Evolutionary rates and stabilizing selection in large-bodied opossum skulls (Didelphimorphia : Didelphidae)

Phenotypic evolutionary rates are reported for cranial characters of eight species of large-bodied Neotropical marsupials: Didelphis aurita, D. marsup ialis, D. albiventris, Philander opossum, P. frenata, Lutreolina crassicaud ata, Chironectes minimus, and Metachirus nudicaudatus. These rates were fou nd to be lower than expected if cranial diversification in these opossums h ad occurred by mutation and genetic drift only, and it is clear that a grea ter diversification was prevented. As all parameters used in estimating rat es were very conservative, the conclusion that stabilizing selection has pr edominated during the evolution of the skull of large-bodied opossums is fa irly robust. We also show that directional selection sustained for 150 gene rations (158 years) or less is capable of producing differences of the same magnitudes as those found between various pairs of species. Therefore, we conclude that even where a particular differentiation has been caused by di rectional selection, neutral rate tests are unlikely to infer it. This is b ecause following a morphological shift, stabilizing selection will progress ively erase evidences of directional selection.