Determinants of geographic distribution in western North American monkeyflowers
Date
2014
Authors
Sheth, Seema Nayan, author
Angert, Amy L., advisor
Funk, W. Chris, committee member
McKay, John K., committee member
Mueller, Rachel L., committee member
Steingraeber, David, committee member
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Abstract
The geographic range of a species represents the basic unit of biogeography. Despite ample evidence that properties of geographic ranges vary among species, we do not fully understand the ecological and evolutionary processes underlying these patterns, thereby hindering our ability to forecast changes in species' distributions in response to changing environments. Key hypotheses about variation in geographic range size among species emphasize the roles of ecological niche properties and the connectivity of suitable habitat. In the first study of my dissertation, I combined primary occurrence data with climate variables to test the relative importance of these hypotheses in 72 species of western North American monkeyflower (genus Mimulus). Climatic niche breadth, via its effect on the amount of suitable habitat, was a strong predictor of geographic range size, whereas climatic niche position (relative to regional climate) and connectivity of climatically suitable habitat were not. Given the role of climatic niche breadth in shaping geographic range sizes in Mimulus, the goal of the second study of my dissertation was to examine the relationship between thermal tolerance (an important axis of niche breadth) and range size experimentally using 5 pairs of closely related Mimulus species with differing range sizes. Within four species pairs, the more geographically widespread species had a broader thermal tolerance than the narrowly distributed species, providing further support for the hypothesis that species with broader niches are able to achieve larger geographic ranges. Further, within each species pair, the species with broader thermal tolerance encompassed greater variation in temperature across its geographic range and higher genetic variation for thermal tolerance than the species with narrower thermal tolerance, supporting the hypotheses that climatic variability and genetic variation in ecologically important traits can explain variation in environmental tolerance among species. Although species vary in range size, every species has a limited geographic range, leading to the question of what prevents a species from expanding its range via niche evolution. Thus, in the third study of my dissertation, I tested whether adaptation at geographic range margins is constrained by insufficient evolutionary potential. To do so, I used artificial selection experiments to quantify genetic variation in flowering time for populations from the northern edge, center, and southern edge of the geographic range of the scarlet monkeyflower (M. cardinalis). Contrary to prediction, southern populations exhibited significantly greater responses to selection (and thus evolutionary potential) than northern or central populations. Together, these results highlight an important role of niche breadth in explaining variation in geographic range size among species, and reveal variation in evolutionary potential that facilitates niche and range expansion within and among species.
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Subject
biogeography
Mimulus
niche
thermal tolerance
evolutionary ecology
geographic range