Ecological preferences

Whereas an allopatric model is consistent with primary speciation of ferns in temperate regions, evidence suggests that, especially in tropical forests, speciation is linked with responses to ecology. With the greatest diversity on Earth occurring in the tropics (see Chapter 14), species are densely packed and may be distributed in response to geological and ecological variation. The edaphic complexity of lowland tropical forests yields patterns of species variation among ferns that correlate with soil features (Tuomisto et al., 2002) and these features may promote segregation of species edaphically at the landscape scale within a uniform-looking forest (Tuomisto et al., 1998, 2003). Montane tropical forests are stratified both elevationally (Gentry, 1988; Lieberman et al., 1996) and within tree canopies (Terborgh, 1985; Richardson et al., 2000) and fern species composition changes along elevational gradients with the greatest diversity occurring at middle elevations (Kluge and Kessler, 2006; Watkins et al., 2006).

In contrast to the pattern of low genetic identities among cryptic or nearly cryptic congeneric fern species in temperate regions (Haufler, 1987; Soltis and Soltis, 1989), isozymic analyses of species in Hawaii (Ranker, 1992a,1992b, 1994; Ranker et al., 1996) and tropical America (Hooper and Haufler, 1997) showed that some morphologically distinct species have very high genetic identities. Combined with an analysis of DNA sequences among members of a putatively monophyletic cluster of Pleopeltis species, it was possible to demonstrate ecological differentiation among these species, and to suggest that changes in ecological preferences promoted species diversification (Haufler et al., 2000). Spe-ciation in this set of Pleopeltis species may have occurred as follows. (1) An ancestral species expands its range, colonizing new areas. Disturbance may play a role in generating opportunities for colonization by providing open habitats for spore germination and gametophyte development. Alternatively, gameto-phytes of tropical species may have growth requirements and abilities that could narrow or widen the possible sites for successful establishment and growth.

(2) Range expansion may result in two outcomes that are significant in speciation. The new peripheral populations can (a) become separated by distance from the main distribution of the species or (b) encounter novel habitats that provide selective pressure for change to occur. (3) In either of these cases, the peripheral population could become genetically differentiated from the main range of the ancestral species. It seems likely that peripheral populations are changing relatively rapidly through selective adaptation to new environmental pressures. (4) These changes are reflected in a modified morphology for the residents of the peripheral populations and the changes may take place quickly. (5) Because the peripheral populations are adapted to the new ecology, they would develop pre-zygotic isolation from their ancestor populations, i.e., residents of the two ecologically differentiated populations may not tolerate the conditions of their neighbors even though they may be within spore dispersal range. Speciation following this scenario would be rapid relative to that described for temperate species (Figure 12.3).

These case studies of ferns and lycophytes provide evidence of considerable variation in the modes of primary speciation. Further, they illustrate the complexities involved in developing and testing hypotheses for the mechanisms that result in the origin of species. There is fertile ground for more studies in both temperate and tropical habitats, and for consideration of the many different patterns and processes by which primary divergence can lead to the diversification of fern and lycophyte species.

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