The Origin of Novelty Through the Evolution of Scaling Relationships

Morphological novelty is often thought of as the evolution of an entirely new body plan or the addition of new structures to existing body plans. However, novel morphologies may also arise through modification of organ systems within an existing body plan. The evolution of novel scaling relationships between body size and organ size constitutes such a novel morphological feature. Experimental studies have demonstrated that there is genetic variation for allometries and that scaling relationships can evolve under artificial selection. We show that an allometry equation derived from Gompertz growth kinetics can accurately reconstruct complex non-linear allometries, and can be used to deduce the growth kinetics of the parts being compared. The equation also shows the relationship between ontogenetic and static allometries. We discuss how changes in the non-linear kinetics of growth can give rise to novel allometric relationships. Using parameters for wing and body growth of Manduca sexta, and a population simulation of the allometry equation, we show that selection on wing-body scaling can dramatically alter wing size without changing body size.