Frequency-dependent survival in natural guppy populations

Where the fish are greener? One of the trickiest problems in evolutionary biology is to explain how natural populations maintain an element of genetic diversity. Of all the proposed mechanisms, theory shows that frequency-dependent selection can be the most potent, yet there is only indirect evidence for its importance in natural populations. An experimental manipulation in natural populations of guppies now shows that there is a significant survival advantage for rare genotypes (exotic colouring in males) in natural populations of guppies. This is perhaps the best experimental evidence yet that frequency-dependent selection can be a potent mechanism maintaining genetic variation in natural populations. Results from an experimental manipulation showed a significant survival advantage for rare genotypes in natural populations of guppies, confirming that frequency-dependent selection can act as a potent mechanism in maintaining genetic variation in natural populations. The maintenance of genetic variation in traits under natural selection is a long-standing paradox in evolutionary biology 1 , 2 , 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.