This is the deviation from perfect bilateral symmetry caused by environmental stresses, developmental instability and genetic problems during development. It is thought that the more perfectly symmetrical an organism is, the better it has been able to handle developmental stress and has more developmental stability. Fluctuating asymmetry (FA), then, may be a measure of good-genes that is difficult or impossible to mask. An honest signal?? Mates with low FA should be preferred. Causes of FA: mutations, inbreeding, homozygosity, poor genetic coadaptation.
KEY QUESTION: IS FA HERITABLE?
Symmetrical men are more attractive to women, particularly during high fertility phases of the ovulatory cycle. Scents from symmetrical men are also more attractive to women. Conversely, men show no preferences among women based on either symmetry or scent. This is interpreted as support for good-genes preferences, with the scents being an "honest signal of phenotypic and genotypic quality of the human male". Men: FA with lifetime number of sex partners, r = -0.34 Women: r = 0.00 Women with low fertility risk, correlation between man's scent rating and attractiveness: r = 0.04 Women with high fertility risk: r = -0.15 Men do not find the scent of symmetric women more attractive "One possible good-genes explanation is that symmetric men may have particularly rare major histocompatiblity (MHC) genotypes making them attractive to many fertile women."(Thornhill and Gangestad 1999).
Blanckenhorn et al. (1998) show that body size is heritable and correlated with fitness while FA is heritable for females but not males. The authors conclude: "The crucial assumption that asymmetry of sexually selected traits reflects overall, heritable developmental stability of an individual is thus only partly substantiated by our data. In contrast, large body size is heritable, associated with high fitness, and consequently could be indicative of good genes." OR (my interpretation), strong sexual selection has eliminated heritability of FA, while body size is epiphenomenal and has not been subject to selection.
Blanckenhorn, W. U., T. Reusch, and C. Muhlhauser. 1998. Fluctuating assymetry, body size, and sexual selection in the dung fly Sepsis cynipsea--testing the good genes assumptions and predictions. J. evol. biol. 11:735-753
Ligon, D. J., R. Kimball and M. Merola-Zwartjes. 1998. Mate choice by female red junglefowl: the issues of multiple ornaments and fluctuating assymetry. Anim. Behav. 55:41-50. Females used only comb size to choose mates, ignored symmetry.
Morris, M. R. 1998. Female preference for trait symmetry in addition to trait size in swordtail fish. Proc. Roy. Soc. London (B). 265:907-911. Nice paper showing that females use both trait size and symmetry when choosing mates.
Oakes, E. J. and P. Barnard. 1994. Fluctuating asymmetry and mate choice in paradise whydahs, Vidua paradisaea: an experimental manipulation. Anim. Behav. 48:937-943. Symmetry did not affect mate choice in these birds.
Thornhill, R. and S. W. Gangestad. 1999. The scent of symmetry: A human sex pheromone that signals fitness? Evolution and Human Behavior 20:175-201.
Tomkins, J. L. and L. W. Simmons. 1999. Heritability of size but not symmetry in a sexually selected trait chosen by female earwigs. Heredity 82:151-157.
copyright ©2001 Michael D. Breed, all rights reserved