Assessing diversification patterns in an ancient tropical lake: Gomphocythere (Ostracoda) in Lake Tanganyika.

Abstract

I examined the distribution of 70 morphological characters for 17 extant species of an ostracod genus, Gomphocythere, in Africa, to test hypotheses concerning character development and speciation patterns. Using heuristic searches conducted with the phylogenetic reconstruction program PAUP, I found 9 trees of 269 steps (CI = 0.45). The skewness of tree length distribution reveals significant phylogenetic structure in the data. Nodes are supported by 2 to 13 character state changes, and these character changes are sometimes reversed or paralleled elsewhere, accounting for much of the homoplasy in the reconstructions. The results suggest the presence of four new species (Gomphocythere coheni, G. downingi, G. wilsoni, G. woutersi). These species are relatively derived within the favored tree. Analyses were done to elucidate the effect of the exclusion of hard and soft part characters and the effects of differential fossil preservation on phylogenetic reconstruction. Eliminating the hard part characters caused the collapse of many branches as polytomies and decreased the agreement of the hard part trees. Excluding the soft part characters increased the number of most parsimonious trees, and decreased the resolution of the trees by creating many unresolved polytomies, but produced similar islands of stability as the complete analysis. I integrated phylogenetic and ecological data sets to examine ecological variables of substrate and depth range and their role in speciation of Gomphocythere in Lake Tanganyika. Using one-way MANOVAs on substrate distribution data, I found significant variability in the substrate distribution of (p =.001) seven species. In addition, principle components analysis and R-mode cluster analysis revealed close associations between certain species with respect to substrate ranges. Correlations exist between species and their ecological tolerances, indicating that environmental fluctuations could have had a profound effect on speciation. In addition, evidence for multiple invasions of Gomphocythere species exist from mapping endemism onto the phylogenetic tree. From this information, a model of speciation emerges in which there have been multiple invasions and subsequent radiations.