Uneven diversity in both numbers of species and anatomical variability represents one of the more perplexing evolutionary patterns across the Tree of Life. Although an increasing number of researchers have begun to understand what drives uneven diversity between species, few researchers have examined how differences within single species can affect these broad scale evolutionary patterns. One source of variation lies in the difference between the sexes. The primary goal of this research is to build upon evolutionary theory by examining how sex differences contribute to species and phenotypic diversity. This research fills a critical gap in our understanding of how variation within species may drive larger patterns of species diversity. Through this research program, scientists will contribute to training the next generation of students through their mentorship of high school students through the UC Santa Cruz Science Internship Program (SIP), undergraduate student training in the Small Mammal Undergraduate Research in the Forest (SMURF) program, public talks through the Santa Cruz Public Library, and undergraduate teaching in Comparative Vertebrate Anatomy.
The skulls of musteloids (badgers, otters, raccoons, skunks, and weasels) will be used as a model to examine the effects of sexual dimorphism on phenotypic disparity at the macroevolutionary scale. Musteloids are a taxonomically rich and phenotypically disparate clade whose lineages exhibit uneven patterns of diversity. The researchers will use 3D geometric morphometrics and develop new comparative methods to test the hypothesis [H1a] that incorporating the effects of sex will reveal increased cranial disparity. Males and females in each species are predicted to occupy mostly non-overlapping parts of cranial morphospace; furthermore, the inclusion of both sexes will fill morphospace that was previously unoccupied when the analysis focused on a single sex. The researchers will then quantify the rates of cranial disparity through time in males and females separately and pooled together (species means) to test the hypothesis [H1b] that incorporating sex enhances the pattern of adaptive radiation (i.e. rapid increases in evolutionary rates near ecological opportunity followed by a slowdown in rates). A rate shift in cranial evolution is predicted to occur just after the onset of the Mid-Miocene Climate Transition when using the pooled male and female dataset. Lastly, the researchers will examine an ecological basis for sexual dimorphism by testing the hypothesis [H2] that the degree of carnivory corresponds with the evolution of sexual dimorphism in cranial shape and size across Musteloidea. Therefore, hypercarnivorous musteloids are predicted to exhibit the highest degree of cranial sexual dimorphism. We suspect that this disparity in hypercarnivores is present to mitigate competition between the sexes.
