Range disjunctions, speciation, and morphological transformation rates in the liverwort genus Leptoscyphus

Bryophytes and angiosperms exhibit similar intercontinental disjunct distributions that have traditionally been explained by continental drift. Such disjunct distributions are, however, typically observed at the species level in bryophytes, whereas they occur at much higher taxonomic level in angiosperms. The corollary of this observation is that morphological evolution in bryophytes is exceedingly slow. These hypotheses can now be explicitly tested with the advent of molecular dating. In this article, we show that the trans-Atlantic disjunctions observed in the mostly tropical liverwort genus Leptoscyphus date back to 5.5 Myr, thus largely postdating the opening of the South Atlantic. The temporal calibration of the phylogeny allows us to estimate for the first time the absolute timing of morphological evolution in bryophytes. The time frame necessary for shifts to occur between character states was estimated on average at ca. 4.05+/-1.86 Myr. As opposed to the traditional view that bryophyte evolution has been triggered by episodic shifts in habitat conditions, our analyses furthermore suggest that morphological and molecular divergence gradually accumulated in the genus, which contrasts with the rapid diversification documented in some tropical trees.