DIAGNOSABILITY VERSUS MEAN DIFFERENCES OF SAGE SPARROW SUBSPECIES

• Published in: The Auk 2002-01, Vol.119 (1), p.26-35
• Main Authors: Patten, Michael A, Unitt, Philip
• Format: Article
• Language: English
• Subjects: Animal wings; Aves; Biological and medical sciences; Biological taxonomies; Birds; Fundamental and applied biological sciences. Psychology; Geographical variation; Ornithology; Population distributions; Population mean; Sparrows; Speciation; Vertebrates; Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution; Zoology
• ISSN: 0004-8038; 1938-4254; 2732-4613
• DOI: 10.1642/0004-8038(2002)119[0026:DVMDOS]2.0.CO;2

A subspecies is a collection of populations within a biological species that are diagnosably distinct from other such collections of populations. That infraspecific designation has motivated a litany of spirited debates over the past half-century, from impassioned pleas for its retention to heated outcries for its abolition. We believe that the vast majority of attacks on the subspecies concept have resulted from displeasure with its improper application, not from serious flaws in the concept itself. The recognition of diagnosable subspecies allows one to address many questions not easily answered otherwise, ranging from dispersal and migration to local selection and adaptation and biogeographic affinities, yet that goal was lost for many years. Many taxonomists in the late nineteenth century and first half of the twentieth century named subspecies on the basis of average differences between populations under study, a procedure at odds with identification of diagnosable populations. To resolve that dilemma, we make explicit the established 75% rule for subspecies recognition, including formalizing the rule and developing a simple statistic to test whether diagnosability is met. The equations can be adapted readily to any level of diagnosability. We apply the concept and the statistic to a revision of the subspecies of the Sage Sparrow (Amphispiza belli). Rather than the seven named subspecies or the five that are generally considered valid, we show that only three aggregates of populations are diagnosable, and thus only three subspecies should be recognized: (1) A. b. belli in chaparral and sage scrub of coastal California, northwestern Baja California, and San Clemente Island; (2) A. b. cinerea in desert scrub of west-central Baja California; and (3) A. b. nevadensis in sagebrush and saltbush of the Great Basin and interior California. Consistent application of the 75% rule will result in fewer trinomials and a more biologically meaningful and taxonomically useful subspecies concept.