GENOMIC SIGNATURE OF THE LILLIPUT EFFECT ACROSS THE K/PG EXTINCTION

Survivorship following major mass extinctions has been associated with a decrease in body size, a phenomenon called the Lilliput Effect. Since body size is a strong predictor of many life history traits (LHTs), which, in turn, influence demography and intrinsic biological processes, pronounced changes in organismal size throughout Earth history may be associated with concomitant changes in nucleotide substitution rates. Here, we document extreme shifts in genome-wide rates of molecular evolution across a recent ~200-taxon genomic dataset, and demonstrate that these shifts are strongly associated with inferred changes in body size when controlling for other LHTs. We show that in the presence of such life history-driven rate variation, relaxed molecular clocks may dramatically over-estimate divergence times. We hypothesize that increases in nucleotide substitution rate deep within the extant avian phylogeny are associated with selection for reduced body size following the Cretaceous-Paleogene transition, consistent with the paleontological prediction of a ‘Lilliput Effect’ in the wake of a major mass extinction. This phenomenon may help resolve persistent divergence time debates across the tree of life, and suggests that selection for certain life history traits may be associated with deterministic molecular evolutionary outcomes.