Frequency-dependent survival in natural guppy populations

The maintenance of genetic variation in traits under natural selection is a long-standing paradox in evolutionary biology(1-3). Of the processes capable of maintaining variation, negative frequency-dependent selection ( where rare types are favoured by selection) is the most powerful, at least in theory(1); however, few experimental studies have confirmed that this process operates in nature. One of the most extreme, unexplained genetic polymorphisms is seen in the colour patterns of male guppies ( Poecilia reticulata)(4,5). Here we manipulated the frequencies of males with different colour patterns in three natural populations to estimate survival rates, and found that rare phenotypes had a highly significant survival advantage compared to common phenotypes. Evidence from humans(6,7) and other species(8,9) implicates frequency-dependent survival in the maintenance of molecular, morphological and health-related polymorphisms. As a controlled manipulation in nature, this study provides unequivocal support for frequency-dependent survival - an evolutionary process capable of maintaining extreme polymorphism.