The so-called "species problem" can be characterized, broadly speaking, as the task of providing a functional species-concept that picks out the "right" kind of biological entities. After decades of extensive debate -- and centuries of taxonomic practice -- no general consensus has been achieved on the individuation and definition of biological units, species arguably being the most prominent and widely discussed. If anything, there is now more disagreement than ever!
The reason for all this increasing controversy can hardly be grounded in lack of information in the form of empirical data. While still far from a comprehensive understanding of evolutionary entities, we now know more about how biological lineages diversify and diverge than we ever did. The problem is rather that much contemporary research points to various kinds of discordance -- the main topic of this conference -- including non-overlapping histories, fragmentation of ecological habitats, and dispersal, whether or not it is human mediated.
In this essay, I shall not say much regarding the source of all this discordance. Rather, I want to focus on a popular strategy to accommodate this plethora of data and make sense of it in the context of biological species and related discussions. This strategy involves presupposing some form of species pluralism. More specifically, my present goal is to examine what species pluralism amounts to and to assess whether it really helps the study of species in the "age of discordance."
My starting point is the observation that while pluralism is at the center of much theoretical discussion, both among philosophers and biologists, the concept often figures in different contexts and with different meanings. Some caution is thus required in presenting the idea, which evolves along two related but distinct strands (Boyd 1999, Hey 2006). On the one hand, species pluralism is sometimes identified with the thesis that different kinds of species can be found in nature, and, consequently, different species concepts are required to account for this diversity. Let's call this thesis "heterogeneity," since it postulates that species-taxa are heterogeneous, in the sense that a variety of phenomena may underlie species entities (Ereshefsky 1999). Biologists often plainly identify this criterion of heterogeneity with pluralism. This, however, is problematic, because it obliterates the distinction between heterogeneity (the mere denial of what Hull (1997, 1999) calls "species universalism" and a much stronger stance that is, in principle, independent of whether prokaryotes and eukaryotes can be clustered in the same kinds of species. This more radical and more controversial thesis involves a commitment to the claim that the assignment of species-level taxa is always relative to a particular scientific theory, aim, or classificatory purpose (Kitcher 1984, Dupre 1981, Dupre 1993, Ereshefsky 1992, Boyd 1999). This stronger pluralism -- which can be characterized as the view that assignments of species-level taxa depend on the organisms and processes being studied and the explanatory target at hand -- is the focus of my present discussion.
After introducing the concept, I clarify some important assumptions concerning species pluralism. First, it is important to keep in mind that, from a general methodological perspective, pluralism is more radical than heterogeneity. Whereas heterogeneity is, in principle, consistent with the existence of a single correct way of clustering a particular group of organisms into species, pluralism overtly rejects this idea because there is no single correct standard for uniting organisms or populations as members of a species. Second, contrary to a common perspective, pluralism per se is not a solution to the species problem; it is a general framework within which to work to (hopefully) find a solution to the debate. As such, we should not expect the assumption of pluralism, in and of itself, to "settle anything." Third, pluralism does not necessarily imply that "anything goes," nor does it require one to completely "forgo objectivity." There might be independent reasons to prefer some theoretical goals over others, leading one to adopt the concepts posited in such frameworks (Kitcher 1989). Finally, a widespread idea is that pluralism can be reduced to a form of antirealism (Hull 1999). This, however, is a mistake as it sets up the wrong kind of opposition. Realism is an extremely weak position that bears very little ontological commitment: virtually anything can be dubbed as real, including abstract, fictional, and theory-dependent entities. As a result, the claim that species are "real" is perfectly compatible with pluralism. The claim that should be contrasted with pluralism is the thesis that species are "concrete entities in nature" (Nathan and Cracraft, forthcoming).
With these important distinctions in mind, in the final part of the essay, I discuss whether pluralism provides the appropriate framework for addressing contemporary debates on the nature of species. Specifically, I argue that, as ecumenical as it may seem, pluralism comes at a high ontological cost. To be sure, the claim is not that pluralism is false. The point is that it is a more substantial metaphysical assumption than is typically recognized. Consequently, pluralism is hard to reconcile with much current work -- both theoretical and empirical --currently pursued in systematics. I conclude by suggesting that the working assumption of pluralism be replaced by a form of metaphysical agnosticism, at least until the current "age of discordance" has gone by.