We then discuss its relative importance in comparison with alternative mechanisms that could be maintaining genetic and phenotypic diversity. The idea that the fitness of an organism is affected by the relative frequencies of the genotypes in

a population was first described by Fisher (1930), suggesting that an inverse relation between the two could maintain stable polymorphisms. This concept was later formalized by other researchers, such as Li (1955), Wright (1956) and Lewontin (1958), who developed mathematical models to describe the mechanism. Evidence that the fitness of a morph depends on its frequency relative to the frequencies of the other morphs was first found by Wright & Dobzhansky (1946) in an experimental Doxorubicin molecular weight population of Drosophila pseudoobscura. PF-02341066 research buy Three different gene arrangements can be found in the third chromosome of this species, and their frequencies were observed to fluctuate over the year in natural populations. Wright and Dobzhansky set up an experimental population with known frequencies of the different genotypes and controlled environmental conditions. They found that the observed changes in frequencies of the phenotypes at different temperatures fitted the predictions of a model where the fitness of the homozygotes decreases

as their frequencies increase, while the fitness of the heterozygotes remains constant. However, Wright and Dobzhansky considered this hypothesis to be an ‘extreme’ one. Since then, there have been several laboratory studies where evidence for NFDS has been found in populations of Drosophila, with morph frequencies fluctuating MCE in a manner that is predictable based on the known effects of frequency on

fitness (Levene, Pavlovsky & Dobzhansky, 1954; Kojima & Tobari, 1969; Anderson & Brown, 1984; Singh & Chatterjee, 1989). A correlation between fitness and frequency has also been found in laboratory studies in crustaceans (Maskell et al., 1977; Duncan & Little, 2007), land snails (Tucker, 1991) and water snails (Koskella & Lively, 2009). This correlation has been found in natural populations as well, and is the commonest form of evidence supporting NFDS in the wild (Reid, 1987; Gross, 1991; Seehausen & Schluter, 2004; Svensson et al., 2005; Olendorf et al., 2006; Bleay et al., 2007; Takahashi & Watanabe, 2010). A few studies have also demonstrated oscillations in morph frequencies over time that can be explained by NFDS (Hori, 1993; Sinervo & Lively, 1996). However, direct evidence for NFDS in the wild is generally scarce because the best way to test for it is to manipulate the frequencies of different morphs in a population, and to obtain reliable measures of fitness from individuals of each morph, both of which pose considerable practical challenges.