Discussion
Although patterns of relationship shared among populations of a single species should not be used to draw general biogeographic conclusions (Brooks and McLennan 1991), area relationships in A. plicata can be instructive if compared with results of other codistributed species to consider more general biogeographic patterns, patterns of phylogeographic association, and thus also patterns of ecological interaction over long periods of time. To do this, it is necessary to assume that the populations sampled adequately reflect underlying diversity within the species, that the hypothesis of phylogenetic relationship can be accepted, and that there is some relationship between present and ancestral biogeographic states. In addition, where the root of the tree is placed at least partly affects the biogeographic patterns.
Phylogeographic and biogeographic patterns from several other codis-tributed groups recovered by other studies allow meaningful comparison with A. plicata. When this is done, two major patterns are evident. First, studies of intraspecific differentiation in salamanders (Routman, Wu, and Tem-pleton 1994) found little congruence between geographic proximity and phylogenetic propinquity. Zink and Dittman (1993) and Routman, Wu, and Templeton (1994) hypothesized that this general pattern could be due to recent colonization events postdating the Pleistocene. This would point to the ephemeral nature of community assemblages in the Central Highlands.
In A. plicata there was some congruence between geographic proximity and phylogenetic propinquity, particularly in the cases of more narrowly circumscribed biogeographic regions such as the Upper Ohio and Middle Arkansas drainages. Such congruence is quite incomplete, however, with several instances of sister group relationships between populations from the two major biogeographic regions in the Central Highlands to the exclusion of sister group relationships between populations within a single one of these regions. This could be taken as further evidence for the transitory nature of community assemblages in the Central Highlands. Based on the second chief pattern that emerges from comparative phylogeography and biogeography, however, this conclusion is not supported.
Instead, phylogeographic patterns in A. plicata are resonant with interspecific biogeographic patterns from numerous clades of aquatic fish of the Central Highlands given in Wiley and Mayden (1985) and Mayden (1988). One salient feature found by this study and that of Wiley and Mayden (1985) and Mayden (1988) is a sister group relationship between taxa in the Interior and Eastern Highlands, assuming parsimonious optimizations of biogeo-graphic states. These regions have been split since the Illinoian glaciation (Wiley and Mayden 1985). To explain this evolutionary pattern as the result of late or post-Pleistocene environmental change, one must posit a large number of dispersal events across biogeographic barriers. A more parsimonious view of the phylogeographic patterns would have A. plicata occurring across the Central Highlands prior to the Illinoian glaciation, with one largely Eastern Highlands clade and one clade homogeneously distributed throughout the Central Highlands. This indicates that A. plicata, like the freshwater fishes considered in Wiley and Mayden (1985) and Mayden (1988), persisted for a long period of time in the Central Highlands without experiencing fundamental evolutionary and biogeographic alteration during the major environmental changes at the end of the Pleistocene. A limited amount of post-Illinoian dispersal did occur in A. plicata, but the overall pattern is of a long association with other taxa in the region, such as fish. That is, evidence exists for phylo-geographic association, although it would be desirable to have a greater range of intraspecific phylogeographic patterns from other taxa in the same region. As mentioned previously, unionid taxa share a parasitic relationship with freshwater fish, but the precise host of A. plicata is not known; indeed, the species probably has several hosts. Therefore, the phylogeographic association between population level geographic divergence within A. plicata and the interspecific geographic divergence replicated in several fish clades may pro vide evidence for strict coevolution or phylogeographic association. The similar patterns of geographic differentiation hold across different levels of the genealogical hierarchy, perhaps due to differences between unionids and fish in their propensity to speciate.
The patterns of coevolution and phylogeographic association and the evidence for biogeographic differentiation with an early Pleistocene signature in modern unionids and fish suggests a long period of maintained ecological interactions associated with evolutionary differentiation. These results provide at least some support for the hypotheses of Jackson (1992), Morris et al. (1995), and Jackson, Budd, and Pandolfi (1996) in the sense that they posited that communities made up of populations of different species can be stable over long periods of time, and in this study populations showed phylogeo-graphic association even when environments changed significantly. Additional data are of course necessary on the nature of these communities, and the types of interactions that prevailed. Association need not be equated with stability, but without long-term association as evidenced by phylogeographic studies, there could not have been community stability. Thus, studies that concentrate on members of the genealogical hierarchy can still make contributions to our understanding of paleoecological hypotheses.
However, the hypotheses of Jackson (1992), Morris et al. (1995), and Jackson, Budd, and Pandolfi (1996) do not receive unambiguous support from this study. Over the Quaternary, the unionid taxon showed evidence of intra-specific differentiation, whereas the fish clades discussed in Wiley and Mayden (1985) and Mayden (1988) speciated. Thus, members of the genealogical hierarchy, the entities that provided the participants in the ecological hierarchy, were not obdurately stable. If a crucial component of the hypothesis of coordinated stasis is complete stability of the members of the genealogical hierarchy that make up a fauna, then these results would have to be seen as a challenge to the hypothesis. Similarly, Lieberman (1994) and Lieberman and Kloc (1997) challenged the predictions that coordinated stasis made about members of the genealogical hierarchy. By contrast, the possibility of ecological association, and thus potentially community stability, even in the face of evolutionary change is indicated by the results of this study, although community stability has not yet been demonstrated, nor can it be demonstrated by this type of study alone. Further, Bambach and Bennington (1996) have suggested that it is unlikely that ecological communities can be stable over long periods of time.
The notion that life is divided up into two largely nonequivalent hierarchies implies that aspects of the coordinated stasis hypothesis can be valid for one but not both of these hierarchies. In addition, this hierarchical structure of nature implies that studies that rely on information from a single hierarchy, such as this one, can provide a means, though not the sole means, of testing a hypothesis that attempts to describe patterns within each of the two hierarchies. Thus, the value of a research program in evolutionary paleoecology, though complicated by the hierarchical structure of nature, seems clear.
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