Ecological Mechanisms of Diversification in Hyalella Amphipods

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9815059Wellborn Research by Dr. Gary Wellborn at the University of Oklahoma will examine the ecological and behavioral bases of evolutionary diversification within the freshwater amphipod Hyallela azteca complex, to determine the role of adaptation to predators and the mechanism of collateral evolution of reproductive isolation. Hyallela is a widespread, phenotypically variable, and taxonomically problematic group of freshwater crustaceans, that exhibits substantial ecotypic variation in morphology and in life history across New World habitats. A salient pattern in Hyallela is that small-bodied, early-reproducing ecotypes occur in habitats that contain size-selective predatory fish, and large-bodied, late-reproducing ecotypes occur in habitats lacking predatory fish. These phenotypic differences are genetic, and the body size and life history differences among natural populations are consistent with the hypothesis of adaptation to habitat-specific predation regimes. In preliminary studies on interfertility, individuals of the different ecotypes were found to be reproductively isolated. Genetic diversity, however, as revealed in preliminary allozyme analyses, did not correlate with ecotypic differences; still, genetic diversity appears to be moderate to high overall in populations of this species complex. The hypothesis under test is that the amphipods have diversified into large and small ecotypes repeatedly, and that reproductive isolation (hence, speciation) has evolved collaterally with ecotypic divergence. Predation-related selection on body size or on age-to-maturation may induce, as an accidental or collateral effect, differences in mating behaviors that reinforce reproductive isolation, and hence speciation. Observational work in North America and laboratory experiments will test various genetic, behavioral, and life-history aspects of the hypothesis. This diversification mode, termed replicate adaptive radiation or parallel speciation, may be common in freshwater taxa, although little appreciated until now.