Meeting Abstract
Natural selection across environmental gradients can lead to locally-adapted phenotypes, but the homogenizing effects of gene flow may mediate this outcome. Island populations are ideal for studying the interaction of divergent selection pressures and gene flow on phenotypic variation as water barriers presumably pose a barrier to dispersal. However, the enhanced dispersal ability of birds may increase gene flow among islands and potentially constrain selection. To infer the relative role of gene flow and selection in shaping phenotypic variation, we assess the degree to which song sparrow (Melospiza melodia graminea) populations distributed along a distinct climate gradient on the Northern California Channel Islands are locally adapted given varying degrees of isolation. We used landscape genomic analyses of SNPs to infer gene flow and population structure across islands. We coupled this with analyses of phenotypic variation in morphological and physiological traits related to thermoregulation. Despite the relatively short distances between the islands, our population genomic data revealed limited gene flow between islands. We found song sparrows have larger bills on hotter islands; a pattern previously suggested to facilitate increased heat dissipation. Flow-through respirometry experiments reveal higher upper critical temperatures of the thermoneutral zone on hotter islands. However, we found no difference in basal metabolic rate (BMR) despite our prediction that birds on hotter islands would have reduced BMR. Collectively these results suggest populations can diverge in morphological and physiological traits, even in the presence of some gene flow.
