Microgeographic divergence in a single-island endemic: evolutionary patterns and conservation implications
Date
2014
Authors
Langin, Kathryn M., author
Ghalambor, Cameron K., advisor
Crooks, Kevin R., committee member
Funk, W. Chris, committee member
Morrison, Scott A., committee member
Sillett, T. Scott, committee member
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Abstract
Understanding the processes that generate biological diversity is the central goal of evolutionary biology. Geographic isolation has traditionally been viewed as the primary scenario favoring evolutionary divergence. However, there is growing appreciation for the role of ecological variation and natural selection in driving adaptive differentiation, even in the absence of geographic barriers to gene flow. My dissertation tests for microgeographic patterns of local adaptation within one of the most range-restricted bird species in North America, the Island Scrub-Jay (Aphelocoma insularis). The species is restricted to Santa Cruz Island in southern California, USA, where it occurs in a diversity of habitat types within its narrow (250 km2) geographic range. Remarkably, I found that Island Scrub-Jays in three separate stands of pine habitat had longer, narrower bills than those in oak habitat, a pattern that mirrors adaptive differences between allopatric populations of the species' mainland congener (A. californica). Adaptive divergence was not constrained by genetic diversity, even though Island Scrub-Jays had much lower levels of neutral genetic diversity than A. californica. Genetic data indicate that Island Scrub-Jays in pine habitat were more closely related to individuals in adjacent oak habitat than to individuals in other pine stands, indicating that each pine stand can be considered an independent case where adaptive divergence has been maintained in the face of some gene flow with adjacent oak birds. Morphological differences were not abrupt across the pine-oak boundary, as bill length declined gradually with distance from pine habitat, a clinal pattern that is also consistent with a scenario of divergence-with-gene flow. Individuals mated non-randomly with respect to bill length within the population, which may be due to a combination of (1) isolation-by-distance (localized dispersal) across the landscape and spatial autocorrelation in bill morphology, and (2) assortative mating at a more local scale based on bill morphology or correlated acoustic signals. These findings provide strong support for the notion that microgeographic patterns of local adaptation may be more common than is currently appreciated, even in mobile taxonomic groups like birds. They also underscore the importance of conserving Island Scrub-Jays across the entire island in order to preserve the species' full range of biological diversity and to facilitate adaptive responses to future environmental changes.
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Subject
sympatry
local adaptation
gene flow
island scrub-jay
adaptive evolution
Aphelocoma insularis