Phylogenetic Constraints on Sexual Dimorphism Secondary sexual dimorphism has important ramifications to ecology and evolutionary biology. Understanding the compartmentalization of phenotypic variation provides insight into evolutionary mechanisms leading to speciation, and has consequences for ecologists, especially those studying variation in ecological attributes of populations. My general interests in morphometric variation in mammals appear early (Willig 1983, 1985; Willig et al., 1986; Willig and Owen, 1987a, 1987b) and recurrently (Gannon et al., 1992; Hollander and Willig, 1992; Willig and Hollander, 1995; Williams et al., 1995) in my career, and are tied to my focus on ecomorphology (Willig, 1986; Willig and Moulton, 1989; Stevens and Willig, 1999a,b). My study of insects is a more recent phenomenon (Sites and Willig, 1994a,b; Sites et al. 1996) born of association with an entomologist with whom I collaborate on ecological projects (Robert Sites at the Enns Museum of Entomology at the University of Missouri at Columbia). Although my early interests in morphometrics considered conventional aspects of differences in central tendency and dispersion, I have become convinced that the most fruitful avenues of investigation will examine the ways in which taxonomic differentiation de-couple associations among phenotypic characters. To this end, I am interested in differences in the character correlation matrix between the sexes (or among stages in hemimetabolous insects) of populations, among populations of a species, and among species in a clade. I have shown for bats (Willig and Hollander, 1995) that although considerable differences may exist in mean and variance of characters, inter-character correlations remain similar within species. Nonetheless, character correlations degrade once populations attain specific status. I am applying similar approaches for understanding ontogenetic trajectories in morphology in insects (Sites and Willig, 2000; Willig and Sites, Unpubl. Ms.) and currently am gathering comprehensive data on phyllostomid bats to more fully examine the degradation of character correlations during speciation.

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