Adaptation at the Range Edge and the Causes of Range Limits in Mimulus laciniatus: The Roles of Gene Flow and Selection.

摘要

Species range limits are paradoxically viewed as either diversifying areas, where niche evolution occurs due to adaptations in novel environments, or as fragile and depauperate sinks, where adaptation is stymied by maladaptive gene flow from central populations. Many studies have described individual components of these scenarios that lend support to both viewpoints. This study seeks to simultaneously examine major hypothesized components (gene flow, selection, and the ecological gradient) for understanding population differentiation at range limits and the effects of gene flow on these dynamics.;The first study was is a review of the literature on the evolution and ecology of species range limits. Species range limits involve many aspects of evolution and ecology, from species distribution and abundance to the evolution of niches. Theory suggests myriad processes by which range limits arise, including competitive exclusion, Allee effects, and gene swamping; however, most models remain empirically untested. Range limits are correlated with a number of abiotic and biotic factors, but further experimentation is needed to understand underlying mechanisms. Range edges are characterized by increased genetic isolation, genetic differentiation, and variability in individual and population performance, but evidence for decreased abundance and fitness is lacking. Evolution of range limits is understudied in natural systems; in particular, the role of gene flow in shaping range limits is unknown. Biological invasions and rapid distribution shifts caused by climate change represent large-scale experiments on the underlying dynamics of range limits. A better fusion of experimentation and theory will advance our understanding of the causes of range limits.;The second study was a test of the role of gene flow on adaptation using experimental breeding in the annual plant, Mimulus laciniatus, at its warm range edge. M. laciniatus inhabits open, rocky seeps between 760 -- 3,270 m and is endemic to the Sierra Nevada Mountains of California. Replicate populations, including replicate edge populations, were sampled along three elevation-based transects across the species range. Plants from all populations were grown in growth chambers for one generation to reduce among-family maternal effects. Gene flow was experimentally simulated by producing crosses within and between warm-limit populations as well as crosses between warmlimit and central range populations. Offspring from these crosses were sown into a common garden at the warm-edge species range limit where lifetime reproductive success was measured. Gene flow, regardless of range location, increased emergence at the range limit, but increased lifetime fitness occurred only when pollen originated from another warm-limit population. We found benefits of gene flow among populations, especially beneficial gene flow between populations occupying the same range limit. Our results emphasize the overlooked importance of gene flow among populations occurring near the same range limit and highlight the potential for prescriptive gene flow as a conservation option for populations at risk from climate change.;In the third study, field surveys and genetic markers were used to test established predictions that edges are more isolated and differentiated ("isolated limits hypothesis") and to place these findings in the context of theory on constraints to adaptation at range limits. In general, there was no pattern of change in population size, but plant density was greater towards margins. No consistent changes in microsatellite diversity was found between the range center and its limits, but there was a reduction in allelic richness in the most peripheral populations, perhaps signaling a step pattern caused by isolation or abrupt environmental changes at range limits. Mean genetic divergence did not increase significantly towards limits, whereas inbreeding increased gradually across the entire species range, from low to high elevation limits. There was little evidence for reduced gene flow at edges as estimated from graph theory analysis. Gene flow appears to be greater among populations inhabiting similar elevations. Vicariance, potentially driven through environmentally mediated selection, was evident across the elevation gradient, even within the same watershed. Depending on the analysis employed (i.e. using continuous or categorical measures of peripherality), one can find support for or against the isolated limits hypothesis. Adaptive constraints at range limits in this system may be due to limited genetic variation at both edges, but are not likely to be due to directional, swamping gene flow from central populations.;The results presented in this dissertation confirm theoretical expectations that gene flow plays a key role in determining the performance of individuals at range limits. Moreover, we have supplied new evidence, from experiments and molecular genetic analyses, that the role of gene flow can switch depending on the environmental sources and destinations of migration. Adaptation across a strong environmental gradient is potentially maintained by preventing maladaptive gene flow through isolation between elevations. Conversely, gene flow among populations inhabiting similar environments at the range margin can provide adaptive genetic variation.