Trait-Fitness Associations via Fecundity and Competition in a Two-Million-Year-Long Fossil Record

The evolution of phenotypic traits is usually studied on generational timescales or across species on million-year timescales. We bridge this conceptual gap by using high-density sampling of a species lineage, Microporella agonistes (Bryozoa, Cheilostomatida), over 2 million years of its evolutionary history, to ask whether trait-fitness associations are consistent with evolutionary trait models often applied to phenotypic time series. We use average fecundity and competitive outcome as two different fitness components, where competitive outcome is a proxy for partial survival. Examining three quantitative traits in multivariate analyses, we present evidence that some traits experienced substantial selective pressures, in part controlled by past environments. A complex interplay of resource competition with an altering set of competitors and past temperatures has contributed to the changing patterns of phenotypes within the focal species. A comparison with congeneric species living in the same regional community suggests that size traits are more temporally variable and less constrained than shape traits. Our analyses also show that while controls on phenotypes are complex and varied in time, ecological and evolutionary processes that unfold on shorter timescales are not inconsistent with macroevolutionary patterns observed on longer timescales.