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 endem ic 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 s ympatrically occurring male colour morphs represent biological species . We have tested this assumption using a combination of behavioural ob servations of courtship and microsatellite DNA analysis for six putati ve species of the Pseudotropheus (Tropheops) complex and three of the Pseudotropheus (Maylandia) complex occurring sympatrically at Nkhata B ay. We were unable to demonstrate assortative courtship for the specie s pairs Pseudotropheus (Maylandia) zebra/P. 'gold zebra' or P. (Trophe ops) 'band'/P. (T.) 'rust' because we were unable to distinguish betwe en the females of these taxa. All other taxa showed clear assortative courtship, except for P. (T.) 'deep', a deep-water species which was r arely observed. Fixation indices (theta(ST) for the infinite allele mo del, and R-ST for the stepwise mutation model) calculated from six mic rosatellite 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 alle lic diversity providing evidence that genetic bottlenecking may have b een of limited importance in the speciation process. Assortative matin g among taxa differing only in male colouration is supportive of theor ies that speciation in these fishes has been driven by sexual selectio n by female choice.