Long-distance dispersal by aquatic insects can be difficult to detect because direct measurement methods are expensive and inefficient. When dispersal results in gene flow, signs of that dispersal can be detected in the pattern of genetic variation within and between populations. Four hundred seventy-five base pairs of the mitochondrial gene, cytochrome b, were examined to investigate the pattern of genetic variation in populations of the stonefly Pteronarcys californica and to determine if long-distance dispersal could have contributed to this pattern. Sequences were obtained from 235 individuals from 31 different populations in the western United States. Sequences also were obtained for Pteronarcella badia, Pteronarcys dorsata, Pteronarcys princeps, Pteronarcys proteus, and Pteronarcys biloba. Phylogenies were constructed using all of the samples. Nested clade analysis on the P. californica sequence data was used to infer the processes that have generated the observed patterns of genetic variation. An eastern North American origin and 2 distinct genetic lineages of P. californica could be inferred from the analysis. Most of the current population structure in both lineages was explained by a pattern of restricted gene flow with isolation by distance (presumably the result of dispersal via connected streams and rivers), but our analyses also suggested that long-distance, overland dispersal has contributed to the observed pattern of genetic variation.
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