Morphometric Variation in Introduced Populations of the Common Myna (Acridotheres tristis): An Application of the Jackknife to Principal Component Analysis

Abstract

-Morphometric variation in samples of common mynas from 11 localities in Australia, New Zealand, Fiji and Hawaii was analyzed with principal component analysis. We attempt to avoid two shortcomings in many previous applications of principal component analysis in morphometrics by analyzing separately variation within and among populations, and by applying the jackknife procedure to reduce subjectivity in interpretation of the principal components. Within populations, only principal component I appears to have a stable orientation and this orientation is common to all localities. Component II may be a simple vector, differing across localities, but similarity in the second and third eigenvalues warns that the associated components could be largely arbitrary. The variance along component I does differ across localities. Among populations, again only the first component displays convincing stability, although component II may be a stable but extremely simple vector. Both within and among populations, component I appears to be a general factor representing size and size-related shape variation. The apparently simple patterns of covariation displayed by the introduced populations may be attributable to bottlenecks in small founding populations and the short time since the introductions were made. Future studies should incorporate some form of testing to confirm putative patterns of character covariation, and doing so probably will require sample sizes much larger than has been the custom. [Skeletal morphometrics; principal component analysis; jackknife; population variation; mynas.] A general justification for a morphometric approach to evolutionary studies arises from the common belief that morphology is a primary and direct means by which organisms interact with their environment; variation in size and shape can have physiological and mechanical consequences (Gould, 1966; Alexander, 1968, 1971; McMahon, 1973, 1975; Pedley, 1977). Support for this belief tends to come from large-scale phenomena. Morphological differences among species, for example, are related convincingly to functional differences (e.g., Lack, 1947; Bock, 1970; Liem, 1973; Abbott et al., 1977). It then is assumed generally that such correspondences apply continuously through finer scales of variation. Thus, we arrive at an assumption of morphometricians that measured differences among individuals have functional consequences to the organisms involved, though empirical support for this assumption is rather sparse (but see Grant et al. [1976] and references therein; Herrera, 1978). By further extension, morphological differences among geographically separated populations are believed not only to affect function but also to correlate with environmental differences, leading to generalizations such as the ecogeographic rules. Finally, it is assumed that both withinand amongpopulation variation is adaptive and, therefore, has been crafted by natural selection. Correlation of these two sources of variation is a central prediction of the synthetic theory of evolution (Sokal, 1978). Our goal herein is to learn how skeletal variation is organized within populations of birds, and whether the pattern of this variation is involved in among-population differentiation. Additionally, we wish to describe the pattern of among-population variation and to determine its relationship to within-population structure. Principal component analysis is a common analytical approach here, but many applications have incorporated two methodological