Biogeography

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.

For Darwinian biologists, biotic distributions were a result of dispersal away from a centre of origin. On an Earth that was thought to be static, they believed the solution to finding why these distributions have occurred was to search for the centre of origin by retracing the steps of genealogical lineages through space and time. They therefore saw each species lineage as

Fig. 6.1. Holarctic distribution of the liverwort Lepidozia reptans, with intrusion in high mountain areas in the tropics (redrawn from Tan & Pocs 2000).

Fig. 6.1. Holarctic distribution of the liverwort Lepidozia reptans, with intrusion in high mountain areas in the tropics (redrawn from Tan & Pocs 2000).

Fig. 6.2. Circum-Subantarctic range of the liverwort genus Herzogobryum (redrawn from Tan & Pocs 2000).
Fig. 6.3. Nearly pantropical distribution of the moss Erpodium biseriatum (redrawn from Tan & Pocs 2000).

having its own unique history and extrapolated common biotic distributions to be the result of the same dispersal processes (see Humphries & Parenti 1999 for review).

In the 1960s, the reappraisal of the nature of the Earth's crust by Wegener's plate tectonics theory provided a solid basis for understanding and explaining the floristic similarities among regions of the world that are widely separated today. In contrast to Darwin's theory, dispersal was thought to overwrite, in a stochastic fashion, the initial pattern established by drift. Dispersal, it was argued, is not capable of falsification. As it could explain any pattern, it could conclusively explain no pattern in particular and was regarded as unsuitable for scientific examination. Dispersal was therefore considered an irrelevant noise that could be disregarded in the search for patterns established by drift (McGlone 2005).

It has, however, been increasingly admitted that dispersal patterns may be surprisingly regular and enduring in source, direction and target areas (McDowall 2004). Recent developments based on studies at the molecular level have, in fact, undermined the pre-eminence that continental drift has enjoyed and we are now in the middle of a dispersalist counter-revolution (De Queiroz 2005).

In this chapter, we describe the general patterns of bryophyte distributions. We discuss how these distributions were achieved by contrasting the hypotheses of continental drift and long-distance dispersal. Finally, we discuss the implications that the competing hypotheses of continental drift and longdistance dispersal have for our understanding of morphological evolution and species diversification.

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