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. Preferential predation on common forms should lead to a rare-morph advantage, resulting in stable polymorphism for the trait concerned (Clarke & O'Donald 1964). This behaviorally induced stability excited many behavioral ecologists, producing an explosion of studies on frequency-dependent choices of prey, mates, and hosts in natural populations [reviewed by Allen 1988; O'Donald & Majerus 1988; Barrett 1988; see Clarke (1962) for shell-pattern polymorphisms in Cepea snails and Turner (1977) for wing-pattern mimicry in tropical butterfly species].

Levin (1972) suggested that pollinators choose disproportionately the most common floral types in plant populations, even if nectar rewards are the same. Whereas predators cause mortality selection in prey, pollinators cause fecundity selection in plants. Because he expected pollinator visitation rate to be positively related to plant fecundity and fitness, Levin predicted that pollinator behavior would lead to common-morph advantage among floral types, i.e., "positive" FDS.

Levin (1972) tested the hypothesis of frequency-dependent pollinator foraging using arrays of two shape morphs of Phlox. He defined relative morph fitness as the mean proportion of outcrossed seeds, quantified as the proportion of heterozygotes among progeny derived from the recessive morph. At low morph frequency, he found the deficit of heterozygous progeny predicted by FDS. This inference assumed that a decrease in the proportion of heterozygote progeny accurately reflected a decrease in the total number of overall outcross matings, however, whereas it could alternatively reflect an increase in the proportion of matings between similar morphs, i.e., assortative visitation (Kay 1978). Therefore, Levin's results did not conclusively show that pollinators prefer common morphs.

Why is it important to test whether pollinator-induced FDS is an important selective force acting in plant populations.? A common-morph advantage will tend to fix alleles (Thomson 1984); therefore, FDS could generate stabilizing selection, severely constraining floral evolution in animal-pollinated plants. Alternatively, if pollinator preferences are reversed, a rare-morph advantage should promote phenotypic diversity. We need to identify the circumstances that can lead to this reversal. Pollinator-driven FDS is also implicated in other selective processes, such as the evolution of mimetic pollination systems and the convergence of floral signals of different plant species (Roy & Widmer 1999).

The aim of this paper is to review the evidence both for frequency-dependent pollinator foraging behavior and for FDS in plant populations. Do pollinators really select in a frequency-dependent way, and if so, why.? Does pollinator preference induce FDS of sufficient strength to account for patterns of monomorphism or polymorphism among floral traits.

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