THE EFFECT OF ABIOTIC AND GENETIC CONSTRAINTS ON DEMOGRAPHIC PERSISTENCE LIMITING SPECIES RANGES IN ENDEMIC SPECIES

Andrea Lorraine Dixon

A species' range is the result of ecological and evolutionary constraints on population persistence. The direct effects of abiotic and biotic factors on individual survival, ecological constraints, are proximate causes of population persistence, while the underlying genetic structure of the species determines evolutionary constraints and ultimately how species will respond to their environment. This work addresses the effects of abiotic and genetic factors on population persistence in two plant species, <italic>Leavenworthia stylosa</italic> and <italic>Mimulus bicolor</italic>. Using demographic censuses and estimations of population-genetic structure, the abundant center hypothesis (ACH) was evaluated as a mechanism limiting <italic>L. stylosa's range</italic>. Patterns of population-genetic structure were generally consistent with predictions, but population demography was not, indicating that the ACH was not a predominate limiting mechanism. In <italic>M. bicolor</italic>, the hypothesis that abiotic factors constrain the range was addressed by estimating population growth in climate-controlled chambers emulating conditions expected within and beyond the range edge. While the environment beyond the southern edge could not sustain individuals, population growth was highest in the environment beyond the northern edge. Population growth was observed at the range center, but declining in edge environments. Given the harshness of the edge environments, the hypothesis that an antagonism between selection and gene flow constrains the range was addressed using tests of local adaptation and maladaptive gene flow in edge environments. Contrarily, there was no signature of local adaptation and gene flow from the center increased hybrid fitness. Coalescent analyses of nuclear and plastid loci demonstrated that edge populations were effectively smaller than the central population and were connected by varying degrees of migration. Specifically, the northern population had low migration (N<sub>e</sub><italic>m</italic> < 1), while the southern population was less genetically differentiated (N<sub>e</sub><italic>m</italic> > 1) from central populations. These results imply that constraints on adaptation may contribute to the northern range limit, but the southern limit reflects an abiotic constraint on population persistence. Taken together, this work advances our understanding of the evolutionary processes underlying population responses to ecological constraints. These processes explain not only how species ranges are currently shaped but help predict how species will respond to a changing climate.

THE EFFECT OF ABIOTIC AND GENETIC CONSTRAINTS ON DEMOGRAPHIC PERSISTENCE LIMITING SPECIES RANGES IN ENDEMIC SPECIES

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