MACROEVOLUTIONARY CONSEQUENCES OF PHENOTYPIC VARIATION: A TEST USING THE DEEP-SEA OSTRACODE GENUS POSEIDONAMICUS

It has been long known that variation is the raw material of evolution. Modern microevolutionary theory successfully integrates patterns of trait variance and covariance into predictive models of evolutionary change. Although these patterns of variation play an important role when evolution is considered on time scales of a few generations, their influence on evolutionary change considered over thousands of generations is still unclear. The purpose of the current research is to use the rich fossil record of the deep-sea ostracode genus Poseidonamicus to test if patterns of phenotypic variation have any detectable effect in directing or limiting the evolutionary trajectory of this clade.

Testing this hypothesis requires documentation of both patterns of phenotypic variation and evolutionary change. The traits considered in this study include a set of linear distances based on the position of homologous skeletal and anatomical landmarks on the ostracode carapace. Patterns of variation are represented by phenotypic covariance matrices among these traits, and evolutionary transitions are computed between multivariate means of different samples.

Evolutionary change is documented at two scales of resolution: calculated directly on the basis of stratigraphic position for multiple samples of the same lineage spanning single cores, and inferred from a hypothesis of phylogenetic relationships among studied samples. This phylogenetic analysis is based on over 60 morphologically homogenous clusters of spatially and temporally restricted specimens, approximately 40 of which had sample sizes sufficient for biometric analysis. These samples are distributed over the 15 named species within this genus over approximately 40 million years, and thus represent a fairly dense sampling of the history of this clade. The evolutionary transitions between these samples will be used to test the null hypothesis that the probability of evolution in a particular morphological direction is independent of the amount of variance in that direction. Rejection of this null hypothesis is expected when 1) evolution is facilitated by high phenotypic variability, and/or 2) populations are effectively prevented from evolving in traits or combinations of traits with low variance.