The diversity of male genitalia observed among many disparate arthropod taxa has stimulated a number of studies investigating which evolutionary factors contribute to this phenomenon. Much of this research, however, is focused on testing patterns of selection on male genitalia only. Because copulatory processes are a joint act between males and females, detailed study of the selection pressures on female morphology seems equally appropriate. In the current analysis, I used geometric morphometrics to quantify intraspecific shape variation of male and female genitalia in the scarab beetle species Phyllophaga hirticula (Knoch). Characters under sexual selection are predicted to be more phenotypically variable than nonsexual characters because of the persistent tendency of sexual selection to eliminate bouts of stabilizing selection. Thus, this analysis tested whether phenotypic variation of male and female genitalia is equivalently higher than shape variation of a nonsexual character, the right elytron. Shape variation was analyzed in SHAPE, a program that transforms two-dimensional outline data into elliptic Fourier descriptors to be analyzed in a principal components analysis. Data from this study supported the hypothesis that male and female genitalia were significantly more variable than the right elytron. In addition, P. hirticula female genitalia were significantly more variable than male genitalia. Comparable phenotypic variation of male and female genitalia suggests that like male genitalia, female genitalia also may be subject to sexual selection and should be considered when testing hypotheses of genitalic evolution.
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Vol. 99 • No. 1