CONNECTIONS BETWEEN EVOLUTIONARY MODE AND TEMPO, EVOLVABILITY, AND TRAIT INTEGRATION IN A COCCOLITHOPHORE LINEAGE

Evolutionary time series of ancestor–descendant populations in the fossil record provide unique opportunities to explore key questions in evolutionary biology that are challenging to address by studying extant taxa alone. In this study, we investigate the multivariate phenotypic evolution of the well-sampled coccolithophore lineage Reticulofenestra umbilicus over a 12 million-year span. We find that the trait dynamics within this lineage are characterized by fluctuating evolution, or stationary trait dynamics, punctuated by a consistent trend towards larger sizes—culminating in a 90% increase in size-related traits. The distinct and clearly delineated evolutionary modes observed in the time series enable us to assess whether there is a correlation between effects of evolvability, rates of phenotypic change, and the mode of evolution. Specifically, we address the following questions: Is the rate of evolution more rapid during periods of directional change as opposed to phases when traits fluctuate around a fixed mean? Does phenotypic change occur more frequently in directions with higher variability (i.e., greater evolvability), and is there a difference in how levels of trait variance predict phenotypic change during directional change compared to intervals of fluctuating trait dynamics? Does the degree of trait integration (i.e., trait correlation) shift over time, and if so, how is this associated with changes in the mode of evolution within the lineage?