Data from: Evolution of body shape in differently colored sympatric congeners and allopatric populations of Lake Malawi’s rock-dwelling cichlids

The cichlid fishes of Lake Malawi represent one of the most diverse
adaptive radiations of vertebrates known. Among the rock-dwelling cichlids
(mbuna), closely related sympatric congeners possess similar trophic
morphologies (i.e. cranial and jaw structures), defend overlapping or
adjacent territories, but can be easily distinguished based on male
nuptial coloration. The apparent morphological similarity of congeners,
however, leads to an ecological conundrum: theory predicts that ecological
competition should lead to competitive exclusion. Hence, we hypothesized
that slight, yet significant, ecological differences accompanied the
divergence in sexual signals and that the divergence of ecological and
sexual traits is correlated. To evaluate this hypothesis, we quantified
body shape, a trait of known ecological importance, in populations of
Maylandia zebra, a barred, widespread mbuna, and several sympatric
non-barred congeners. We found that the barred populations differ in body
shape from their non-barred sympatric congeners and that the direction of
shape differences was consistent across all barred versus non-barred
comparisons. Barred species are generally deeper-bodied which may be an
adaptation to the structurally complex habitat they prefer, whereas the
non-barred species have a more fusiform body shape, which may be adaptive
in their more open micro-habitat. Furthermore, M. zebra populations
sympatric with non-barred congeners differ from populations where the
non-barred phenotype is absent and occupy less morphospace, indicating
potential ecological character displacement. Mitochondrial DNA as well as
published AFLP data indicated that the non-barred populations are not
monophyletic and therefore may have evolved multiple times independently.
Overall our data indicate that the evolution of coloration and body shape
may be coupled as a result of correlational selection. We hypothesize that
correlated evolution of sexually selected and ecological traits may have
contributed to rapid speciation as well as the maintenance of diversity in
one of the most diverse adaptive radiations known.