Darwin and the Nature of Species

4. Common Descent and Natural Classification

65 Chapter 4 Common Descent and Natural Classification It might be thought that Darwin’s emphasis on genealogy for classification , on common descent, completely undermines the analysis in the previous chapter. For example, in the Origin Darwin repeatedly emphasized that the “Natural System” of classification, so debated and desirously sought after by his fellow naturalists, is in fact a genealogical one. As he put it, “all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike” (420). Does not Darwin’s emphasis on genealogy entail that he thought of taxa, both species and higher, as vertical entities, as branches in the Tree of Life? What only aids this supposition is that many biologists today, irregardless of Darwin, think that if species are to be evolutionary entities, real entities in nature rather than timeless abstract entities, then they must be thought of as vertical entities, indeed as individuals. Ghiselin (1974) and Hull (1978), for example, have routinely argued that everything real is either a class or an individual, classes are timeless entities, species evolve, therefore species must be individuals. (I have inveighed against the class/individual distinction as a false dichotomy in Stamos 1998, 456–459, 2003, 214–215.) Following Ghiselin and Hull, a number of authors have claimed that if species are historical entities then they must be individuals. For example, Eldredge (1985) claims that “punctuated equilibria puts the icing on the cake in the argument that species are real historical entities, comparable in a formal manner to individual organisms” (122). Moreover, one can find the related claim that the importance of history reconstruction requires a vertical species concept. Frost and Wright (1988), for example, claim that “only individuals (⫽ entities) exist independently of definition and have histories. Thus, only individuals should be included in biological taxonomies” (201). Returning to Darwin, in the Origin he seems to be saying the same thing when he says “If we extend the use of this element of descent,—the only certainly known cause of similarity in organic beings,—we shall understand what is meant by the natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes” (456). To this we may add what he says near the very end, viz. “when we regard every production of nature as one which has had a history . . . how far more interesting , I speak from experience, will the study of natural history become!” (485–486). Moreover, this was a view that Darwin long held. For example, in a letter to George Waterhouse (July 26, 1843) he wrote “According to my opinion . . . classification consists in grouping beings according to their actual relationship, i.e. their consanguinity, or descent from common stocks . . . . it is clear that neither number of species—nor grade of organization ought to come in, as an element” (Burkhardt and Smith 1986, 375–376). In Descent (1871 I) Darwin states, with regard to the natural system, that “This system, it is now generally admitted, must be, as far as possible, genealogical in arrangement. . . . The amount of difference between the several groups—that is, the amount of modification which each group has undergone—will be expressed by such terms as genera, families , orders, and classes” (188). By focusing on common descent for natural classification, it might seem that Darwin went so far as modern taxonomists of the now dominant cladistic variety, who insist that taxa (both species and higher) must be monophyletic. This latter term generally has two meanings. The more general sense is that of single origin, meaning that a taxon cannot have multiple origins (as in repeated polyploidy). In phylogenetic systematics, polyphyletic groups are not considered natural groups. The other, more strict sense of monophyly (which is not always applied to species but always to higher taxa), is that of a taxon consisting of the common ancestral organisms and all descendant organisms. If some of the descendant organisms 66 DARWIN AND THE NATURE OF SPECIES are not included in the group, then the group is called paraphyletic, and most (but not all) of those who subscribe to phylogenetic systematics eschew paraphyletic taxa since it brings in the criterion of similarity. When we take a closer...