Adaptive introgression during environmental change can weaken reproductive isolation

Anthropogenic climate change is an urgent threat to species diversity1,2. One aspect of this threat is the merging of species through increased hybridization3. The primary mechanism for this collapse is thought to be the weakening of ecologically mediated reproductive barriers, as demonstrated in cases of ‘reverse speciation’4,5. Here, we expand on this idea and show that adaptive introgression between species adapting to a shared, moving climatic optimum can readily weaken any reproductive barrier, including those that are completely independent of climate. Using genetically explicit forward-time simulations, we show that genetic linkage between alleles conferring adaptation to a changing climate and alleles conferring reproductive isolation (intrinsic and/or non-climatic extrinsic) can lead to adaptive introgression facilitating the homogenization of reproductive isolation alleles. This effect causes the decay of species boundaries across a broad and biologically realistic parameter space. We explore how the magnitude of this effect depends on the rate of climate change, the genetic architecture of adaptation, the initial degree of reproductive isolation, the degree to which reproductive isolation is intrinsic versus extrinsic and the mutation rate. These results highlight a previously unexplored effect of rapid climate change on species diversity.