GENETIC STRUCTURE OF CORAL REEF ORGANISMS : GHOSTS OF DISPERSAL PAST

SYNOPSIS. Molecular genetic studies are revealing the presence of cryptic taxa, and patterns of gene flow in coral reef species, that do not correspond to present day ocean circulation patterns. Concordant borders of genetic inhomogeneity in several taxa emphasise the influence of historical barriers to gene flow. The persistence of genetic differences between sites apparently connected by present-day currents provides evidence for lack of effective contemporary gene exchange. A review of the limited data available to date cannot be conclusive, but suggests that present patterns of genetic variation in the Indo-Pacific have resulted from highly pulsed dispersal events associated with range expansion during interglacial periods. Thus, population genetic structure appears to be dominated by events associated with global climate change and sea level fluctuation during the last 1–3 million years, rather than vicariant geological events in the early Caenozoic. Regional speciation outside the tropical Indo-West Pacific and movement of these species into that region may have played a more important role in producing diversity in that region than traditionally recognised. Some genetic variants have arisen before, and have persisted through, several cycles of climate change. The genetic structure of populations is likely to have been maintained for several thousand years after they were first established, during or immediately after range expansion, by the occurrence of co-adapted gene complexes of some form, and because of more limited opportunity for dispersal than has been assumed to date.