CONTRASTING PATTERNS OF SPECIATION IN REEF CORALS AND THEIR RELATIONSHIP TO POPULATION CONNECTIVITY

Evolutionary theory predicts that gene flow between large established populations of demographically stable species prevents population differentiation and thereby acts as a constraining force to speciation. Over geologic time, species with large population sizes and wide geographic distributions are expected to have low speciation rates and long species durations, and to be better able to survive environmental perturbations. Here we use morphological, genetic, and fossil data to compare the relationship between speciation rate, species duration, and population connectivity in two common Caribbean reef corals, the Montastraea annularis complex and Montastraea cavernosa, in an effort to better predict how this currently endangered group of organisms will respond to the many environmental factors that are increasingly threatening coral reefs. Samples were taken from living and fossil populations at scattered locations across the Caribbean region. Genetic data consist of DNA sequences for three AFLP loci, ITS alleles, and two polymorphic nuclear loci, b-tub1 and b-tub2. Geometric morphometric analyses show that both corals form similar morphologically-distinct clusters (=morphotypes); however, in the M. annularis complex, morphotypes agree with genetic data, but in M. cavernosa, they do not. These results suggest that the M. annularis complex consists of multiple cryptic species whereas M. cavernosa is a single species. Moreover, morphological disparity and evolutionary innovation are concentrated at the geographic margin of the distribution of the M. annularis complex, but genetic diversity is lower in peripheral populations of M. cavernosa. In general, speciation rates are higher, and species durations and population connectivity (estimated using F_ST) are lower in association with the M. annularis complex than M. cavernosa. The differences in evolutionary patterns between the two corals may be partly related to reproduction (hermaphroditism in the M. annularis complex vs. gonochorism in M. cavernosa).