Based on distributional, ecologic, morphologic, and vocal data, Klicka et al. (1999) argued in favor of species status for the form taverneri, long regarded as a subspecies of the Brewer's Sparrow (Spizella breweri). For several reasons we disagree with their conclusion: lack of evidence for the reproductive isolation of taverneri from breweri, loss of information on the close relationship and allopatric distribution of the two taxa that would accompany their elevation to species, and violation of the principle of taxonomic balance. Until the demonstration of vocal or display differences relevant to pair formation and maintenance, taverneri and breweri are best regarded as reproductively compatible subspecies.
In 1925, in the days when most new avian taxa were described as binomina, Spizella taverneri was described by Swarth and Brooks as a new species. Grinnell et al. (1930) ranked it as a subspecies of Spizella breweri and this rank was accepted by all subsequent AOU checklists. Recently Klicka et al. (1999) raised the question again and decided, even though somewhat hesitatingly, that the taxon deserved full species rank. Their analysis of the differences between S. breweri and S. taverneri is exemplary in the amount of detail and information provided, so that even a nonspecialist is enabled to evaluate the situation.
S. taverneri is apparently completely geographically isolated from breweri, and all efforts to find an area of interbreeding or overlap have so far failed. The closest approach of the two taxa is about 150 km. The rank of taverneri, according to recognition criteria for species and subspecies under the biological species concept, therefore, cannot be determined by direct observation, but must be inferred, making use of the traditional criteria of such an inference (Mayr and Ashlock 1991:100–105). S. taverneri clearly is an incipient species, as are all geographically isolated populations, and we must infer whether or not it has already reached species level. Several considerations help us in making our decision.
Evidence for reproductive isolation
The analyses of distribution, ecology, and morphology provided by Klicka et al. (1999) usefully defined the allopatric distributions and size characters by which taverneri clearly differs from breweri. Unfortunately, these features have little or nothing to do with reproductive isolation, the only valid criterion by which to infer genetic independence and species status. Instead, information on pairing behavior is required as a basis for inference. Displays used in pair formation are apparently unknown in these sparrows. Thus we are left with possible differences in vocalizations as the chief potential reproductive isolating mechanism. Vocal characteristics in sparrows, as in other oscines, are apparently learned. Because such cultural transmission allows for considerable copying error, levels of variability are enhanced well beyond those seen in suboscines with innate, stereotyped voices (Kroodsma 1996).
Increased variability presents a formidable obstacle in any attempt to establish differences in songs between these closely related forms of sparrows. In our opinion the preliminary analysis of advertising song by Klicka et al. (1999) has not begun to describe even intra-individual variation, let alone population differences, and until such analyses are completed taverneri is best viewed as a subspecies, reproductively compatible with S. breweri.
Considering that a classification is an information storage system, we must ask, would it provide more information if taverneri were treated as a species or as a subspecies? When taverneri is treated as a species, all one learns is that it is different from breweri, but not even how different. Indeed, one might be misled into believing that taverneri is as different from breweri as are the other species of Spizella from each other. By contrast, when taverneri is treated as a subspecies one is at once provided with two pieces of important information. The first is that breweri is its nearest relative, a piece of information of considerable value in a genus with several other species. The other is that subspecies status of taverneri provides valuable geographical information by telling us that taverneri and breweri are allopatric. Both advantages are lost if taverneri is raised to full species rank.
The principle of balance
All entities (taxa) within a Linnaean category should be as equally different from each other as possible. For instance, one should not raise a genus to family rank when this family would be far less distinct than the other related families are from each other. When one compares the ranking criteria in different classes and phyla, one notices that this principle is often violated, but it is usually adhered to in the ranking within a class. The question then is, is taverneri as distinct from breweri as the other species of Spizella are from each other, and if not, does this justify treating it as a subspecies? In the base pairs of the mitochondrial DNA, taverneri and breweri differ on the average by only 0.13%. By contrast, two other species pairs of Spizella differ by 5.9% or 6.1%. Hence, the difference between taverneri and breweri for this character is more than an order of magnitude smaller than that between other species of Spizella. In a recent survey of subspecies differences in sexual vertebrates, Avise and Walker (1999) found that subspecies often differed by more than 2% and sometimes even more than 3% of their mitochondrial base pairs. Is the small difference in the base pairs of the mitochondrial DNA of taverneri and breweri necessarily proof of only subspecific difference? Not necessarily! For example, Johnson and Zink (1983) and Cicero and Johnson (1995) found that relatively minuscule genetic distances separate the Red-breasted Sapsucker (Sphyrapicus ruber) and Red-naped Sapsucker (Sphyrapicus nuchalis), two taxa best defined as species on the basis of assortative mating in sympatry and strikingly different plumage signals (Johnson and Johnson 1985). Similarly, divergent vocal behavior (Borror 1972, James 1981) distinguishes the Cassin's Vireo (Vireo cassinii) from the Blue-headed Vireo (Vireo solitarius), despite the relatively trivial genetic distances that separate them (Murray et al. 1994, Johnson 1995, Cicero and Johnson 1998). In both of these examples, the crucial question for deciding species status was whether the differences provided evidence for essential reproductive isolation. We propose that the same question should be asked in attempting to determine the systematic status of S. taverneri. Lack of data proving essential reproductive isolation, therefore, rather than slight genetic difference, is the paramount reason supporting the conclusion that taverneri should continue to be listed as a subspecies of breweri.
We thank Javier A. Rodríguez for the Spanish translation of the abstract.