Do xeric landscapes increase genetic divergence in aquatic ecosystems?

1. Previous investigations of the ecological genetics among amphipods in so uth-western U.S.A. suggested a xeric landscape promoted genetic divergence among passively dispersed freshwater invertebrates, thereby enhancing speci ation events. We predicted that less divergence would occur among amphipod populations across similar geographic distances in mesic regions. 2. Eight Hyalella azteca populations were sampled along a 200-km transect i n western Oregon, U.S.A. Genetic distances among populations were estimated from randomly amplified polymorphic DNA (RAPD). Genetic analyses indicated much less divergence among Oregon populations than among Arizona populatio ns in an arid environment. 3. Behavioural observations support the genetic data: Oregon populations ex hibit little differentiation in swimming behaviour, whereas Hyalella popula tions in Arizona exhibit extremes in swimming behaviour. 4. These results provide preliminary support for the hypothesis that a xeri c landscape promotes genetic and behavioural divergence among amphipods. Ma ny aquatic invertebrates classified as panmictic populations may encompass genetically distinct groups; those isolated by a xeric landscape are especi ally prone to diversification.