EVOLUTIONARY CONSTRAINT OF THE DIVERSIFICATION OF AVIAN LIMBS

Genetic and developmental factors (e.g. traits influenced by the same gene), and functional factors (e.g. traits of the same bio-mechanical apparatus) may integrate an organism’s traits so that selection cannot optimize its form for a given environment, creating tension between processes that promote anatomical diversification and those that promote integration. In this study, we used geometric morphometric methods to assess the extent to which integration has constrained the diversification of limb elements during the evolution of birds.

Flightless birds have evolved many times, producing unrelated species that share similar limb morphologies, especially in the forelimb. In this study, we compared patterns of covariances in limb bones of flying and flightless birds as a measure of integration using geometric morphometrics, Mantel tests, Principal Components Analyses, Common Principal Components Analyses, Two Block-Partial Least Squares, and cluster analyses to determine whether patterns of integration are different in flying and flightless birds because of reduced functional demand or whether the two groups have a shared pattern of integration that is consistent with shared genetic and developmental constraints across all birds.

A majority of analyses showed that flying and flightless birds have similar integration patterns within elements of both the fore- and hind limb. However, a number of the analyses, especially when phylogenetically independent contrasts were used, showed that patterns of integration between elements are different for flying and flightless birds.

The results support the hypothesis that integration patterns within the elements of avian limbs is constrained by developmental and genetic factors, regardless of flight ability. However, integration between bones may be influenced by functional factors. Therefore, the disparity of functional selection acting of the limbs of flying and flightless birds may promote diverse integration patterns within the limbs of the two groups, while shared genetic and developmental factors constrain the evolution of individual limb elements across all birds.