A scaling theory of trait evolution

A scaling model named the "geometric-similarity-first model" is developed to explain allometric trait divergence over recent evolutionary time. In the model's first step, traits undergo geometric scaling with a population's change in body size. In step 2, directional natural selection re-optimizes trait shapes such that traits showing positive ontogenetic allometry undergo positive evolutionary allometric scaling while traits that exhibit negative ontogenetic allometry go through negative evolutionary allometric scaling. Five predictions of the model were tested using morphological data for three locomotor-relevant traits in pygopodid lizards. The dataset, which was based on 1,756 museum specimens representing 31 species, supported all of these predictions. 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 some other well-studied species are discussed.