According to an evolutionary model of sexual selection based on intersexual conflicts of interests over mating decisions, increased costs to females from mating can lead to a suite of characters resulting from escalating adaptations and counter-adaptations. The model predicts certain characters will evolve in the following sequence: 1) female resistance to mating, 2) male grasping devices to overcome female resistance, and 3) female morphological counter-adaptations to male grasping devices. Females of numerous species of Dytiscidae (Coleoptera) resist mating attempts by swift and erratic swimming. Males of some groups possess a grasping device in the form of sucker-shaped adhesive setae on the pro- and mesothoracic legs which are used in mating to adhere to females. Also, females of some groups have conspicuous modifications of the dorsal cuticle in various forms which appear to interfere with the male adhesive setae. In this study, the predicted pattern of these characters is tested in a cladistic analysis using characters from adult and larval morphology, behavior, and DNA data from the gene wingless. The analysis resulted in a single most parsimonious cladogram which indicates that female resistance to mating arose once. Male sucker-shaped setae arose one time in a clade nested within those taxa with female resistance. Within this clade occur the several groups that exhibit independently-derived modified female cuticular structure. Thus, the pattern of characters exhibited in the phylogeny of the Dytiscidae is consistent with their evolution as predicted by the sexual selection model. A female morphological response to a male grasping device, rare in other taxa, is seemingly present in several independent lineages of Dytiscinae in various forms of roughened cuticle which presumably interfere with the function of the male sucker-shaped setae.
Keywords: cladistics, sexual selection
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