| Biogeographic patterns, such as distribution of genetic variation or pathways of habitat invasion, may depend on geographic and temporal scales. Examining biogeographic patterns at different scales can uncover processes that predominate at these scales, such as extent of dispersal or impact of geological events. This study investigates distribution of genetic variation among populations of the copepod Eurytemora affinis, and reconstructs pathways of freshwater invasion at different geographic scales. Within the northern hemisphere E. affinis spans a wide geographic range, including regions of North America, Europe, and Asia. Such a broad distribution contrasts with limited dispersal capability of this small crustacean. Phylogenetic analysis based on DNA sequences from two mitochondrial genes, 16S rRNA (450 bp) and cytochrome oxidase I (652 bp), reveals highly divergent clades that are geographically structured on different continents and within North America. Interpopulation matings between divergent clades show evidence of hybrid breakdown. Patterns of distribution and location of phylogeographic breaks correspond with those of other species complexes, but sequence divergences are much greater for E. affinis. High levels of sequence divergence reflect either unusually large divergence times (dating to the Miocene) or extremely high rates of molecular evolution. While premodern dispersal among regions has been limited, the predominantly saline and brackish E. affinis has recently invaded many inland freshwater habitats, possibly through human-mediated transport. Alternative hypotheses may account for the invasion of fresh water. Independent freshwater invasions may occur readily from saltwater habitats. In contrast, freshwater invasion from saltwater may be rare, and dispersal among freshwater habitats may predominate. Phylogenetic reconstructions show that at large geographic scales, in different continents and in different river drainages, freshwater invasion has occurred independently. In contrast, at finer geographic scales, within a river drainage, dispersal may have occurred among freshwater habitats. This study is the first to demonstrate multiple, independent invasions of fresh water occurring over rapid ecological time scales. |
