Introgressive hybridization between two species of waterstriders (Hemiptera : Gerridae : Limnoporus): geographical structure and temporal change of ahybrid zone

Where the distribution ranges of the waterstriders Limnoporus notabilis and L. dissortis meet in western Canada, extensive hybridization and introgres sion occurs. Multivariate ordination analyses of genetic and morphometric d ata by principal component analysis revealed that a single axis separating the two parent species could account for nearly all the variation in both d ata sets. Maps of principal component scores for both data sets revealed ge ographical patterns of variation reflecting specific topographic features i n the region. Comparisons of morphometric data from some of the samples col lected in the 1980s and from the same sites revisited in the 1990s revealed substantial changes. An 'island' of dissortis-like populations inside the range of L. notabilis in interior British Columbia expanded, and a marked l ocal protrusion of notabilis-like phenotypes into the range of L. dissortis on the east slope of the Rocky Mountains diminished during the decade betw een collections. We conclude that introgressive hybridization between these two species of waterstriders is a spatially complex and highly dynamic pro cess.