Baker's law and the island syndromes in bryophytes

Summary The evolution of island syndromes has long served as a model to understand the mechanisms accounting for phenotypic differentiation. Combining literature data with actual observations, we determine whether typical syndromes such as the loss of dispersal power and the bias towards self‐compatibility ( B aker's law) apply to vagile organisms, using bryophytes as a model. The life‐history traits ( LHT s) observed in oceanic island floras were statistically different from those observed on continents, evidencing the evolution of island syndromes. In contrast, LHT s of continental and continental island floras were similar, pointing to differences in migration intensity between continents, continental islands and oceanic islands. The proportion of bisexual species was significantly higher on oceanic islands than on continents. A significant proportion of species that are unisexual or bisexual on continents shifted towards exclusive bisexuality on oceanic islands, suggesting that B aker's law applies to bryophytes. The underlying mechanisms, however, probably differ from in situ selection for selfing. The proportion of species producing specialized asexual diaspores, which are assumed to play a role in short‐distance dispersal ( SDD ), was higher on oceanic islands than on continents. The proportion of species producing spores, which are involved in long‐distance dispersal ( LDD ), exhibited the reverse trend, suggesting a shift in the prevalent reproductive strategy to favour SDD on oceanic islands. Approximately 50% of the species, however, maintained the ability to produce sporophytes on oceanic islands, and the relative frequency of fertile shoots within collections of four model species was even higher on islands than on continents. Synthesis . Bryophytes exhibit typical island syndromes, indicating that migration rates between oceanic islands and continents are not sufficient to prevent the effects of genetic drift and contradicting the view that the sea does not impede migration in the group. Significant shifts in life‐history traits ( LHT s) towards increased production of specialized asexual diaspores and decreased sporophyte production on oceanic islands indeed point to a global loss of long‐distance dispersal ( LDD ) ability. The maintenance of traits characteristic for LDD in a large number of species has, however, substantial consequences for our understanding of island plant evolution, and in particular, for our vision of islands as evolutionary dead ends.