How do mate-finding Allee effects affect evolutionary rescue?

Harsh environmental change can put populations at risk of extinction, requiring rapid adaptation for persistence. In sexually reproducing populations, the challenge of finding mates at low densities can impose a strong demographic Allee effect. When such a population falls below the Allee threshold, either through a reduction of the population size or an increase in the threshold, successful adaptation relies on overcoming the Allee effect, which gets increasingly difficult as the population declines. Despite mate-finding Allee effects being common, most models of evolutionary rescue assume that mating is assured even at low densities. Here, we set up a population genetic model for evolutionary rescue of a population below its Allee threshold. For the analysis, we combine stochastic computer simulations with mathematical arguments. As expected, mate limitation can severely impede rescue but the extent differs across sexual systems. We further show that mate limitation shifts the optimal sex ratio for dioecious populations, alters optimal evolutionary routes when there are trade-offs between increasing mate-finding efficiency and fecundity, and enhances the importance of standing genetic variation relative to de novo mutants. Overall, our results highlight the importance of accounting for positive density dependence in the assessment of a populations scope for evolutionary rescue.