Genetic rescue without genomic swamping in wild populations

Gene flow is an enigmatic evolutionary force because it can limit adaptation but can also help populations escape inbreeding depression. Manipulating gene flow for conservation purposes is a controversial, but potentially powerful management strategy. We use multigenerational pedigrees and genomics to test demographic and evolutionary consequences of manipulating gene flow in two isolated wild Trinidadian guppy populations. We found that on average, hybrids lived longer and reproduced more. Despite overall genome-wide homogenization, alleles potentially associated with local adaptation were not entirely swamped by gene flow. Our results suggest that combining new genomic variation from immigrants with potentially adaptive variation from the recipient population resulted in highly fit hybrids and subsequent increases in population size. Contrary to the prevailing view that gene flow constrains adaptation, our study shows that immigration can produce long-term fitness benefits in small populations without swamping locally adaptive variation.