DIFFERENCES IN RATES OF MORPHOLOGICAL EVOLUTION IN EAST AFRICAN CICHLIDS FROM STOCHASTIC BRANCHING MODELS

Patterns of morphological disparity are important to interpreting diversification and adaptive radiation in East African cichlids. However, comparisons of cichlid disparity often do not consider the effects of clade age or stochasticity. Here, a model of branching morphological evolution allows assessment of the relative contributions of differing turnover and morphological change rates, clade ages, and stochastic variation to the observed patterns of disparity in four endemic tribes of Lake Tanganyika. Simulations are used to compare the probabilities of generating the observed disparity of the four tribes. The sensitivity of the model to conditioning assumptions is assessed using simulations with no conditions, and simulations requiring clade survival to the present, achievement of the minimum species richness, or achievement of richness between the minimum and maximum species richness of the actual tribes. The conditioning does not affect the interpreted direction of rate differences across the tribes, although it does affect the statistical significance of these interpretations. The disparity of the tribes Ectodini, Lamprologini, and Tropheini are within reasonable stochastic variation of each other, and thus differences in disparity among these three tribes most likely result from stochastic variation and possibly age differences. However, the fourth tribe, Cyprichromini, has substantially lower disparity relative to its age and thus likely experienced a lower rate of morphological change when compared with the other three groups. Correlations of disparity with clade age among six Tanganyikan tribes support this interpretation. The model indicates that differences in turnover rates are most likely not responsible for differences in disparity. Therefore, the most meaningful comparisons of disparity in East African cichlids will be those that account for both clade age and stochastic variation, as well as rates of morphological evolution.