DISSERTATION RESEARCH: Phylogenetic Persistence of Modularity of the Rodent Skull

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The developmental architecture (DA) of complex morphological structures causes traits to develop as integrated units or independent modules. Persistent questions are whether DA influences the divergence of morphological features that often accompanies organismal diversification, and whether DA itself evolves. The aim of this research is to address such issues by comparing DA over a large group of organisms for which a robust phylogeny is available. Finding that more closely related, morphologically distinct species share a common DA would argue for its persistence. For the present study, the DA of the craniomandibular complex will be compared across an extensively sampled group of ecologically and morphologically diverse rodents, the Sigmodontinae, to determine whether DA persists despite diversification. The comparisons will be based on a quantitative analysis of DA, being DA revealed by statistical covariances among traits. The structure of those covariances provides an estimate of a group's DA. Such covariance structures will be contrasted across species, using an approach that compares them to models deduced from hypotheses of developmental mechanisms. Should all species within a lineage fit the same model, that will be taken as evidence for the persistence of DA. These broadly comparative analyses will be based on samples of adult specimens. The hypothesis that developmental information persists until the adult stage will be tested by comparing samples of specimens on different developmental stages. A novel approach will be employed in these analyses to compare the effects of processes that generate or eliminate variation through development.