DIAGNOSABILITY OF SUBSPECIES: LESSONS FROM SAGE SPARROWS (AMPHISPIZA BELLI) FOR ANALYSIS OF GEOGRAPHIC VARIATION IN BIRDS

Auk, The, Jan 2006 by Cicero, Carla, Johnson, Ned K

THE VALIDITY AND utility of subspecies is an enduring subject of controversy in systematic ornithology. In a set of commentaries in The Auk more than two decades ago, numerous authors contributed personal views on avian subspecies and reaffirmed the validity of the concept despite its frequent misapplication (Barrowclough 1982, Gill 1982, Johnson 1982, Lanyon 1982, Mayr 1982, Monroe 1982, O'Neill 1982, Parkes 1982, Phillips 1982, Storer 1982, Zusi 1982). More recently, the subspecies rank was reviewed in light of molecular data (Zink 2004), with the conclusion that named subspecies commonly mislead taxonomy, evolutionary studies, and conservation policy. Because morphology and molecules may show discordant patterns of geographic variation (e.g. Zink 1996, Fry and Zink 1998), and because subspecies are traditionally defined on the basis of morphological criteria, rigorous analysis of morphology is crucial for proper classification at the subspecies level.

Patten and Unitt (2002) reviewed the debate and contended that taxonomists too often have diagnosed avian morphological subspecies on the basis of calculated mean differences among populations rather than an objectively defined level of diagnosability. Although admitting that "the lower boundary for defining a valid diagnosable subspecies is arbitrary" (Patten and Unitt 2002:28), they proposed that the level of diagnosability should be defined formally for the trait of interest so that 75% of its distribution in one set of populations falls outside of 99% of the distribution of the other set of populations being compared (the "75% rule"; Amadon 1949). Patten and Unitt (2002) used museum specimens of subspecies of Sage Sparrow (Amphispiza belli) to illustrate their thesis and claimed that A. b. canescens Grinnell, 1905-a name long applied to breeding populations in the San Joaquin Valley and Mojave Desert of California and the Grapevine Mountains of Nevada -is not diagnosable from A. b. nevadensis by the 75% rule despite significant differences in size (mainly wing length), as demonstrated in their study and others (Grinnell 1905, Johnson and Marten 1992). Hence, they synonymized A. b. canescens under A. b. nevadensis.

Overall, we agree with Patten and Unitt (2002) regarding the importance of diagnosability, and we recommend their review to systematists and others wishing to place morphological subspecies on a more objective footing than has often been the practice. However, because their results for A. b. canescens and A. b. nevadensis are at such variance with morphological differences reported by Johnson and Marten (1992) for specimens in the Museum of Vertebrate Zoology (MVZ, University of California, Berkeley), as well as with data for additional males and females from this collection, we suspected that their analyses and findings masked real patterns of geographic variation. In particular, two important issues stood out: (1) measurements were lumped by subspecies across the geographic range of specimens examined (presumably classified according to existing identifications on specimen labels), and (2) specimens were noted to be from the "breeding range" but apparently included times of the year when Sage Sparrows are not breeding (e.g. reference to "breeding males of A. b. canescens" from 28 July in southern California, or to "A. b. nevadensis" from 9 September in eastern California [Patten and Unitt 2002:32]; both of these dates are well outside the known breading season for these subspecies [Johnson and Marten 1992, Martin and Carlson 1998]).

We reiterate a long-standing truth that "unless specimens are clearly from a known breeding population they are irrelevant for analyses of geographic variation" (Zink and Dittmann 1992:765). Failure to restrict analyses to such individuals obscures potential variation within subspecies, such as clinal variation from northern to southern populations of the wide-ranging A. b. nevadensis. Furthermore, the null hypothesis in such studies should be that the species is geographically invariant (Johnson 1980, Cicero 1996). Accordingly, the existence of any variation must first be proved by examining geographic areas of breeding birds without regard to named subspecies, and then evaluated in light of the distributional limits of those taxa. This is the only approach that allows investigators to exclude potentially contaminating foreign specimens from local gene pools or denies in which variation is being assessed. Finally, a priori reliance on specimen identifications from museum labels to establish limits of trait variation for a particular subspecies is circular.

To clarify the findings of Patten and Unitt (2002), we requested copies of their original data, and they graciously complied. We reciprocated by sending copies of our own data sheets for measurements of Sage Sparrows (Johnson and Marten 1992, C. Cicero and N. K. Johnson unpubl. data). In addition to photocopies from Patten's notebook with data on museum specimens examined, they sent a spreadsheet that included "every specimen.. .used in [their] analysis" (M. A. Patten pers. comm.) with the exception of four individuals (he was unable to determine which four were missing). Patten also provided new means and standard deviations for wing chord from these data, which "do not differ materially from those in [the] published analysis" (M. A. Patten pers. comm.). We corroborated that statement by recalculating means and standard deviations for the data provided in the spreadsheet and then comparing them to the published data (table 2 in Patten and Unitt 2002); the only differences were a mean wing chord of 70.7 ± 2.81 (n = 43) versus 70.9 ± 2.88 (n = 45) for male A. b. canescens, and 66.9 ± 2.41 (n = 40) versus 67.2 ± 2.77 (n = 42) for female A. b. canescens.