The evolution and fate of diversity under hard and soft selection

How genetic variation arises and persists over evolutionary time despite the depleting effects of natural selection remains a long-standing question. Here, we investigate the impacts of two extreme forms of population regulation-at the level of the total, mixed population (hard selection) and at the level of local, spatially distinct patches (soft selection)-on the emergence and fate of diversity under strong divergent selection. We find that while the form of population regulation has little effect on rates of diversification, it can modulate the long-term fate of genetic variation, diversity being more readily maintained under soft selection compared to hard selection. The mechanism responsible for coexistence is negative frequency-dependent selection which, while present initially under both forms of population regulation, persists over the long-term only under soft selection. Importantly, coexistence is robust to continued evolution of niche specialist types under soft selection but not hard selection. These results suggest that soft selection could be a general mechanism for the maintenance of ecological diversity over evolutionary time scales.