Negative frequency-dependent selection on polymorphic color morphs in adders

Color pattern polymorphism occurs when more than one form is found within the same population. It is widespread in a variety of taxa, leading us to ask what maintains this variation. One stabilizing mechanismis negative frequency-dependent selection, also known as apostatic selection, in which the fitness of a phenotype decreases with its frequency.(1-6) Negative frequency-dependent selection has been proposed as one of the most powerful selective forces in maintaining phenotypic and genetic diversity in both plant and animal populations.(1,5) Despite its importance and experimental evidence,(1,2,6) no study has documented that natural selection due to predation may result in negative frequency-dependent selection in a wild undisturbed vertebrate population. Here, we report the results of a long-term study, spanning 37 years from 1984 to 2020, of two distinct color morphs, zigzag and melanistic, within a population of adult adders (Vipera berus) on the island of Hallands Vadero in southern Sweden (Figure 1). Our results strongly suggest that the color pattern polymorphism is maintained by negative frequency-dependent natural selection in both males and females.