TESTING BIOTIC VS ENVIRONMENTAL FACTORS IN THE EVOLUTION OF MARINE BIVALVES: CHARACTER DISPLACEMENT AND CONVERGENCE AMONG RELATED SPECIES

Many closely related species overlap in their geographic ranges. But closely related species are expected to share similar ecological requirements given their genetic similarity, which begs the question of how they can share geographic space. Competition between sympatric related species for shared resources may result in a pattern of ecological trait divergence among species pairs. Conversely, stronger selection from their shared environment may drive convergence on similar ecological traits. To evaluate the relative balance of these two selective pressures, in one of the first analyses of this kind in a marine system, I investigated the spatial and temporal patterns of ecologically significant aspects of shell form in the Venerid bivalve genus Chione.

Today, 10 of 16 species pairs exhibit similar shell outlines in sympatry (taken here as a proxy for similarity in burrowing ecology), which suggests that environmental selection for habitat-specific traits is stronger than any intraclade competition driving divergence of these traits. The 6 remaining species pairs do show different burrowing ecology in sympatry, but in 5 instances those differences derive from the presence of a single species with markedly different shell form for the genus. At the lineage level, trajectories of trait evolution among Chione species in the Miocene-Recent fossil record also suggest a predominant role for environmental selection. Eastern Pacific species exhibit stasis in burrowing ecology from their origination to the present day, and extant species from the Western Atlantic show convergent trait trajectories both towards each other and the ecomorphological forms previously occupied by extinct relatives.

Together, these population and lineage level results imply a lesser role for intraclade competition in shaping shell morphology. Instead, environmental selection, be it from abiotic or “extra-cladal” biotic pressures (e.g., predation), appears to be the dominant factor shaping burrowing ecology within this marine bivalve clade. Combined biogeographic, morphometric, and paleontological analysis provides a powerful approach for evaluating the long-held hypothesis of character displacement against alternatives that may prove to be more important in structuring marine clade morphology.