Ecological transitions in a freshwater habitat gradient drive diversification in a freshwater amphipod species complex.
Wellborn, Gary*,1, Broughton, Richard1, 1 University of Oklahoma, Norman, OK
ABSTRACT- Although freshwater habitats exist along a continuum of size and permanency, physical and biotic processes generate distinct ecological discontinuities, or transitions, along the gradient, resulting in ecologically discrete habitat types that are replicated across the landscape. Transitions on the freshwater habitat gradient may promote phenotypic diversification and speciation when lineages exhibit habitat shifts and adaptation. Because ecologically similar habitat types recur spatially, diversification in freshwater may frequently be characterized by convergent and parallel evolution. Here we examine adaptive diversification in a freshwater amphipod group for which much is known about the functional basis for differential success across the transition from habitats with no or low levels of vertebrate predation to habitats with intense size-selective predation by fish. Recent genetic studies demonstrate that North American Hyalella amphipods form a complex of many, currently undescribed, species. In Michigan, Oklahoma, and Oregon, species of Hyalella occur as large-bodied and small-bodied species types that are associated with occupancy of the different habitat types. We show that species are closely similar in body size and life histories within species types, but are distinctly different between types. These species types segregate by habitats with the small species occupying habitats with intense size-selective predation by fish, and the large type in habitats with no vertebrate predators or with less intense vertebrate predation. We conducted a molecular phylogenetic analysis of the evolutionary history of species by sequencing an 1187 base pair region of the cytochrome c oxidase I gene from mitochondrial DNA. The analysis yielded high support for nodes of major clades, and character state optimization indicated that diversification has involved convergent and parallel evolution in the two distinct phenotypic species types. We argue it is likely that early reproduction and small body size evolved independently in three lineages, as late reproducing, large sized ancestral lines invaded and adapted to habitats containing predatory fish that impose intense size-selective predation.
Key words: speciation, freshwater, Hyalella, adaptation
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