EARTH SYSTEMS, EUSTASY, AND EVOLUTION: COMPUTATIONAL MODELING OF THE EFFECTS OF SEA LEVEL CHANGE ON TRAIT EVOLUTION AND MORPHOLOGICAL DISPARITY IN BERMUDAN LAND SNAILS

Many evolutionary processes are driven by biogeographic change and, thus, by Earth systems. Sea level change, tectonic uplift, basin filling, and climate cycles all impact the geographic ranges of species. These processes can, therefore, directly influence rates of evolution, population diversification, and speciation by altering metapopulation dynamics. Range expansions and contractions, for example, can change the genetic composition of a species by winnowing the geographic diversity of its local populations or by subdividing its populations to facilitate allopatric speciation.

The effects of sea level cycles on evolution and morphological diversity in Bermudan land snails were evaluated using computational models. Bermuda is a seamount in the Atlantic capped by carbonates. About five percent of its broad summit is currently above sea level forming an island chain of eolianite deposits. The rest is less than 50 m below current sea level and has been exposed during most of the Quaternary except for periodic interglacial inundations. Bermuda is home to an endemic Quaternary radiation of land snails of the genus Poecilozonites, the evolutionary patterns of which were influential on Gould’s model of speciation by punctuated equilibrium.

Combining Raup’s shell coiling equations with a population-level stochastic model of neutral drift, dispersal, gene flow, and extirpation, I simulated the evolutionary changes in Poecilozonites through the last half million years. During long phases of low sea level, drift processes produce geographically random morphological patterns in an essentially panmictic species covering the sea mount. When interglacial transgressions flood the island, populations are extirpated and subdivided. Geographic disparity in shell morphology always increased, often dramatically, during transgressive events, as did rates of evolutionary change. Measured through time in a single locality, morphological disparity was lowest in the central area of the summit, greater on those areas that always remain above sea level, and greatest on the periphery in areas currently under water. The greatest morphological disparity and fastest rates of evolution thus occur in places and times that are unlikely to be sampled in the fossil record.