EVOLUTION IN A PAEDOMORPHIC LINEAGE. II. ALLOMETRY AND FORM IN THE MEXICAN AMBYSTOMATID SALAMANDERS.

Abstract

Allometry, broadly defined as the study of size and its consequences (Gould, 1966), is one of the primary tools of morphometric analysis. By quantifying the relationship between size and shape both within and among species, morphological null hypotheses, or expectations, can be generated and compared with the known morphology of related taxa (Sweet, 1980). Populations that fail to conform to allometric expectations may then be singled out as deserving further explanation or attention. Previously (Shaffer, 1983, 1984) I investigated the phylogeny of the Mexican ambystomatid salamanders, using an electrophoretic data base which is independent of morphological evolution. A major result of that study is that reproduction in the larval morphology has evolved several times, independently, in different salamander populations. What these electrophoretic analyses cannot show is that these various non-transforming salamanders take on a wide variety of shapes and sizes, some of which have been considered of specific and even generic-level importance. In addition, the phenomenon of larval reproduction has had a long history in the group, with well known fossil material from the Pliocene (Tihen, 19 5 5). Two conflicting hypotheses can explain the morphological diversity among these non-transforming populations. Since each is apparently derived from a nearby, transforming population, it may be that the differences in paedomorphic morphology simply reflect variations in ancestral larval growth trajectories. Alternatively, the non-transforming salamanders may have completely new shapes not predicted from ancestral growth curves. In this study I report the results of both bivariate and multivariate morphometric analyses to distinguish between these two hypotheses. The bivariate analyses offer a simple graphical interpretation of growth predictions, while the multivariate analysis (discriminant function analysis) treats many aspects of the organism simultaneously. A final discriminant function analysis considers patterns of morphological variation among all of the non-transforming Mexican ambystomatids, and attempts to separate shared ancestral from possible convergent components of variation.