A PAIRWISE COMPARATIVE METHOD AS ILLUSTRATED BY COPULATION FREQUENCY IN BIRDS

The pairwise comparison of the value of the dependent variable between pairs of populations of one species or between pairs of congeners differing with respect to the character state of an independent variable is a very simple solution to two major problems of comparative studies, namely, poorly known phylogenies and unduly inflated sample sizes caused by using dependent observations. We illustrate the usefulness of this approach by testing the hypothesis that among birds frequent copulations between pair members may be one of several paternity-insurance mechanisms. The difference between each pair of observations was used as an independent observation in a sign test under the null hypothesis that equal numbers of observations should occur in each direction. A pairwise comparative approach may be superior to other comparative methods because few confounding variables are likely to influence the conclusions, the assumption of parsimony is more likely in closely related taxa, and problems of poorly known phylogenies are reduced.