Rapid speciation can occur on ecological time scales and interfere with ecological processes,
resulting in species distribution patterns that are difficult to reconcile with ecological
theory. The haplochromine cichlids in East African lakes are an extreme example of rapid
speciation. We analyse the causes of their high speciation rates. Various studies have
identified disruptive sexual selection acting on colour polymorphisms that might cause
sympatric speciation. Using data on geographical distribution, colouration and relatedness
from 41 species endemic to Lake Victoria, we test predictions from this hypothesis. Plotting
numbers of pairs of closely related species against the amount of distributional overlap
between the species reveals a bimodal distribution with modes on allopatric and sympatric.
The proportion of sister species pairs that are heteromorphic for the traits under disruptive
selection is higher in sympatry than in allopatry. These data support the hypothesis that
disruptive sexual selection on colour polymorphisms has caused sympatric speciation and help
to explain the rapid radiation of haplochromine species flocks.