Using Phylogenetic Studies to Test Paleoecological Hypotheses

One area in which phylogenetic studies have made an important contribution to the understanding of the evolution of ecological interactions is in the study of the coevolution of hosts and their parasites (Brooks and McLennan 1991). In these studies, the search for congruence between host and parasite phyloge-nies does not imply unwavering verisimilitude of ecological interactions but rather patterns of constant association and isolation with concomitant diversification in host and parasite. These patterns of association, isolation, and diversification ensure some ongoing ecological interaction between host and parasite organism, but do not specify its nature.

Studies of molecular phylogeography share an obvious kinship with coevo-lutionary studies. Separate analyses are conducted on different species with overlapping geographic ranges to look at how different populations are related to one another. Two intergrading results of such studies are possible. If populations in different species always show the same pattern of biogeographic differentiation, then these populations were continually associated, became isolated at roughly the same time, and underwent concomitant intraspecific differentiation (if a non-ad hoc approach to the analysis of biogeographic patterns is accepted). I will term this phylogeographic association. This pattern indicates the important role that earth history factors play in structuring evolution. By contrast, if different phylogenies show different patterns of intraspecific differentiation, then these populations were not continually associated and did not become isolated and undergo differentiation at the same time. Population 1 of species A might be associated with population 2 of species B at time x and with population 3 at time y. I will term this phylogeo-graphic nonassociation. This pattern indicates that earth history factors do not play an important role in structuring evolution. Let us assume further that we knew that in each of these cases, populations and organisms of these species were interacting ecologically. What would either of these patterns tell us about the nature of ecological interactions through time?

The conclusions depend on the way that scientists believe nature is structured. Some work in hierarchy theory as applied to paleobiology has divided life up into two hierarchies, the genealogical and the ecological (see Eldredge 1985, 1989, and references cited therein). These contain largely separate entities. For example, species and clades belong to the genealogical hierarchy, and ecosystems and the biosphere belong to the ecological hierarchy. However, in certain instances, entities can appear in both hierarchies. For example, organisms and populations both interact ecologically, as members of the ecological hierarchy, and replicate, as members of the genealogical hierarchy.

Considering this, a pattern of phylogeographic association implies coincidence between genealogical descent, and potentially, ecological interactions at the population level through time. A pattern of phylogeographic nonassocia-tion implies disjunction between genealogical descent and ecological interactions at the population level through time. However, if different populations of a single species tend to be ecologically commensurate, then even without genealogical coincidence between populations across geographic space, similar ecological interactions may be preserved through time. This would go against the notion that species are typically broken up into different populations that have their own distinctive adaptations and ecological preferences (Eldredge 1985, and references cited therein). Therefore, only if we view the species rather than the population as the entity that provides the significant context for ecological interactions can we say that incommensurate phylogeo-graphic patterns among populations of different associated species imply maintenance of ecological interactions through time. Of course, when comparing a phylogeny at one level of the genealogical hierarchy (for example, the population level) with phylogenies at a different hierarchical level (for example, the species level), difficulties can arise. If there is phylogeographic nonas-sociation, there would be no evidence for consistency of ecological interactions through time, regardless of the hierarchical level at which one views significant ecological interactions initiating. By contrast, phylogeographic association, even between clades of populations on the one hand and species on the other, would provide evidence for coincidence of ecological interactions through time (though not necessarily their similarity, of course). However, the groups would show differences in their propensity to speciate.

Thus, it is clear that phylogeographic studies of populations have the potential to reveal something about the constancy of ecological interactions through time. Without a pattern of phylogeographic association, ecological interactions cannot have been maintained through time. How can these phy-logenetic studies be extended to the analysis of paleoecological hypotheses such as coordinated stasis? Previously, Lieberman (1994) and Lieberman and Kloc (1997) conducted phylogenetic studies involving genealogical entities at the species level to test aspects of the hypothesis of coordinated stasis. In particular, the hypothesis of coordinated stasis as set out in Brett and Baird (1995) and Morris et al. (1995) predicted that the establishment of the different faunas defined in Brett and Baird (1995) should be a roughly singular event associated with a particular episode of biogeographic emigration following extinction. For one of the paradigm examples of coordinated stasis, the Middle Devonian Hamilton Group fauna, phylogenetic evidence indicated that the initiation of at least part of the Hamilton Group fauna could not be confined to a single event. Rather, different taxa that comprised the Hamilton Group fauna actually arrived from different regions at different times.

The hypothesis of coordinated stasis as discussed in Morris et al. (1995) also invoked the mechanism of ecological locking, the close coupling of ecological interactions through time, as a process that might preserve the stability of faunas recognized to prevail over long periods of time by Brett and Baird (1995) and Morris et al. (1995). It is clear that phylogeographic analyses of populations offer a partial test of this aspect of coordinated stasis, but the nature of the fossil record makes the phylogenetic analysis of populations of species extremely difficult or perhaps impossible. Thus, a study of extant taxa is required, with molecular methods offering a potential means of looking at evolutionary relationships at the population level. In the study presented herein, the search was for patterns of phylogeographic association, based on the view that the population level, rather than the species level, is the hierarchical level from which sig nificant ecological interactions are initiated. Failure to recover patterns of phy-logeographic association, although not a complete refutation of the coordinated stasis hypothesis, would be counter to the predictions of that model in the sense that ecological interactions were not maintained over long periods of time. Results from phylogenetic studies also have additional bearing on the hypotheses that are discussed further in the following sections.

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