Genetic variation in fluctuating asymmetry

Abstract

The study of fluctuating asymmetry has recently become a topical subject in the biological sciences (Markow, 1994). Fluctuating asymmetry (FA) is widely thought to reveal something deep about the relationship between genotype and environment. One of the major claims is that FA is a sensitive indicator of both genetic (e.g., inbreeding, hybridisation, spread of new mutants) and environmental (e.g., pH, temperature, pollutants) stress (review Parsons, 1990). Another is that the degree of fluctuating asymmetry closely reflects fitness (Clarke, 1995). These ideas have led to the promotion of FA as a useful measure of well-being at both the individual and population levels (Leary and Allendorf, 1989). But the link between FA and stress (or fitness) is not as consistent as many reviewers suggest. There is plenty of circumstantial evidence but few definitive experiments. For example, despite correlational evidence, uncertainty still surrounds the suggestion that FA increases with inbreeding and the degree of homozygosity. In this case, carfully controlled and well-replicated experiments have not shown an association of FA with inbreeding (Clarke et al., 1992; Fowler and Whitlock, 1994). There is also practically no unambiguous evidence that FA is a more sensitive measure of stress than other measures like trait size (an exception is M@ller, 1992). This experimental deficit does not mean that FA is not a useful index. For instance a substantial body of evidence has now accumulated which shows that FA in male sexual traits is indicative of condition and is used by females in their mate choice (Mnller, 1994; Swaddle and Cuthill, 1994a; 1994b). However not all sexual traits behave in the same manner (Mnller and Pomiankowski, 1993; Tomkins and Simmons, 1995). It appears as if the utility of FA, like other measures (e.g., size, variance), varies with the functional importance and history of selection on a trait. These qualifications need to be borne in mind when extrapolating from individual and population measures of FA.