Genetic rescue despite outbreeding depression

Loss of adaptive capacity and risk of inbreeding depression in small populations have been cited as causes of extinction for decades. Population viability and evolutionary potential can be restored by augmenting small populations with genetically diverse immigrants – genetic rescue. However, conservationists are often hesitant to attempt genetic augmentation owing to concerns that introduced alleles could cause outbreeding depression or displacement of locally advantageous alleles. This concern has caused conservation geneticists to recommend against genetic augmentation when presented with any evidence of outbreeding depression. However, population genetics theory demonstrates that genetic rescue is possible despite outbreeding depression if recombination and natural selection can proceed efficiently. Here I present a few simple models combining population genetics and density-dependent population dynamics to illustrate how extinction can be prevented by genetic rescue, even when immigrants and F1 offspring have low fitness. The theory is not new, but its implications are underappreciated. The specters of outbreeding depression and genetic swamping are probably overestimated. An additional, unexpected result is that a single large translocation can result in less genetic swamping than a smaller translocation followed by multiple generations of assisted gene flow. When possible, genetic augmentation and its fitness consequences should be experimentally investigated when population declines are caused or exacerbated by genetic problems. The main challenge in practice is to manage the risk of extinction during the transitional period in which low-fitness genotypes are common in an augmented population. However, this is also the main challenge for managing declining populations without genetic augmentation.