Assessing morphological differences in an adaptive trait: A landmark-basedmorphometric approach

East African cichlid fishes have evolved a stunning array of oral jaw morph ologies. To better understand the adaptive evolution of this trait, we perf ormed a morphological analysis of the jaws of two closely related species f rom Lake Malawi that have very different modes of feeding. Labeotropheus fu elleborni forages along the substrate with a "biting" mode of feeding, whil e Metriaclima zebra feeds in the water column with a "sucking" mode. We ana lyzed each of the four skeletal elements that make up the oral jaws: the de ntary, articular, premaxilla, and maxilla. In addition, we performed the sa me analysis on the neurocranium, an element closely associated with the ora l jaws. We used the thin-plate spline method to quantify morphological diff erences, which allowed us to relate our results to the functional biology o f 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 chara cter, hybrid progeny are statistically different from both parental species . These results suggest an additive mode of action of the alleles responsib le for these phenotypes. J. Exp. Zool. 289:385-403, 2001. (C) 2001 Wiley-Li ss, Inc.