Evolvability and nonevolvability of allometric slopes

Abstract

The science of morphological allometry centers around relationships between the size of organismal structures and overall body size. Morphological allometry usually is concerned with two parameters, the slope of log–log plots of trait parameters on body mass or size (reflecting how trait size varies with body size), and the intercept of such plots (providing measures of body-size–independent trait parameters). Morphological allometry is a central area of study for organismal and evolutionary biologists, because body size explains a large proportion of the variance in size of most body structures, and because we still lack a broad mechanistic understanding of the processes that produce allometric slopes. One of the debated themes of allometric research has been whether allometric slopes are determined by natural selection or represent constraints on biological variation. In PNAS, Bolstad et al. (1) address this question directly by testing whether the allometric slope of wing vein length on wing area of Drosophila melanogaster could be changed by artificial selection (yes, it was changed), and also show that after selection on allometric slopes, but not their intercepts, populations returned rapidly to control values when artificial selection ceased (Fig. 1). Bolstad et al. conclude that the rapid return of allometric slope to control values provides evidence for a pleiotropic constraint on the slope; that is, the genetic, developmental, and functional bases for different traits are not independent, so that selection on one allometric relationship affects others and overall fitness. They state “… this hypothesis could provide a general explanation for the striking evolutionary conservatism of allometric power laws, while still leaving open the possibility for allometries to be optimized by natural selection over long time scales through compensatory mutations” (1).