Phenotypic evolutionary rates are reported for cranial characters of eight species of large-bodied Neotropical marsupials: Didelphis aurita, D. marsupialis, D. albiventris, Philander opossum, P. frenata, Lutreolina crassicaudata, Chironectes minimus, and Metachirus nudicaudatus. These rates were found to be lower than expected if cranial diversification in these opossums had occurred by mutation and genetic drift only, and it is clear that a greater diversification was prevented. As all parameters used in estimating rates were very conservative, the conclusion that stabilizing selection has predominated during the evolution of the skull of large-bodied opossums is fairly robust. We also show that directional selection sustained for 150 generations (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 directional selection, neutral rate tests are unlikely to infer it. This is because following a morphological shift, stabilizing selection will progressively erase evidences of directional selection.

Nephila edulis is a spider with large body size variability in males and females. Genital characters show negative allometric values compared to somatic characters. In males, the embolus (the most important structure for sperm transfer) had a significantly lower coefficient of variation than body size. This suggests that male genitalia are under stabilizing selection favouring intermediate size. Female N. edulis showed a trend similar to males regarding allometric values in genitalia. In females, however, the variation coefficient in a specific genital character crucial for successful copulation did not differ from that of indicators for overall body size. This suggests that in Nephila the genitalia of the females experience less stabilizing selection than those of the males. In male and female genitalia, the mode of selection seems to cause developmental instability not in degrees of fluctuating asymmetry but in the degree of data scatter which indicates a lower coefficient of determination.