Variability of a vestigial structure: First metacarpal in Colobus guereza and Ateles geoffroyi

Abstract

A tenet of evolutionary theory is that, under conditions of stabilizing selection, phenotypic variability is inversely related to selection intensity. Correspondingly, a nonfunctional, vestigial structure is expected to be highly variable relative to its functional homologue. This study tests the hypothesis that species with a vestigial pollex, Colobus guereza and Ateles geoffroyi, have a first metacarpal whose length is both highly variable relative to, and poorly correlated with, the lengths of the other metapodials. The results are consistent with the hypothesis, though this combination of traits is also found in Presbytis rubicunda and Presbytis cristata. The latter two species have functional, albeit miniature, pollices. This study also demonstrates a general, inverse relationship among anthropoid primates between relative length of the first metapodial and its relative variability. These results suggest that elevated variability accompanies structural reduction.