Speciation in North American Chickadees: II. Geography of mtDNA Haplotypes in Poecile carolinensis

Abstract

The Carolina Chickadee (Poecile carolinensis) of southeastern North America comprises two geographically structured, monophyletic clusters of mitochondrial DNA (mtDNA) haplotypes (Gill et al. 1993). Here, we report the mtDNA identities of specimens of P carolinensis from new localities in southwestern Alabama west across southern Mississippi. The molecular identities of the new specimens define a clinal, not parapatric, transition from eastern haplotypes to western haplotypes. The results also document the first genetic cline structure between morphologically undifferentiated populations of a bird species. Methods.-Earlier specimens from a coarse sampling transect from Texas to Georgia (collected in 1990) revealed that the divergent R carolinensis haplotypes respectively occupy extensive ranges east and west of the Mobile Bay / Tombigbee River system of southwestern Alabama. These samples also focused attention on the Mobile Basin/ Tombigbee River as the place of contact and possibly parapatric transition from eastern to western haplotypes (Gill et al. 1993). To define the extent of coexistence of haplotypes and to test whether the contact was truly parapatric or a more gradual step cline, we collected additional specimens in February 1993 and February 1995, mostly from 10 new localities west from the Tombigbee River in southwestern Alabama and across the state of Mississippi (Fig. 1). We froze heart and liver samples within 30 min of collection. Specimens were preserved as traditional study skins and tissue vouchers in the collections of the Academy of Natural Sciences of Philadelphia. Sequence compositions of eastern versus western haplotypes differ by an estimated 2.7%, which is about one-half the divergence (5.2%) between R carolinensis and the Black-capped Chickadee (Poecile atricapillus). These unpublished divergence estimates are based on comparisons of 1,068 bp of the cytochrome-b gene from mitochondrial DNA (F. Gill, B. Slikas, and F. Sheldon unpubl. data). Earlier estimates of divergence based on calculations from