Recent theoretical work indicates that the geographic ranges of species may be determined by evolutionary constraints. For species that expand over large geographic areas, populations at the range must continually evolve. This process of evolution and expansion in edge populations may eventually be halted by a variety of factors including the loss of genetic variation or substantial gene flow that swamps selection. Both factors may be precipitated by relatively large changes in environmental conditions over relatively small distances, thus causing either a shift in selection that rapidly erodes genetic variation and/or an increase in gene flow that overwhelms selection. However, relatively little is known about the pervasiveness of evolutionary constraints in limiting species ranges.
Results/Conclusions
In this study, we used a combination of field experiments and eco-informatic analyses to determine the degree of trait change required for range expansion and to identify areas where large trait change over short distances could evolutionarily constrain the expansion of the common weedy annual Cocklebur (Xanthium strumarium). Cocklebur is well suited for this research because we have identified an unambiguous range limiting trait in our previous research: the evolution of flowering phenology is important both for local adaptation with the range and would be sufficient for northern expansion beyond the range. In this study, we first used a combination of reciprocal transplants within, and phenological manipulations beyond, the species range to identify the number of degree days between flowering and the end of the growing season required to maximize fitness in all environments. We then used this optimal number of degree days to predict the critical daylength under which a population should be induced to flower to maximize relative fitness (Cocklebur is a qualitative short-day species). To test this predictive method, we compared our predicted optimal critical daylengths to previously published observed critical daylengths from populations across the U.S. and found a close correlation between them. Finally, we used data from over 3000 WMO weather stations to construct a map of optimal critical daylength isoclines for Cocklebur across North America and compared it with Cocklebur's current range. Several areas of large critical daylength shifts over short distances corresponded to the species northern border, indicating that evolutionary constraint may be an important force limiting Cocklebur's geographic range.
