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Developmental stability, sexual selection and speciation

Fluctuating asymmetry occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. Since both sides of a bilaterally symmetric trait are the result of the actions of a single genome, fluctuating asymmetry represents an epigenetic measure of the sensitivity of development to stress. Different morphological traits may show a direct relationship between their functional importance and their degree of developmental canalization. This may explain why some characters show high degrees of fluctuating asymmetry, and why these characters more often become exaggerated secondary sexual ornaments. The degree of fluctuating asymmetry is generally larger in small marginal populations living in novel environments, and this will particularly lead to relatively large degrees of asymmetry in the least developmentally canalized traits. More stringent selection against heterozygotes in marginal populations may 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 characters, because it is more difficult to make large traits (a good genes argument), a large trait is more easily perceived (a sensory bias preference), and because symmetry signals ability to cope with stress (a good genes argument). The low degree of developmental stability and the large amount of genetic variance in secondary sexual characters in small, marginal populations could set the scene for rapid development of divergence and speciation in marginal populations.