Biogeography is the science that aims to describe the spatial distributions of biota (a pattern) and understand the means by which these distributions were achieved (a process). Biogeography is a field that existed long before evolutionary biology and indeed helped in founding the evolutionary ideas of Charles Darwin and Alfred Wallace, among others (Humphries & Parenti 1999). Biogeography and evolutionary biology therefore interface with each other, as the discovery of the mechanisms regulating species distributions involves an understanding of species dispersal ability, evolutionary rates and diversification mode, which are among the main foci of the sciences of evolution.
In general, bryophyte species have broad geographic ranges that often span more than one continent (e.g. Figs. 6.1–6.3). Some, termed as ‘cosmopolitan’, are even widespread across all continents. Bryophyte species thus tend to show wider distributions than vascular plants. In fact, many bryophyte species exhibit the same disjunctions that are well known in flowering plants at the generic level. For example, 43% of the moss species found in North America are also found in Europe, while 70% of the species found in Europe also occur in North America (Frahm & Vitt 1993). By contrast, 48% of the genera, but only 6.5% of the species, are shared between the North American and European vascular flora (Qian 1999). Two competing hypotheses, namely repeated intercontinental dispersal and continental drift, have traditionally been proposed to explain the broad and highly disjunctive distributions typical of bryophyte species.