DEVELOPMENTAL STABILITY, SEXUAL SELECTION AND SPECIATION

Fluctuating asymmetry occurs when an individual is unable to undergo i dentical development of an otherwise bilaterally symmetric trait on bo th sides of its body. Since both sides of a bilaterally symmetric trai t are the result of the actions of a single genome, fluctuating asymme try represents an epigenetic measure of the sensitivity of development to stress. Different morphological traits may show a direct relations hip between their functional importance and their degree of developmen tal canalization. This may explain why some characters show high degre es of fluctuating asymmetry, and why these characters more often becom e exaggerated secondary sexual ornaments. The degree of fluctuating as ymmetry is generally larger in small marginal populations living in no vel environments, and this will particularly lead to relatively large degrees of asymmetry in the least developmentally canalized traits. Mo re stringent selection against heterozygotes in marginal populations m ay further break down developmental stability and linkage groups which would lead to increased genetic variance. Females may prefer to mate with males having large, but relatively symmetric morphological charac ters, because it is more difficult to make large traits (a good genes argument), a large trait is more easily perceived (a sensory bias pref erence), and because symmetry signals ability to cope with stress (a g ood genes argument). The low degree of developmental stability and the large amount of genetic variance in secondary sexual characters in sm all, marginal populations could set the scene for rapid development of divergence and speciation in marginal populations.