MITOCHONDRIAL-DNA HOMOGENEITY IN THE PHENOTYPICALLY DIVERSE REDPOLL FINCH COMPLEX (AVES, CARDUELINAE, CARDUELIS-FLAMMEA-HORNEMANNI)

Breeding redpoll finches (Aves: Carduelinae) show extensive plumage an d size variability and, in many cases, a plumage polymorphism that is not related to age or sex. This has been ascribed to extreme phenotypi c variation within a single taxon or to moderate variability within di stinct taxa coupled with hybridization. The predominant view favors th e recognition of two largely sympatric species: Carduelis flammea, com prised of four well marked subspecies-flammea, cabaret, islandica, and rostrata; and C. hornemanni, comprised of two subspecies-hornemmani a nd exilipes. We studied representative samples of these putative subsp ecies (except islandica) for variation in mitochondrial DNA (mtDNA). U sing 20 informative restriction enzymes that recognized 124 sites (642 base pairs [bp] of sequence or approximate to 3.7% of the molecule), we identified 17 RFLP haplotypes in the 31 individuals surveyed. The h aplotypes formed a simple phylogenetic network with most clones diverg ing by a single site difference from a common haplotype found in almos t half of the individuals. Within populations and taxa, levels of mtDN A diversity were similar to those observed in other avian species. The pattern of mtDNA divergence among populations was statistically unrel ated to their geographic or traditional taxonomic relationships, and t he estimated distance between the two traditionally recognized species was very small relative to those typically observed among avian siste r species. Redpolls are highly vagile and have very large effective po pulation sizes that may be relatively stable over the long term. Under these conditions, an absence of mtDNA differentiation between reprodu ctively isolated, but recently diverged, taxa is not necessarily surpr ising. Hybridization between imperfectly isolated taxa might also have impeded mtDNA differentiation or contributed to its loss. Other expla nations for the conflicting patterns of genetic homogeneity and phenot ypic polymorphism in redpolls include models of genetic polymorphism a t a major locus having broad pleiotropic or epistatic effects, and var ious ecophenotypic models (e.g., niche polymorphism, threshold respons e mechanisms, phenotypic modulation, developmental conversion). Distin guishing among these possibilities will require detailed knowledge of the genetic or ecophenotypic basis of phenotypic variation in redpolls .