Intersections of evolution and ecology of polymorphisms from the viewpoint of demographic performance

Genetic polymorphisms in a population may enhance demographic performance, as ecological niche partitioning among phenotypes can reduce negative interactions such as resource competition and predation risk. However, an increase in maladaptive or harmful phenotypes can reduce population performance, causing confusion among researchers about the ecological consequences of genetic polymorphism. Here, we propose a general framework for understanding the population-level ecological effects of genetic polymorphisms arising from adaptive evolution (balanced polymorphism) and stochastic processes (temporal polymorphism). To this end, we use the yardstick effect of diversity, which isolates the ecological effect of genetic diversity itself on population performance (e.g. growth rate and carrying capacity). In a population with balanced polymorphisms maintained by selection, genetic polymorphisms tend to have a positive diversity effect on its performance. By contrast, the diversity effect of temporal polymorphisms, which are maintained by gene flow, mutation and similar processes, is predicted to be zero, although such polymorphisms may increase genetic load and thereby reduce population performance. A negative diversity effect is only predicted when temporal polymorphisms are subject to positive frequency-dependent selection. This simple, systematic framework integrates the evolutionary mechanisms that establish genetic polymorphisms and their demographic functions, thereby enabling predictions of their ecological consequences. This article is part of the theme issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.