent on the assumption that sympatrically occurring male colour morphs represent biological species. We have tested this assumption using a combination of behavioural observations of courtship and microsatellite DNA analysis for six putative species of the Pseudotropheus (Tropheops) complex and three of the Pseudotropheus (Maylandia) complex occurring sympatrically at Nkhata Bay. We were unable to demonstrate assortative courtship for the species pairs Pseudotropheus (Maylandia) zebra/P. 'gold zebra' or P. (Tropheops) 'band'/P. (T.) 'rust' because we were unable to distinguish between the females of these taxa. All other taxa showed clear assortative courtship, except for P. (T.) 'deep', a deep-water species which was rarely observed. Fixation indices (theta(ST) for the infinite allele model, and R-ST for the stepwise mutation model) calculated from six microsatellite DNA loci demonstrated significant deviations from panmixia in all pairwise comparisons of putative species, indicating little or no gene flow between populations. All taxa showed high levels of allelic diversity providing evidence that genetic bottlenecking may have been of limited importance in the speciation process. Assortative mating among taxa differing only in male colouration is supportive of theories that speciation in these fishes has been driven by sexual selection by female choice.