Assessing morphological differences in an adaptive trait: a landmark-based morphometric approach.

East African cichlid fishes have evolved a stunning array of oral jaw morphologies. To better understand the adaptive evolution of this trait, we performed a morphological analysis of the jaws of two closely related species from Lake Malawi that have very different modes of feeding. Labeotropheus fuelleborni forages along the substrate with a "biting" mode of feeding, while Metriaclima zebra feeds in the water column with a "sucking" mode. We analyzed each of the four skeletal elements that make up the oral jaws: the dentary, articular, premaxilla, and maxilla. In addition, we performed the same analysis on the neurocranium, an element closely associated with the oral jaws. We used the thin-plate spline method to quantify morphological differences, which allowed us to relate our results to the functional biology of the species. We find many aspects of shape change that relate directly to the functional design of the cichlid head. The same series of measurements was made on hybrids between Labeotropheus and Metriaclima. For every character, hybrid progeny are statistically different from both parental species. These results suggest an additive mode of action of the alleles responsible for these phenotypes.