It has long been recognized that sexually dimorphic traits -- traits that are systematically different between members of different sex in the same species, such as peacocks' tail feathers -- tend to vary a great deal among individual males, and that much of this within-sex variation depends on individual condition. Indeed, theory predicts that sexual dimorphism will evolve based on condition dependence so that, among traits, a more pronounced difference between male and female should be associated with a stronger response to variation in condition.
Russell Bonduriansky (University of New South Wales in Sydney) tested this prediction using the giant stilt-legged fly Telostylinus angusticollis, which breeds on rotting tree trunks along Australia's east coast. Analysis of multiple body shape components revealed a remarkable congruence between the degree of sexual dimorphism and the strength of condition dependence, accounting for nearly all of the variation in these traits.
"There is no sexual dimorphism without condition dependence in this species -- they are conceptually and biologically inseparable," says Bonduriansky. "The evidence suggests that sexual dimorphism is a pleiotropic consequence of genes that generate condition dependence." These findings call for a unification of evolutionary and genetic models of sexual dimorphism and condition dependence.
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Reference: Bonduriansky, Russell, "The evolution of condition dependent sexual dimorphism." The American Naturalist: January 2007.
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