The fundamentals of mechanics provide a strong basis for the hypothesis that allometry could be an important constraint on evolutionary changes in shape. Empirical evidence that allometry accounts for sufficient shape variation to be an effective constraint is much weaker. Numerous studies claim to show that most shape variation is correlated with size, but nearly all these studies use morphometric methods that confound size and shape. Consequently, the proportion of the shape variation correlated with size remains unknown. To address this issue, I examined ontogenetic change and adult variation in lower jaws of the fox squirrel, Sciurus niger. Jaw morphologies were quantified using traditional distance measurements and geometric shape variables computed from landmark coordinates. The same analyses were performed on each data set. As expected, analyses of the distance measurements found that allometry accounted for nearly all ontogenetic variation. The same analysis on the coordinate data found that allometry accounts for <50% of the ontogenetic variation in shape. In analyses of adults, allometry explained >50% of the variation in distance measurements but <25% of the variation in shape. These results confirm that analyses of distance measurements confound size and shape and that this can lead to erroneous claims about the importance of allometry.
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