Morphological divergence in endemic gastropods from Lake Tanganyika; implications for models of Species Flock Formation

Patterns of variability in gastropod shell morphology were used to examine modes of morphological divergence and their implications for intra-lacustrine divergence. Two thiarid gastropods endemic to Lake Tanganyika, that are both stenotopic and rock-dwelling, were investigated because they are believed to be equally subject to environmental barriers to dispersal. A model of allopatric divergence, facilitated by habitat fragmentation, predicts that variation among populations should be large relative to the variation within them, and that organisms equally subject to environmental barriers to dispersal should exhibit similar magnitude and character of morphological divergence. Spekia and members of the Lavigeria species flock appear only in rocky, wave-battered shoals and neither gastropod is known to exhibit wide dispersal. Intervening reaches of sandy and muddy substrates are thought to be barriers to gene flow. Analyses of variance of factor scores reveal that interpopulation morphological variance is greater than intrapopulation variance for both genera, suggesting that divergence is allopatric. However, Spekia shows little morphological variability compared to shallow-water Lavigeria. In graphical analyses offactor scores, Lavigeriaforms discrete clusters of morphology related to differences in environment, geographic distribution, and timing of larval broods, all indicative of speciation. The model of allopatric divergence controlled by environmental barriers to dispersal must be reviewed because of two incongruent results: sympatry of divergent morphs of Lavigeria, and the observation that members of Lavigeria show much greater endemic divergence than members of Spekia, even though they are thought to be equally poor dispersers.