Theory predicts that species undergoing range expansions will evolve increased dispersal ability at the expanding edge and also adapt to local environmental conditions. In temperate regions, species must respond to environmental cues that indicate seasonal change. The northern tamarisk leaf beetle, a biological control agent of the invasive plant tamarisk, has spread southwards in North America, where daylengths that signal to the beetle to prepare for winter are shorter than in the north. We tested theoretical predictions regarding dispersal ability and also evaluated adaptation to daylength of the beetle across its range in North America. To do this, we used laboratory flight mills to measure dispersal ability of populations from across the range. To understand local adaptation to daylengths across latitude, we defined a new trait, days until diapause (insect hibernation), and evaluated genetic variation in days until diapause for a northern focal population in both its home environment and a novel, southern environment. We also measured days until diapause in both northern and southern environments for eight populations collected at varying latitudes.
Results/Conclusions
We found no increase in dispersal ability for populations at the expanding edge of the range. We found substantial genetic variation in days until diapause in the focal population in its home environment, but no significant variation in the novel environment. When comparing populations across the range, we found significant differences in the diapause behaviors of northern core beetles and southern edge beetles in each environment, such that each population performs best in its local environment. Our findings show that rapid adaptive evolution of diapause timing is possible only when populations are near their home environment. Thus, range expansion must proceed slowly enough that genetic variation on which selection can act is maintained. Strong selection on diapause timing may be precluding the evolution of increased dispersal ability at the expanding edge. These results exemplify the importance of examining evolution in multiple traits during range expansion, especially across heterogeneous environments. We propose that these results will be exemplary of the dominant role of adaptation in determining the rate of range expansion across environmental gradients, while theoretical predictions of increasing dispersal ability will be restricted to homogeneous environments. This research will lead to improved predictions of species movement across the globe.