firm that it is trivial to diagnose species, subspecies, or even populations, even in the presence of abundant shared polymorphisms; a recent study of native and introduced house finch populations in North America was able to readily diagnose populations that had been separated for 50–100 years (Wang et al., 2003), even in the absence of fixed genetic or morphological differences. A naïve evaluation of the genetic patterns in this study would have reasonably inferred the existence of three diagnosable “species” of native and introduced House Finches within the continental United States and Hawaii. A similar ease of diagnosibility using large-scale multilocus approaches is observed for geographic populations of humans (Tishkoff and Kidd, 2004). Thus, diagnosibility will become highly problematic as the resolving power of multilocus approaches increases; the specter of statistical significance without biological significance will be perennial.
The criterion of monophyly advocated by proponents of the phylogenetic species concept (PSC) also suffers from arbitrariness, particularly given the disproportionate emphasis on mtDNA. It is not surprising that a number of workers have advocated the use of mtDNA in species delimitation, given its rapid attainment of reciprocal monophyly (relative to nuclear genes) and frequent ability to diagnose populations (Moore, 1994; Moritz, 1994, 2002; Wiens and Penkrot, 2002). Reciprocal monophyly of mtDNA is attractive because it is an objective criterion that can be applied to any animal system. However, use of mtDNA alone to delimit species has been criticized, because the complete organismal history is not captured and because many other loci in the genome may not exhibit reciprocal monophyly (Hudson and Coyne, 2002; Sites and Crandall, 1997; Wiens and Penkrot, 2002).
Another gene-tree-based criterion for species delimitation calls for finding reciprocal monophyly among a large majority (≈95%) of sampled nuclear genes (Avise and Ball, 1990; Baum and Shaw, 1995). Because nuclear DNA (nDNA) achieves reciprocal monophyly much slower than mtDNA, this criterion is considered conservative (Hudson and Coyne, 2002; Sites and Crandall, 1997). However, to date, and certainly at the time this species concept was put forward, there have been no avian data sets consisting of multiple independent gene trees with which to test the utility of this approach. A recent study of speciation in three Australian grassfinches (Poephila) using 30 independent nuclear loci provides a test of this concept. One of the taxa (Poephila cincta) examined is phenotypically and geographically very distinct from the other two, and its species status has never been questioned (Schodde and Mason, 1999); the two allopatric western lineages of long-tailed finches (Poephila acuticauda/ hecki) are distinguished by fewer characters but are nonetheless diagnosable morphologically and have been designated phylogenetic species (Cracraft, 1986). Coalescent estimates of population divergence times sug-