Rigor and Species Concepts

Auk, The, Jul 2006 by Zink, Robert M

Rigor and Species Concepts.-Within a review of work on avian vocalizations by affiliates of his own institution, Remsen (2005) turned his attention to the debate over species concepts. This topic is very controversial and has strong advocates on both sides. Thus, readers should take literally Remsen's warning that "this is not the place for another review of species concepts...." Readers will recognize that his "review" does not present a balanced overview of the principal issues in the debate, owing to Remsen's allegiance to the biological species concept (BSC). It is also important to address controversial issues constructively. Here, I respond to his concerns and criticisms and illustrate my opinion that the BSC continues to be a poor choice for organizing our knowledge of biodiversity.

The debate in a nutshell.-Remsen believes that if two taxa, diagnosed by some phenotypic or genotypic data, can interbreed to some (unspecified) degree, they must be classified as the same species. This is the crux of the BSC. Under the phylogenetic species concept (PSC), diagnosably distinct taxa with independent evolutionary histories are considered species regardless of whether they are reproductively isolated from other phylogenetic species. Adoption of one or the other concept leads to major differences in our understanding of avian species diversity.

Importance of interbreeding.-Remsen perpetuates the notion that advocates of species concepts other than the so-called "biological" species concept (Mayr 1963) consider the phenomenon of reproductive isolation unimportant. In particular, he remarks that "proponents of the PSC [phylogenetic species concept] explicitly denounce the use of interbreeding in classification" (Remsen 2005:406). This does a disservice to the papers he cites, because readers unfamiliar with them will assume incorrectly that he has understood and reported their content accurately and not out of context. Advocates of the PSC have always acknowledged that interbreeding occurs among individuals of the same species, but its existence (actual or presumed) does not justify uniting taxa that are otherwise diagnosable. There is good reason for this, because the ability to interbreed is an ancestral condition (Rosen 1979).

In modern systematics, one does not unite taxa based on their shared possession of an ancestral condition. Apparently because he does not like the outcome, Remsen has decided to ignore this part of phylogenetic systematics and use his own rules. Advocating that we discard this fundamental rule will ensure that nonsister taxa are united by their joint possession of ancestral (sympleisomorphic) conditions; this is not accepted at any level of taxonomic organization. For instance, classifying the Baltimore Oriole (Icterus galbula) and Bullock's Oriole (I. bullockii) as conspecific because they hybridize creates a nonsensical taxon, as they are not sister taxa (Freeman and Zink 1995). Other than Remsen's incorrect assertion that the recognition of such paraphyletic taxa would be an infrequent occurrence under the BSC, Remsen has no solution to this problem created by, and exclusive to, the BSC. Supporters of the BSC have not answered this question since Rosen (1979) first pointed it out: if non-sister taxa hybridize "freely," are they the same biological species, to the exclusion of other, more closely related taxa that are reproductively isolated? Under the BSC version advocated by Remsen, the answer is yes, which places more importance on the potential future outcome of current interbreeding rather than on, perhaps, tens of thousands of years of genetic isolation. To Remsen, this is preferable to a classification that accurately reflects evolutionary history. I fail to see rigor in Remsen's method.

Allopatric populations.-It has been noted numerous times that the BSC provides little basis for judging the species status of allopatric populations. Advocates of the BSC such as Remsen (2005:407) acknowledge this problem: "As for the well-known problems of the BSC in classifying allopatric populations...." This is an important issue, because it is likely that most avian diversity arose as a result of allopatric differentiation. Using a species concept that is not objective in allopatry therefore makes little sense. Remsen suggests that a solution to this problem is the protocol of Isler et al. (1998, 2005), which identifies a set of vocal attributes that characterize antbird taxa known to be reproductively isolated, and then uses them as a benchmark for allopatric forms. Remsen (2005) believes that this approach will codify the long-standing notion that one can make "rigorous" guesses as to whether allopatric populations are reproductively isolated. This may work for a few groups after intensive study (indeed, the Isler et al. [1998] study took many years to complete), but it offers no more than approximate guidelines, not an actual set of rules, and it will be group-specific. Also, suboscines are a special case, because their vocalizations are innate; Remsen's guidelines may not be applicable to the more than 4,000 species of oscine passerines, let alone to other orders for which the genetic-innate contribution to songs is unknown. Application of Remsen's method to the Goldenwinged Warbler (Vermivora chrysoptera) and Bluewinged Warbler (V. pinus) may be instructive. These two species use the same vocalizations in allopatry as they do in sympatry, where they interbreed extensively. Thus, there is no reason to maintain them as separate species. However, they are morphologically distinct, and the American Ornithologists' Union (AOU) check-list committee, which like Remsen strictly follows the BSC, classifies these phylogenetic species as separate biological species. Here, apparently, tradition takes precedence over rigor.