Rapid evolutionary change in introduced populations of Lythrum salicaria L. (Lythraceae) across a climatic gradient in eastern North America

In this thesis, I investigate adaptive evolution of life history traits associated with range expansion in introduced populations of Lythrum salicaria L. (Lythraceae), a widespread invader of North American wetlands. I surveyed variation in flowering time, size at flowering and reproductive output in the field for populations located across most of its latitudinal range in eastern North America and investigated the genetic basis of traits in a common environment greenhouse experiment. I found significant, latitudinal variation in flowering phenology and corresponding variation in size at flowering in the field survey and confirmed substantial genetic variation for these traits among populations in the greenhouse. Plants in populations from higher latitudes flowered earlier and at a smaller size than those from populations at lower latitudes. As predicted, I found evidence for a demographic trade-off between time to flowering and reproductive output mediated by size at flowering in the field survey. However, I found little evidence that this apparent demographic cost was reflected in terms of geographical variation in population size and density. Contrary to expectations, evidence that the demographic trade-off is a result of a genetic correlation between time and size at flowering was weak. This may have been due to genetic mixing during biological invasion, when multiple introductions bring alleles together from geographically distant parts of the native range allowing new gene combinations that do not occur in the native range. The patterns of trait variation that I observed in both the field survey and common greenhouse experiment are consistent with the idea that introduced populations have rapidly evolved in response to climatic selective pressures. (Abstract shortened by UMI.)...