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.