Understanding the origin of modern and fossil lacustrine species swarms has been problematic for evolutionary biologists. Various lines of evidence, primarily from studies of nonmarine cichlid fishes, suggest that a detailed analysis of dispersal mechanisms and brooding behavior in lacustrine organisms is critical to the solution of this problem. The results of studies of interpopulation variability in brooding and nonbrooding taxa from Lake Tanganyikan ostracodes support the hypothesis that gene flow between populations of lacustrine brooders may be greatly inhibited by extreme localization of gene pools. Significant interpopulation variability occurs among populations of the brooding ostracode Mesocythere irsacae in the absence of habitat barriers, whereas sympatric nonbrooding populations of a cytherideid ostracode, n. gen., n. sp. A show minimal variation over the same range. Extreme genetic isolation may cause rapid speciation and extinction in such populations, owing to the vagaries of small population sizes. Lacustrine environments tend to be geologically ephemeral and efficient dispersal is important for continual recolonization of appropriate habitats. When a lake persists for geologically long periods of time, however, selective pressures for excellent dispersal mechanisms may relax. Under such circumstances, clades of lacustrine brooding organisms, which are often poor dispersers, would radiate and suffer extinctions at accelerated rates.