Genetic variation and reproductive isolation among phenotypically divergent amphipod populations

The freshwater amphipod Hyalella azteca appears to form at least two distinct species in southeast Michigan. A small-bodied ecotype occurs in habitats with predatory centrarchid fish, while a large-bodied ecotype is found in habitats without the predators. Gene flow between the ecotypes appears to be limited, with a high degree of reproductive isolation implied. No consistent genetic difference exists between the ecotypes, however.