| Most aquatic ecosystems in the southwestern United States are isolated by a xeric landscape. Despite isolation, many populations of the amphipod Hyalella azteca have maintained species cohesion. One exception to this pattern exists in Montezuma Well, Arizona, where two Hyalella species occupy adjacent habitats. I used behavioral observations, field distribution data, and genetic distance estimates based on Random Amplification of Polymorphic DNA (RAPD) to test alternative hypotheses for how speciation occurred.; Despite close proximity in Montezuma Well, each of the two congeneric populations show greater genetic affinities with different populations from outside the Well than with each other. Behavioral observations and laboratory swimming trials corroborate results of genetic analyses. Hyalella azteca that share genetic affinities with H. azteca in Montezuma Well cling to the roots of emergent macrophytes like Nasturtium (watercress). In populations that share genetic affinities with H. montezuma, H. azteca swims and perches among foliage of the submersed macrophyte Myriophyllum (watermilfoil). The former group of populations is designated as clingers, and the latter group as swimmers. These results support the hypothesis that independent introduction of divergent populations led to coexistence of congeners in Montezuma Well, an allopatric mechanism of speciation.; In addition to illuminating the mechanisms of speciation by amphipods in Montezuma Well, genetic distance data suggest differentiation in levels of gene flow among the Hyalella populations examined. A comparison of genetic distances versus geographic distances for clingers, indicates no correlation. On the other hand, a regression of genetic distance on geographic distance for swimmers indicates a strong relationship.; The steep slope of the regression line between genetic distance and geographic distance for swimmers suggests a potential role of the xeric landscape in limiting gene flow. An investigation of Hyalella populations from a 200 km transect across mesic western Oregon indicates that genetic distance increases with geographic distance, but more gradually than in Arizona. This result supports the hypothesis that a xeric landscape may catalyze differentiation among freshwater invertebrate populations. |
