Comparative Quantitative Genetic Analysis of Cranial Capacity and Craniofacial Morphology in Two Closely Related Primate Species

Evolutionary change in a character is due to a combination of the direct effects of selection on that character and the indirect effects of selection on characters that are genetically correlated with the character in question (Lande and Arnold 1983 ; Cheverud 1996 ). Therefore, selection on a single component within a correlated unit will result in coordinated evolution of the entire complex (Lande 1979 ; Cheverud 1982, 1984 ; Falconer and MacKay 1996 ). Due to the ubiquity of phenotypic correlations among traits, natural selection appears to act on many characters simultaneously. However, genetic correlations among traits can pose constraints on phenotypic evolution because these correlations are capable of altering both the rate and direction of evolution from the optimal path defi ned by natural selection (Cheverud 1984 ; Wagner and Altenberg 1996 ). If traits that are inherited together ultimately evolve together, then the challenge in explaining the correlated appearance of particular characters and understanding coordinated evolution lies in identifying genetically integrated morphological traits.