ENVIRONMENTALLY INDUCED DIFFERENTIAL LEVELS OF MORPHOLOGICAL VARIABILITY

Morphology is a key issue to paleontology as it is used for species definitions, measuring evolutionary rates, and biostratigraphy. Animal form is inherently variable, and variation exists across a hierarchy that ranges from organs to populations, as well over ontogenetic time. But even recent studies ignore (or actively remove) morphological variation from analyses, largely due to the resulting statistical complexities. Here, we show that a single mechanism, protein malnutrition, impacts variation within individuals differently than it does among individuals.

Male and female Rattus norvegicus consumed either a low or control protein diet. Radiographs, taken every 2-3 days, were the basis for 19 linear distances that depicted the craniofacial skeleton through ontogeny. The growth of each distance, for each individual, was fit with a Gompertz model, which provides biologically meaningful growth parameters (e.g. growth rate). For within-individual analyses, we compared the residuals of model fit for each measurement, for each individual. Among-individual variation was assessed by the coefficient of variation for each measurement.

Although both females and malnourished animals were significantly smaller in size, there was no significant effect of sex or diet for within-individual variation. However, the growth parameters among the low protein animals had significantly higher amounts of variation than among the controls. While each individual adhered to its trajectory equally well (low intra-individual variation), the differences among individuals was affected by diet. This was not the case for sex. Males and females, despite different end-points for growth, had the same amount of variability in growth parameters among individuals.

Because these striking differences in variability existed within lab rat populations, animals bred for homogeneity, the effect in natural populations could be even higher. These results pertain directly to debates about canalization and developmental plasticity, two opposing concepts invoked as important navigators for evolutionary transitions. We also suggest that variation can be environmentally induced.