Sustaining genetic variation in a small population: evidence from the Mauritius kestrel

We obtained measures of genetic diversity in 10 kestrel species at a suite of 12 microsatellite loci. We estimated the relative effective size (N-e) o f the species using a Markov chain Monte Carlo (MCMC) approach, which joint ly estimated the locus specific mutation rates as nuisance parameters. Ther e was surprisingly high genetic diversity found in museum specimens of the Mauritius kestrel. Being an endemic species on a small island, it is known to have a long history of small population size. Conversely, kestrels with a continental distribution had N-e estimates that were only one order of ma gnitude larger and similar to each other, despite having current population sizes that were between one and three orders of magnitude larger than the Mauritius kestrel. We show how many of the theoretical results describing t he effective size of a subdivided population can be captured in terms of th ree rates which describe the branching pattern of the gene genealogy, and t hat they are useful in estimating the time to migration-drift and mutation- drift equilibrium. We use this approach to argue that population subdivisio n has helped retain genetic diversity in the Mauritius kestrel, and that th e continental species' genetic diversity has yet to reach equilibrium after the range changes following the last ice age. We draw parallels with Hewit t's observation that genetic variation seems to survive species' range comp ression and is rather vulnerable to range expansion.