Pollinator responses to frequency – definitions and importance

Frequency-dependent selection (FDS) occurs when the relative fitness of a genotype or phenotype is a function of its frequency in the population (Wright 1948; Clarke 1962). In behavioral ecology, FDS usually indicates that the identity of the fittest genotype (or phenotype) reverses at some intermediate frequency (Heino et al. 1998). When rare genotypes have an advantage, in this narrow sense of FDS, such selection will result in stable polymorphic equilibria (Clarke & O'Donald 1964). This “negative” FDS has captured the interest of many evolutionary biologists (Ayala & Campbell 1974).

An example of a floral polymorphism believed to be maintained by FDS is heterostyly, a suite of floral traits including reciprocal style- and stigma-length polymorphisms. These polymorphisms can increase the amount of pollen carried to alternative phenotypes, causing rare morphs to have increased outcross mating opportunities (Heuch 1979; Eckert et al. 1996). Such selection arises purely from the architecture of the sexual organs, even if pollinators forage randomly among phenotypes. Levin (1972), however, suggested that behavioral preferences of the pollinators themselves might induce FDS among floral traits.

During the 1960s and early 1970s, a number of studies suggested that behavioral preferences were frequency-dependent (Ayala & Campbell 1974). Allen & Clarke (1968) showed that predators, especially birds, selected proportionately more of the most common prey types in a color-varying prey population, even if energetic rewards were equivalent for the different types.