Frequency dependence limits divergent evolution by favouring rare immigrants over residents

In a study using stickleback fish, negative frequency-dependent selection favours rare immigrants over common residents, weakening the effect of divergent natural selection. Rare migrants limit divergent selection Natural selection nudges populations in different environments to diverge. This is divergent selection (DS). But there is also negative frequency-dependent selection (NFDS), which means that in certain circumstances rare varieties in a population will have a selective advantage. For example, in a large population of frogs, any frog that's actually a prince will get a lot of attention. DS and NFDS are rarely studied together as they occur on different spatial and temporal scales. Whereas DS describes the broad sweep of selection over generations, NFDS concerns the minutiae of, for example, mate choice. However, NFDS can be stronger than—and even oppose—divergent selection. Daniel Bolnick and William Stutz demonstrate this effect in populations of a freshwater fish—the three-spine stickleback. By transplanting lake-bred sticklebacks into streams, and vice-versa, they found that immigrants did much better than expected, provided that they were few in number, and constrained divergent selection. Two distinct forms of natural selection promote adaptive biological diversity. Divergent selection occurs when different environments favour different phenotypes, leading to increased differences between populations 1 . Negative frequency-dependent selection occurs when rare variants within a population are favoured over common ones 2 , increasing diversity within populations 3 . These two diversifying forces promote genetic variation at different spatial scales, and may act in opposition, but their relative effects remain unclear because they are rarely measured concurrently. Here we show that negative frequency-dependent selection within populations can favor rare immigrants over locally adapted residents. We reciprocally transplanted lake and stream ecotypes of threespine stickleback 4 into lake and stream habitats, while manipulating the relative abundance of residents versus immigrants. We found negative frequency-dependence: survival was highest for the locally rare ecotype, rather than natives. Also, individuals with locally rare major histocompatibility complex (MHC) class IIb genotypes were infected by fewer parasites. This negative frequency-dependent selection will tend to favour rare immigrants over common residents, amplifying the effect of migra