My current research focuses on the effects of ecological, evolutionary, and historical processes on the sizes and limits of species’ geographic ranges. In particular, I am examining the extent to which differences in ecological niche characteristics can explain differences in geographic range size among species. Further, I am studying whether range expansion via niche evolution is constrained by species’ abilities to respond to natural selection. Finally, I am studying whether the responses of ancestral lineages to historical events have led to differences in species’ current geographic distributions.
The geographic range of a species has been regarded as the basic unit of biogeography, yet it remains unclear why some species have very limited ranges and others are able to become extremely widespread. Using species in the “monkeyflower” genus Mimulus (Phrymaceae) that occur in western North America, I am testing the hypothesis that narrowly distributed species have more limiting niche characteristics than widely distributed species. I am testing this hypothesis with two approaches. First, I am combining locality information from herbarium specimen records with broad-scale climate data to quantify species’ geographic range size and climatic niche attributes. Second, I am focusing on closely related pairs of Mimulus species that differ in geographic range size to quantify niche characteristics by measuring fitness across a range of controlled environments.
Constraints to range expansion via niche evolution
Independent of whether niche properties are related to geographic range size across species, the question of what prevents species from expanding their ranges by broadening their niches remains. Within species, populations at range margins may exhibit lower adaptive potential either as a result of small population size, or because directional selection has fixed favored alleles. If so, genetic variation in ecologically important traits and response to selection should be lower in marginal populations than in populations at the center of a species’ range. To test this hypothesis, I am conducting common garden experiments with Mimulus species in section Erythranthe.
A full understanding of the current distributions of species requires consideration of historical factors that have shaped lineages over time. Inference of demographic history based on coalescent theory coupled with estimations of suitable habitat for species during the Last Glacial Maximum of the Pleistocene can provide rigorous tests of the relative importance of long-distance dispersal and vicariance in explaining species’ current distributions. To examine the role of history in shaping species’ current distributions, I am focusing on Mimulus species in section Erythranthe that occupy the deserts of the southwestern U.S. and western Mexico.