Islands within an island: repeated adaptive divergence in a single population

Physical barriers to gene flow were once viewed as prerequisites for adaptive evolutionary divergence. However, a growing body of theoretical and empirical work suggests that divergence can proceed within a single population. Here we document genetic structure and spatially-replicated patterns of phenotypic divergence within a bird species endemic to 250 km2 Santa Cruz Island, California, USA. Island scrub-jays (Aphelocoma insularis) in three separate stands of pine habitat had longer, shallower bills than jays in oak habitat, a pattern that mirrors adaptive differences between allopatric populations of the species’ mainland congener. Variation in both bill measurements was heritable, and island scrub-jays mated non-randomly with respect to bill morphology. The population was not panmictic; instead, we found a continuous pattern of isolation by distance across the east-west axis of the island, as well as a subtle genetic discontinuity across the boundary between the largest pine stand a...