Meeting 6: The Species Problem & the Metaphysics of Species (2/18)

Readings:

• S&D Chapter 9: “Species” **
• Hull, “A Matter of Individuality” [CIEB §18] **
• Kitcher, “Species” [PDF]
• Crane, “On the Metaphysics of Species” [PDF] *
• Slater, “On the Contentious Metaphysics of Species” [PDF]
• Slater, “Against the Individuality of Species” [PDF] **

There is a sense in which evolution both simplifies and complicates biological taxonomy. For some, it simplifies it by making sense of the interrelatedness of different species, the hierarchical, nested structure of biological similarity, and so forth. On this conception, species should be thought of as "hunks of the genealogical nexus" or "segments of the tree of life". Of course, this broadly historical perspective does not tell us much about how to segment the tree of life. On the other hand, it sometimes seems as though similarity (in some sense) is of primary importance for classification. Common historical origin might help explain this, but we needn't regard history as of primary importance for taxonomy. So what are species?

In an oft-quoted letter to Joseph Hooker (24 December 1856), Darwin took the difficulty of reaching agreement about a correct conception of species as well-nigh insuperable:

It is really laughable to see what different ideas are prominent in various naturalists’ minds when they speak of species; in some, resemblance is everything and descent of little weight — in some, resemblance seems to go for nothing, and Creation the reigning idea — In some, descent is the key, — in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the indefinable.

I think he was too quick to give up, but it’s notable that the same sort of disagreements persist: on one recent count, there were over twenty different species concepts being actively pursued. Is just one of these the correct one? Or perhaps we haven't yet come up with the correct species concept. . . . The issue of whether there is just one privileged conception of species (or biological taxa generally) is often referred to as the debate between "monism" and "pluralism" about species. Kitcher contends that we should be pluralists about species — there is no uniquely correct way of dividing up biological reality. Yet, this doesn't mean that species are not real or that there are objective divisions between different biological groups. So he claims. We'll want to talk about this.

The question What are species? has a different sense. Rather than asking after the conditions under which some organisms belong to a species taxon, we might be interested to know into what ontological category these taxa belong. What is an "ontological category"? This in itself is a somewhat fraught question, but the basic idea is that it is a fundamental category of being: for example, objects, processes, events, properties, sets, numbers, propositions, and so forth might all be considered as such categories. If I ask you What are tables? You might respond by trying to tell me the specific conditions under which some pieces of wood compose a table. Or you might start much more generally by telling me that tables are a certain sort of object, thus distinguishing them from properties or events. Tables and wars are in different ontological categories: the former are objects, the latter are events.

Now then: What sort of thing are species? To what ontological category do they belong? We've already seen some concern about treating species as natural kinds (to some, as certain kinds of sets or classes). Perhaps we should consider an alternative. In this spirit, Michael Ghiselin proposed a "radical solution to the species problem":

It would appear that the philosophy of taxonomy is about to undergo a major upheaval. Symptomatic is its Gordian knot, the species problem. Some years ago (Ghiselin, 1966a) I attempted to cut it with the sword, casually remarking that, in the logical sense, species are individuals, not classes. (1976, 536)

The idea seems to be that if we treat species as classes (that is, sets or collections of organisms), we force ourselves into Mayr’s dreaded typological thinking. For the reality of the class would only be secured by some defining essence shared by all (and only) its members. But typology in this sense appears implausible. Perhaps some things have essences (gold, water, electrons, &c.), but the essentialism appears ill-suited to the “blooming, buzzing confusion” of the biological world.

The case was taken up by David Hull, an influential philosopher of biology (now emeritus at Northwestern), who offered many reason for treating species as individuals (though it is not always clear what precisely he means by this term). You might think as you’re reading whether his reasons for defending the thesis shed more light on the details of his thesis. What are his reasons? Are the good?

This is where Kitcher’s paper on “Species” picks up: he suggests that Hull fails to offer us good reason for accepting the species-as-individuals (SAI) thesis. Kitcher defends the view that species are certain kinds of sets, arguing that this thesis becomes a straw-man in Hull’s essay. But Kitcher takes the species problem seriously. Perhaps we go wrong to assume that there is just one privileged species concept. But does a plurality of species concepts imply that species are not real? He thinks not. We can be pluralistic realists about species. I suspect that we may want to put this case off for another week or so.

Despite Kitcher's criticisms of Ghiselin and Hull, SAI is more popular than ever. My paper "On the Contentious Metaphysics of Species" critiques some recent arguments for SAI in the context of the early arguments. Of course, even if our criticisms hit their mark, SAI might still be true (don't confuse the truth of a thesis with its pedigree). This is roughly the sense I’ve gotten from many philosophers of biology over the years: "yes, Hull makes an inconclusive case for SAI — but it’s still the best metaphysics of species." Even Kitcher later contended that the issue doesn't bear on anything: one can equally well construe species as individuals or sets. I disagree. My paper "Against the Individuality of Species" makes the case that SAI incurs certain expenses that biologists should not front! As this case is primarily metaphysical (trading in particular on particular theses about ontological vagueness), those non-philosophers of you might find things a bit opaque (hence its being optional). But if you’re interested in getting some further background on vagueness, I can recommend Roy Sorensen’s SEP entry and Achille Varzi’s (2001) article “Vagueness, Logic, and Ontology” (particularly the section on ontological versus semantic vagueness).

Study Questions

• Explain how the SAI thesis might fit into the debate about typology versus population thinking.
• How does Kitcher’s assertion that “there is no inconsistency in claiming that species are sets and denying that the members of these sets share a common property” (310) answer one of Hull’s arguments for SAI?
• Describe and evaluate Kitcher's arguments for pluralism.
• Explain some objections to the biological species concept.
• Do you think that pluralism is compatible with realism about species?
• Describe Kitcher's lizard example and what it is intended to show.
• What is the difference between historical and structural explanations? How does this distinction bear on the case for pluralism?
• Does pluralism involve the claim that any species concept is acceptable? Why or why not?
• Why is problematic for SAI-ists to adduce the necessary spatiotemporal connectedness of species as evidence for SAI?
• What is Crane's argument for SAI and what difficulties does it face?