GROUP SIZE FACTORS AND GEOGRAPHIC VARIATION OF MORPHOMETRIC CORRELATION

Within-locality correlations among eight morphological traits of the fundatrix and among 15 morphological traits of the alate fundatrigeniae of the aphid Pemphigus populicaulis Fitch were examined for among-locality variation. A jackknife procedure revealed highly significant differences among correlation matrices representing 34 local samples from eastern North America. Most bivariate correlations also differed significantly among samples. Although very low correlations cannot differ, for moderate or strong correlations the average magnitude of correlation is not a good predictor of the degree of interlocality difference. Nearly half of the variation among localities is in the level of "overall correlation," which is positively correlated with intralocality size variation. Despite significant differences among localities, spatial autocorrelation tests failed to reveal any significant geographic pattern in correlations. Monte Carlo experiments suggest that if geographic patterns in the correlations were of the same magnitude as those for the means of these traits, some of these patterns would have been detected. Factor analyses of the pooled within-locality correlation matrices, after oblique rotation to simple structure, reveal appendage and body-size factors for both alates and stem mothers. In general, correlations between traits with high loadings on the same factor differ among localities, whereas substantial correlations that do not vary geographically are between traits that are not well resolved by the factor analyses. It is hypothesized that the apparently random geographic differences in correlation are caused by chance differences in the mode of response to short-term selection. Geographic patterns in trait means are established by selection, but, because local population sizes are finite, similar response to selection occurs by diverse physiological or developmental mechanisms in different populations, just as replicate lines in artificial selection experiments may achieve the same response by diverse mechanisms. These diverse mechanisms of response will have diverse effects on genetic variance and covariance, causing correlation patterns to vary geographically. Several forces will oppose continued divergence of these patterns.