New world nine-primaried oscine relationships: Constructing a mitochondrial DNA framework

ABSTRACT.-Historically, a paucity of comparative morphological characters has led to much debate regarding relationships within and among the major lineages of New World nine-primaried oscines. More recently, DNA-DNA hybridization studies have provided novel and testable hypotheses of relationships, although no consensus has been reached. For 40 songbird taxa, we obtained 1,929 base pairs (bp) of DNA sequence from the mitochondrial cytochrome-b (894 bp) and NADH dehydrogenase subunit 2 (1,035 bp) genes. Phylogenetic analyses confirm the monophyly of this assemblage as traditionally defined. The lineages delineated historically on morphological grounds are retained; finches (Fringillinae) are sister to a well-supported Glade (Emberizinae) containing blackbirds (Icterini), sparrows (Emberizini), wood-warblers (Parulini), tanagers (Thraupini), and cardinal-grosbeaks (Cardinalini). However, each tribe individually is either paraphyletic or polyphyletic with respect to most recent songbird classifications. Our results suggest that Euphonia is not a tanager but perhaps represents a derived form of cardueline finch. Piranga, traditionally considered a typical tanager, is a cardinaline in all of our analyses. Calcarius falls outside the sparrow lineage in all of our analyses, but its true affinities remain unclear. Elements of four different AOU families are represented in our Glade Thraupini. The inclusion of several "tanagerfinches" (Haplospiza, Diglossa, Tiaris, Volatinia, Sporophila) and a nectarivore (Coereba) in this Glade is consistent with findings from other molecular phylogenies in suggesting that convergence in feeding specializations among some lineages has confounded traditional morphological classifications. We obtained a novel arrangement of relationships among tribes in our "best" topology; Cardinalini is sister to the rest of the Emberizinae assemblage (as defined by Sibley and Ahlquist [1990]), and Thraupini is sister to a Glade containing Icterini, Emberizini, and Parulini. Despite nearly 2,000 by of sequence for each taxon, and a high degree of stability across most weighting schemes and analytical methods, most nodes lack strong bootstrap support. The ND2 gene provided higher resolution than did cytochrome b, but combining genes provided the most highly supported and resolved topology. We consider the phylogeny a working hypothesis to be used as a guide for further studies within the nine-primaried oscine assemblage. Received 6 November 1998, accepted 4 August 1999.

THE NEW WORLD nine-primaried oscines, with about 1,000 species, represent roughly 10% of all living species of birds. This group is "by almost unanimous agreement, an assemblage of families of close relationship and common ancestry" (Tordoff 1954a:274). Although this statement still rings true, the systematic relationships within and between the major groups (i.e. families of AOU [1998], tribes of Sibley and Monroe [1990]; unless otherwise specified we use the nomenclature of the latter throughout this paper) of nine-primaried oscines continue to be among the most problematic systematic questions within an avian order.

A long history of attempts to clarify relationships among members of this assemblage has included comparative studies of external morphology (Ridgway 1901, 1902), jaw musculature (Beecher 1953), pelvic musculature and serology (Stallcup 1954), cranial and palatal characters (Tordoff 1954b), and appendicular myology (Raikow 1978). Such works have led to a general agreement regarding the taxonomic boundaries of this passerine assemblage, but they have yielded only a handful of useful characters with which to define the relationships among the component groups (Mayr and Amadon 1951, Sibley 1970, Feduccia 1996). Of these characters, many are often inconsistent with one another (Bledsoe 1988). This result has led to a series of linear classifications (see Sibley and Ahlquist [1990] for a comprehensive review) that differ only slightly from one another. The traditional view of nine-primaried oscine evolution held that "the major groups are products of adaptive radiations into discrete adaptive zones defined mainly in terms of feeding specializations" (Raikow 1978:34). Early classifications based on this perspective had in common the following six "core" groups: the cardueline finches (Sushkin 1925), wood-warblers, blackbirds, sparrows, cardinal-grosbeaks, and tanagers. However, overlapping bill morphologies between the latter three have led to relatively arbitrary boundary distinctions (e.g. Sclater 1886, Ridgway 1902, Tordoff 1954b). As a consequence, genera with "finch type" bills have been shuffled about taxonomically as have a few other genera (e.g. Cyanerpes, Spiza, Icteria) that failed to conform to one of the general adaptive (trophic) types. Because of the extreme morphological uniformity (i.e. paucity of phylogenetically informative characters), earlier oscine classifications lacked insight into among-group relationships, and with the exception of Raikow's (1978) cladistic analysis, few offer much in the way of testable phylogenetic hypotheses. Classifications based on a new source of data, DNA-DNA hybridization comparisons (Bledsoe 1988, Sibley and Ahlquist 1990), have provided a fresh perspective that has challenged traditional views by demonstrating that the adaptive radiation of New World nine-primaried oscines is characterized by convergence at many levels. These molecular phylogenies have provided novel insights concerning the phylogenetic placement of difficult (anomalous) genera and also furnished testable hypotheses of relationships among groups. More recently, the direct sequencing of mitochondrial DNA (mtDNA) has provided an abundance of characters for the construction of phylogenetic hypotheses (e.g. see Mindell 1997).