An analysis of the relative roles of plasticity and natural selection in the morphology and performance of a lizard (Urosaurus ornatus)

Evolutionary ecologists have devoted substantial attention to understanding which factors dictate processes of mortality within populations. Our goal was to understand the dynamics of natural selection on two performance traits (bite force and sprint speed) and associated morphological variables. We first quantified performance and morphology for a sample of marked tree lizards (Urosaurus ornatus) at the middle of the breeding season. We then sampled the same population in the nonbreeding season to determine which of the original lizards survived, and we also remeasured morphological and performance variables for surviving lizards. We found evidence for directional selection favoring fast sprinters in male lizards, but also a nonsignificant stabilizing trend that disfavored the very fastest lizards. However, we also detected substantial seasonal plasticity in bite force and head width, suggesting that an analysis of selection on only preselection (breeding season) values may be overly simplistic. Urosaurus males and females with low bite forces (and narrow heads) in the breeding season generally increased their bite forces and head widths during the nonbreeding season. In contrast, lizards that were initially strong biters in the breeding season diminished in head width and declined dramatically in bite force (up to about 35%). We suggest that seasonal plasticity could act as a retarding force for selection on performance, and could dampen seasonal and year-to-year fluctuations in selective pressures. We argue that this phenomenon may be particularly likely for performance traits important for social interactions related to breeding, such as bite force.