Aquatic plants exhibit striking taxonomic, morphological and ecological diversity. This variation limits the ability to pose general hypotheses with regards to evolutionary processes in aquatic plants. Here we ask whether the population structure, reproductive systems, gene flow and patterns of genetic differentiation in aquatic plants are likely to differ in any significant way from terrestrial plants. Defining the limits of aquatic plant populations is best attempted using demographic and genetic techniques for estimating effective population size (N(e)). Data available for terrestrial species suggest that N(e) in many annual aquatics is likely to be small, a fraction of the census number. In highly clonal species, especially those with water-dispersed vegetative fragments, effective population sizes may differ widely from those of related terrestrial taxa. However, measuring N(e) in such species will probably require approaches more similar to those used to study vagile parthenogenetic animals than those used in plant populations. Reproductive systems in aquatic plants, though well described, have only begun to receive quantitative study. Levels of inbreeding and other mating-system parameters have been measured in several emergent species but are lacking for floating-leaved, submerged or free-floating taxa. Extensive clonal propagation presents analytical difficulties but also provides experimental opportunities for studying mating-system variation, particularly the relationship between large clone size and self-fertilization. Limited sexual reproduction has been observed in many highly clonal, aquatic species; there has been little attempt, however, to investigate the extent to which sterility can be attributed to genetic and environmental factors, or to explore whether sterility accumulates in clonal lineages. Gene flow in aquatic plants may be greatly affected by the discrete and patchy nature of many aquatic habitats and the directional transport of propagules in running waters. While the extent of gene movement may be influenced by habitat structure, genetic consequences of local and long-distance dispersal are likely to depend on the type of propagule involved. Transport of vegetative fragments may lead more frequently to successful gene establishment than dispersal of seed, and may, in part, explain the extensive geographical ranges of many clonal aquatic species. A survey of electrophoretic variation in 81 aquatic taxa revealed that the distribution of genetic diversity within and among populations of emergent species, as in their terrestrial counterparts, appears to be determined primarily by their breeding systems and life histories. In contrast, data for several submerged groups suggest widespread genetic monomorphism. The data, however, are limited, making interpretation of this pattern difficult, especially in cases where uniformity at isozyme loci appears to be associated with morphological and physiological differentiation. Further microevolutionary studies of aquatic plant populations may help to clarify the apparent conservative macroevolutionary pattern exhibited by certain aquatic plant families.
