A new explanation for unexpected evolution in body size

Abstract

Bigger is apparently frequently fitter, and body size is typically heritable, so why don't animals in wild populations evolve towards larger sizes? Different explanations have been proposed for this apparent "paradox of stasis." A new study of snow voles in the Swiss Alps finds higher survival in animals with larger body mass and heritability of body mass, but, surprisingly, a genetic decline in body mass is also indicated. The authors suggest a novel explanation for this observation: the appearance of positive phenotypic selection is driven by a confounding variable of the age at which a juvenile is measured, whereas the evolutionarily relevant selection actually acts negatively on mass via its association with development time. Thus, genes for larger mass are not actually "fitter" because they are associated with longer development times, and juvenile snow voles with longer development times run the risk of not completing development before the first winter snow. However, the genetic decline in body size is not apparent at the phenotypic level, presumably because of countervailing trends in environmental effects on the phenotype.

Fig 1. The hypothesised associations between traits and fitness in snow voles.

“+ve” denotes a positive relationship; “-ve” denotes the negative relationship between development time and juvenile survival. Green boxes are traits that have been measured or (in the case of asymptotic mass) estimated; white boxes are “missing” traits. One-headed arrows imply direct effects; two-headed arrows represent covariance. Blue lines represent entirely nongenetic associations; red lines have some genetic basis. Solid lines represent estimated parameters; dashed lines represent inferred associations. Fitness is assessed by juvenile survival, relative to the population average. Φ_juv,e is the nongenetic covariance between juvenile survival and observed mass; Φ_juv,g is the additive genetic covariance (see Figure 3 in Bonnet et al. [1] for values). Photo copyright T. Bonnet.