Do key innovations outweigh ecological context in the evolution of life-history traits? Insights from spiny lizards (Sceloporus).

Life-history traits are central to the long-term persistence of phenotypes. In reptiles, the transition to viviparity-viewed as a key innovation-and habitat use-an essential component of the ecological context-have been identified as two major drivers of phenotypic evolution. Yet their relative contributions and operating scales remain unclear. We used Bayesian phylogenetic mixed models (univariate and multivariate) and phylogenetic comparative analyses to compare the effects of parity mode and habitat use on the evolution of life-history traits in Sceloporus. We assembled data for nine traits (e.g. female snout-vent length, clutch/litter size, offspring size), parity mode, and habitat use across 88 species. We found a contrast between present-day trait associations and transition dynamics: parity mode was associated with variation in transition rates among habitat-use states, whereas habitat use better explained the multivariate phenotypic architecture. Reproductive allocation in Sceloporus was structured not only by the classic offspring size-number trade-off, but also by how reproductive effort is distributed through time. At the multivariate level, habitat use best captures the size-fecundity architecture, but when additional reproductive traits are included, parity mode dominates covariation. We outline potential mechanisms and propose a general framework to understand and generate testable hypotheses about phenotypic evolution in Sceloporus and other reptile lineages.