A scaling theory of trait evolution

A graphical scaling model, named the "geometric-similarity-first model," is developed to explain allometric trait divergence over recent evolutionary time. According to the model, traits first undergo geometric scaling with body size changes. In step two, traits go through evolutionary allometric scaling owing to a "re-optimization" process caused by directional selection. Three predictions of the model are: geometric similarity precedes evolutionary allometry, geometric similarity is a temporary phase, and all allometric traits undergo geometric scaling during initial divergence. All predictions were supported by data for three locomotor traits in pygopodid lizards. An implication of these results is that geometric scaling, driven by natural or sexual selection, may be a mechanism for peak shifts on an adaptive landscape. Given the ubiquity of body size variation in nature, this hypothetical process, termed "niche scaling," may be important to ecological diversification. Applications of the model to Darwin's finches and the Irish Elk are discussed.