Sorenson et al., 2003), and the frequency of cases of sympatric speciation and divergence due to hybrid incompatibilities or reinforcement is an open question (Coyne and Orr, 2004). Now, a draft of the complete chicken genome and >2.8 million chicken SNPs have been determined roughly 60 years after Mayr’s landmark book (International Chicken Genome Sequencing Consortium, 2004; International Chicken Polymorphism Map Consortium, 2004). This treasure-trove of genes and genetic markers will no doubt spur rapid advances in both the geography and genetics of speciation in birds. This article reviews recent studies of extrinsic and intrinsic aspects of speciation in birds, focusing specifically on systematic and mechanistic issues that challenge the universality of the allopatric speciation paradigm.
Genetic data are serving an ever-increasingly important role in the delimitation of species, yet considerable controversy remains over which criteria to apply to this age-old problem (Avise and Ball, 1990; Coyne and Orr, 2004; Cracraft, 1983; Hey, 2001; Sites and Marshall, 2003; Wiens and Penkrot, 2002). Indeed, determining which of myriad species delimitation methods and species definitions is most appropriate for one’s focal taxa remains one of the paramount challenges in systematics, with important consequences for evolutionary biologists as well as for conservation biologists (Crandall et al., 2000; Moritz, 2002; Sites and Crandall, 1997). The fact that many avian sister taxa occur in allopatry—particularly in Gondwanan continents such as South America and Australia (Bates et al., 1998; Cracraft, 1991)—makes interpretation of biogeographic histories more straightforward but can also make attempts at species delimitation particularly challenging. It has long been recognized that the biological species concept (BSC), with its emphasis on reproductive isolation, is inapplicable in many allopatric situations because there is no opportunity to test for reproductive isolation, rendering the concept arbitrary (Zink and McKitrick, 1995). Species concepts emphasizing genetic clustering of forms can be equally arbitrary (reviewed in Irwin and Price, 1999). Diagnosibility—the ability to delimit and identify distinguishing character states for a given collection of individuals or taxa, usually but not always in a phylogenetic context—has been proffered as a general consideration when delimiting species (Cracraft, 1983). Although diagnosibility is sometimes construed as being equivalent to “fixed” character or genetic differences between taxa, alternate fixations are not a requirement for diagnosibility. The rise in sophisticated statistical genetic algorithms and large-scale multilocus analyses of variation in birds and other taxa con-