Patterns of gene flow and genetic variation inform mechanisms underlying climatic limits of a species' range

Background/Question/Methods

Theoretical models have suggested that gene flow coupled with selection are critical determinants of species’ range limits.  To evaluate these models, population size, genetic diversity, and contemporary gene flow were examined along three transects spanning the entire warm-to-cold elevational range of the annual plant, Mimulus laciniatus, in the California Sierra Nevada Mountains. By examining both range edges, climate patterns were separated from those of peripherality per se. 

Results/Conclusions

Plant density increased gradually towards both climate limits. Despite this increased density, populations at both climate limits had reduced genetic diversity, suggesting increased drift, selfing, and/or selection at limits. Populations occupying similar climates were more genetically similar, perhaps owing to elevation-based selection or phenological differences.  Warm- and cold-climate limits likely stem from limited genetic variation, a result supported by a prior experimental study at the warm edge in this system. Neither the earlier nor this approach supports contemporary, maladaptive center-edge gene flow as a mechanism generating range limits, as predicted by some models.