Hypervariable segments of the control region of mtDNA as well as part of the cytochrome b gene of Dunlins were amplifed with PCR and sequenced directly. The 910 base pairs (bp) obtained for each of 73 individuals complete another of the few sequencing studies that examine the global range of a vertebrate species. A total of 35 types of mtDNA were detected, 33 of which were defined by the hypervariablecontrol-region segments. Thirty of the latter were specific to populations of different geographic origin in the circumpolar breeding range of the species. The remaining three types indicate dispersal between populations in southern Norway and Siberia, but female-mediated flow of mtDNA apparently is too low to overcome the effects of high mutation rates of the control-region sequences, as well as population subdivision associated with historical range disjunctions. A genealogical tree relating the types grouped them into five populations:Alaska, West Coast ofNorth America, GulfofMexico, western Europe, and the Taymyr Peninsula. The Dunlin is thus highly structured geographically, with measures of mutational divergence approaching 1.0 for fixation of alternative types in different populations. High diversity of types within populations as well as moderate long-term effective population sizes argue against severe population bottlenecks in promoting this differentiation. Instead, population fragmentation in Pleistocene refuges is the most plausible mechanism of mtDNA differentiation but at a much earlier time scale than suggested previously with morphometric data.The amplification and direct sequencing of highly polymorphic regions of mtDNA provide a potentially rich source of variation at the nucleotide level for determining the molecular population structure within species and the phylogeny of intraspecific lineages. mtDNA is the molecule of choice for such studies because it is nonrecombining and maternally inherited (but see ref. 1) and has a high average rate of evolution. Analyses of sequence variation of the human mtDNA genome, for example, have shown that the noncoding control region harbors the most variability (2-5) and that this variation is located principally in two hypervariable segments (6, 7). In this paper we report the nucleotide sequences ¶ of two hypervariable segments of the control region of73 individual Dunlins and contrast these results with those from a segment of the more slowly evolving cytochrome b gene. We demonstrate that these mtDNA segments can not only elucidate the population genetic structure of this long-distance migrant shorebird over much of its circumpolar breeding range in the arctic tundras of the Northern Hemisphere but also can distinguish subpopulations within composite flocks of birds at more southerly wintering sites or during migration. Additionally, gene flow between populations of breeding birds can be readily detected and quantified.The Dunlin is among the most polytypic species of highly vagile shorebirds, with up to nine subspecies recognized on the basis of v...

Abstract.-Comparison of mitochondrial DNA (mtDNA) control-region sequences of 155 dunlins from 15 breeding populations confirmed the existence of five major phylogeographic groups in the circumpolar breeding range of this migratory shorebird species. Time estimates of the origin of groups, based on sequence divergences and a molecular clock for birds, suggest a scenario of repeated fragmentation of populations in isolated tundra refugia during the late Pleistocene. The distribution of about three-quarters of all detected molecular variance between phylogeographic groups attests to the strongly subdivided genetic population structure in dunlins that is being maintained by natal philopatry. Each mtDNA phylogeographic group can be related to a morphometrically defined subspecies, but several other recognized subspecies are not supported by monophyletic mtDNA lineages within their purported ranges. More detailed analysis of several European populations reveals low amounts of gene flow and the partitioning of a substantial fraction of molecular variance between them. This ongoing evolution of population-genetic structuring within the European phylogeographic group most likely started with the last retreat of the ice sheets some 10,000 years ago. Dunlins thus provide one of the clearest examples of the linkage between historical and contemporary components of mtDNA phylogeographic structuring in birds.Key words.-Dunlin, mitochondrial DNA, molecular systematics, philopatry, phylogeography, Pleistocene.Received January 21, 1994. Accepted October 4, 1994.The population-genetic structure of a species is the result of an interplay of microevolutionary and demographic processes acting among and within populations, and the historical biogeography of genetic lineages. Whereas gene flow retards the genetic divergence of populations, geographic isolation, genetic drift, philopatry, high rates of mutation, and possibly selection for local adaptation contribute to pronounced intraspecific phylogeographic structure in animal species (Avise et al. 1987). Because of its high mutation rate, haploid maternal inheritance, and lack of recombination, mitochondrial DNA (mtDNA) has been used extensively to investigate the matriarchal population-genetic structure of species (e.g., Avise et al. 1992;Bowen et al. 1992). Although most previous analyses of sequence variation in mtDNA are based on the detection of restriction-fragment-Iength polymorphisms, amplification and direct sequencing of homologous mtDNA fragments provide a more precise means to infer a genealogy of haplotypes. Analysis of the most variable part of the mtDNA genome also offers increased sensitivity for detecting very recently evolved population-genetic structure (Wenink et al. 1993).We previously described the existence of strong global population-genetic structure in a migratory shorebird, the dunlin (Calidris alpina), based on fast-evolving mtDNA control-region sequences (Wenink et al. 1993). Five major clades were detected among 73 dunlins that were sampled at widely d...

Historical population collapses caused by rinderpest epidemics are hypothesized to have resulted in notable genetic losses in populations of the African buffalo. Polymorphism in the major histocompatibity complex (MHC) DRB3 gene was probed by means of restriction analysis of the sequence encoding the peptide-binding region. Nucleotide substitution patterns agreed with a positive selection acting on this fitness-relevant locus. Buffalo populations from four National Parks, situated in eastern and southern Africa, each revealed a surprisingly high allelic diversity. Current high levels of heterozygosity may be reconciled with historical bottlenecks by assuming that local extinctions were followed by fast recolonization, in accordance with the high dispersive capabilities of buffalo. The specific amplification of DRB3 alleles also enabled the assignment of individual genotypes. For each population sample a deficiency in the expected number of heterozygous animals was found. As overdominant selection on the MHC is predicted to yield an excess of heterozygous individuals, this may not be a locus-specific effect. Several other explanations are discussed, of which increased homozygosity caused by nonrandom mating of buffalo in populations seems the most probable.

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