THEORETICAL FRAMEWORK
Diversification.
Why do certain groups of organisms exhibit increases in their rate of lineage production?  Understanding underlying causes of rate shifts in diversification is one of the great challenges that evolutionary biologists face (Farrell et al. 1991; Hulbert, 1993; Hey, 1992; Sanderson and Bharathan, 1993; Gilinsky and Good, 1991; Slowinsky and Guyer, 1989; 1991, to name but a few).  More general questions like ìwhy are there so many species of Xî, have funneled down to a host of more specific questions regarding causes, such as key innovations (e.g., Doyle and Donoghue, 1993; Sanderson and Donoghue, 1994) , climate change, adaptive zones (e.g., Mitter, 1988), and niche vacancy.
Pinpointing shifts in diversification rate and identifying their driving mechanisms requires information on 1) phylogeny, 2)biogeography and 3)time.  More specifically, addressing questions about causes of diversification rate shift demands at least: 1) sufficient taxic coverage for group in question and assurance of monophyly; 2) knowledge of that monophyletic groupís full geographic range; 3) the larger phylogeny (sister group position, diversity and geographic distribution); 4) estimate of extinction rate; and 5) estimate of origination time for monophyletic group in question including all internal nodes (Sanderson and Donoghue, 1996). Breaking down diversification into these components helps identify model organism systems for which there is hope of attaining essential data.  However, finding such model systems has proven difficult since it is often impossible to obtain full data sets, especially for numbers 4 and 5 each of which present unique challenges (cf. Sanderson and Donoghue, 1996; others?).  Despite the difficulties, studies of diversification rate shifts should include components of both time and space (phylogenetic and geographic) so the ìwhenî and ìwhereî may be understood simultaneously.  Otherwise such studies will remain incomplete.  Larger groups of wide geographic distribution (geologically older having complicated extinction histories) are intrinsically more difficult for addressing diversification, since many of the elements listed above may never be reliably estimated (but see Farrel, 1998).  However, small groups of narrow geographic range could provide reasonable solutions.  Also, younger lineages currently undergoing diversification may not have experienced appreciable extinction (such that #4 can be removed from the equation).  It is not surprising then that the relatively few, exceptional studies have managed successfully by either having a complete and unambiguous fossil record as in the case of Neogene horses (Hulbert, 1993) or by using narrowly distributed, young island groups like Hawaiin silverswords (Baldwin and Sanderson, 1998).  The former case is perhaps more exceptional than the latter, of which there are almost certainly others, and search efforts should pay off.  In fact, this proposal identifies one such model island group, described in detail below.
 Studies of diversification are arguably still in their infancy, in part due to lack of such model systems.  Yet, assuming that these systems can be identified and appropriately used, it is the onus of the researcher to fill in the missing data.  Having a solid phylogeny and reasonable time estimates allow for an understanding of where and when the rate shift happened on the phylogeny (thereby allowing for examination of coincident key innovations, for example, and perhaps part of the cause).  Ultimately, however, to answer in full why (mechanisms) the rate shift happened, one also need know where in geographical space the shift occurred (to examine key opportunities).  Such data are harder to come by where geological history is complicated, as on continents, but perhaps simpler with organismal systems on islands.

Islands & Bryophytes natural laboratories for
a model system:

Historical, physical, and ecological constraints influence dispersability, establishment, and persistence of species through time.  Oceanic island systems are natural laboratories for the study of such constraints, which in turn may yield information about the mechanisms responsible for diversification and subsequent biogeographic patterns.  Islands may have more tractable patterns than those on continents because of the relative youth and dateable ages of the land surfaces.  This appeal of oceanic islands has encouraged work of pioneering flowering plant botanists and other biologists since before Darwin (Carlquist, 1974; Wallace, 1800ís Darwin, 1800ís, other citations).  Today, trends in biogeographic patterns for certain organismal lineages such as island angiosperms are well known (e.g., Carlquist, 1974;  Baldwin, 1997; Swenson and Bremer, 1997). Some angiosperm studies (notably Baldwin and Sanderson, 1998), have managed to fill the necessary gaps in the data (achieving suitable knowledge for the 5 elements listed under ìDiversificationî) and have reached exciting conclusions regarding rate shifts, the mechanisms responsible, and the rapid radiations that ensued.  Studies like these are few, but there is good reason to believe that other model systems exist.  Very little is known about patterns shown by islands bryophytes; even less is known about the mechanisms responsible for their dispersal and subsequent diversification.  Yet, bryophytes, being small, economically unimportant plants, and unlikely to have been introduced by humans, may prove to best reflect natural biogeographic patterns of dispersal and vicariance.  In short, bryophytes can serve as model organisms for studies of diversification and biogeographic patterns on islands.