Meeting Abstract
Studying the process of local adaptation is key to understanding population response to environmental change. Although differential selective pressures drive adaptive divergence, the degree to which populations are able to move toward local adaptive optima can be hampered by demographic processes, such as gene flow among populations, fluctuations in population size, and colonization history. Savannah Sparrows occupy salt marsh habitats along the Pacific coast, but phylogeographic analyses indicate salt marsh populations in California and Mexico differ in their degree of isolation from inland counterparts. Analyzing traits associated with osmoregulation within this phylogeographic framework provides a strong approach for inferring the role of demographic factors in shaping adaptive outcomes. Given that young populations experiencing more gene flow should exhibit reduced adaptive divergence, we predicted that California salt marsh sparrows, which are genetically similar to interior birds, would exhibit less divergence in traits linked to osmoregulatory performance than Mexican populations, which exhibit a longer history of isolation. To address this we collected data on kidney morphology (mass, medulla area, nephron number) and function (plasma and urine osmolality) from Mexican salt marsh, California salt marsh, and interior populations. We found an increase in the size and number of kidney traits as well as the plasma and urine osmolality of all salt marsh populations relative to interior birds. California populations, however, were less divergent from interior populations compared to the Mexican populations, suggesting that the lack of genetic divergence in California salt marsh populations has impeded adaptive differentiation to salt marsh environments.
