Minimizing extinction risk through genetic rescue.— According to the genetic rescue hypothesis, immigrants can
improve population persistence through their genetic contribution alone. We investigate the potential for such rescue
using small, inbred laboratory populations of the bean beetle (Callosobruchus maculatus). We ask how many
migrants per generation (MPG) are needed to minimize the genetic component of extinction risk. During Phase 1,
population size was made to fluctuate between 6 and 60 (for 10 generations). During this phase, we manipulated
the number of MPG, replacing 0, 1, 3, or 5 females every generation with immigrant females. During Phase 2, we
simply set an upper limit on population size (.10). Compared with the 0–MPG treatment, the other treatments were
equivalently effective at improving reproductive success and reducing extinction risk. A single MPG was sufficient for
genetic rescue, apparently because effective migration rate was inflated dramatically during generations when
population size was small. An analysis of quasi–extinction suggests that replicate populations in the 1–MPG
treatment benefited from initial purging of inbreeding depression. Populations in this treatment performed so well
apparently because they received the dual benefit of purging followed by genetic infusion. Our results suggest the
need for further evaluation of alternative schemes for genetic rescue