It's not easy living on the edge—of a species' range, that is. Preferred habitat tends to be more sparsely distributed and of lower quality. Population densities tend to be lower. And genetic diversity can be an issue because of the stochasticity associated with evolution in small, isolated populations (i.e., genetic drift).

Many organisms have reduced genetic diversity towards the edge of their range. But the vast majority of known cases involve species with continent-sized geographic distributions (e.g., the Canada lynx), and it's not clear the degree to which the same problem exists for species with more limited distributions.

In a recent study my colleagues and I published in the journal Heredity, we document the same pattern—reduced genetic diversity towards the edge of a species' range—in a bird restricted to a single island. After catching over 500 Island Scrub-Jays across Santa Cruz Island, the species' only home in southern California, we found that individuals were more homozygous towards the outskirts of the island. This was surprising (even to us) given that the species is composed of a single population and there's no evidence that certain parts of the island are closed off to dispersal from other parts of the island.

Low genetic diversity is troublesome from a conservation perspective because it could negatively impact the fitness of individuals (inbreeding depression) and/or the evolvability of populations (e.g., evolutionary responses to climate change). Our results show that even within a single population, the processes affecting genetic diversity may vary depending on where you look on the landscape.

A charismatic Island Scrub-Jay was featured on the front cover of the issue that our research appeared in:

KM Langin, TS Sillett, WC Funk, SA Morrison, and CK Ghalambor. 2017. Partial support for the central–marginal hypothesis within a population: reduced genetic diversity but not increased differentiation at the range edge of an island endemic bird. Heredity 119: 8-15 [link]