Biogeography of discontinuously distributed hydrophytes : a molecular appraisal of intercontinental disjunctions

The extraordinarily wide distributional ranges of aquatic flowering plants have long stimulated phytogeographical discussion. Although aquatic plants occur rarely among the angiosperms, they represent a disproportionately large number of taxa with broad distributions including various intercontinental disjunctions that are manifest even at the species level. Throughout the nineteenth and early twentieth centuries, long-range dispersal by waterfowl was the prevailing explanation for widespread aquatic plant distributions. This explanation gradually fell into disfavor as biologists raised doubts as to the ability of waterfowl to transport propagules across the extensive transoceanic distances between the continents on which an assortment of aquatic taxa now reside. During the twentieth century, the development of biogeographical displacement theory, i.e., "continental drift," steadily began to supplant dispersal as the preferred explanation for discontinuous angiosperm distributions. Our study assesses the dispersal/displacement hypotheses from a temporal standpoint using molecular estimates of divergence time for a diverse sample of phylogenetically related aquatic taxa that exhibit discontinuous intercontinental distributions. With few exceptions, we found divergence times that are far too recent to implicate continental drift as a major determinant of discontinuous distributions in aquatic plants. We suggest that long-distance dispersal by birds should continue to be regarded as a viable explanation for widely disjunct aquatic plant distributions, although such dispersal is likely to have involved a combination of overland as well as transoceanic migratory routes.