Dispersal and vicariance are often contrasted as competing processes primarily responsible for spatial and temporal patterns of biotic diversity. Recent methods of biogeographical reconstruction recognize the potential of both processes, and the emerging question is about discovering their relative frequencies. Relatively few empirical studies, especially those employing molecular phylogenies that allow a temporal perspective, have attempted to estimate the relative roles of dispersal and vicariance. In this study, the frequencies of vicariance and dispersal were estimated in six lineages of birds that occur mostly in the aridlands of North America. Phylogenetic trees derived from mitochondrial DNA sequence data were compared for towhees (genus Pipilo), gnatcatchers (genus Polioptila), quail (genus Callipepla), warblers (genus Vermivora) and two groups of thrashers (genus Toxostoma). Di¡erent area cladograms were obtained depending on how widespread and missing taxa were coded. Nonetheless, no cladogram was obtained for which all lineages were congruent. Although vicariance was the dominant mode of evolution in these birds, approximately 25% of speciation events could have been derived from dispersal across a preexisting barrier. An expanded database is now needed to estimate the relative roles of each process. Applying a molecular clock calibration, nearly all speciation events are of the order of a million or more years old, much older than typically presumed.

The California Gnatcatcher (   Polioptila californica) has become a flagship species in the dispute over development of southern California's unique coastal sage scrub habitat, a fragile, geographically restricted ecosystem with high endemism. One aspect of the controversy concerns the status of the subspecies of this bird in southern California coastal sage scrub that is currently listed as threatened under the U.S. Endangered Species Act. To investigate the recent population history of this species and the genetic distinctiveness of subspecies and to inform conservation planning, we used direct sequencing of mitochondrial DNA (mtDNA) for 64 individuals from 13 samples taken throughout the species' range. We found that coastal sage scrub populations of California Gnatcatchers are not genetically distinct from populations in Baja California, which are dense and continuously distributed throughout the peninsula. Rather, mtDNA sequences from this species contain the signatures of population growth and support a hypothesis of recent expansion of populations from a southern Baja California refugium northward into the southern coastal regions of California. During this expansion, stochastic events led to a reduction in genetic variation in the newly occupied range. Thus, preservation of coastal sage scrub cannot be linked to maintaining the genetic diversity of northern gnatcatcher populations, despite previous recognition of subspecies. Our study suggests that not all currently recognized subspecies are equivalent to evolutionarily significant units and illustrates the danger of focusing conservation efforts for threatened habitats on a single species.

We compared mitochondrial DNA sequences for six species distributed across the aridlands of North America to document phylogeographic patterns and assess levels of congruence. The Curve-billed Thrasher (Toxostoma curvirostre) and Canyon Towhee (Pipilo fuscus) show genetic divisions between the Sonoran and Chihuahuan Deserts, whereas the Cactus Wren (Campylorhynchus brunneicapillus), Black-tailed Gnatcatcher (Polioptila melanura), and Verdin (Auriparus flaviceps) do not. Most likely, species without phylogeographic structure only recently colonized their entire current range. Therefore, although these species are today part of a widespread avifauna, species' distributions were historically different from today. In Baja California, the Cactus Wren and the Verdin show phylogeographic breaks at 28°–30°N, consistent with a division previously described in the LeConte's Thrasher (Toxostoma lecontei) and in some members of the herpetofauna. These genetic divisions were likely caused by isolation resulting from a mid-peninsular seaway that existed one million years ago. Hence, these species appear to have been broadly sympatric for at least one million years. In contrast, the California Gnatcatcher (Polioptila californica) lacks such a phylogeographic division, and apparently only recently expanded into the northern part of its current range. Thus, not all species in Baja California have had similar histories, although further sampling might reveal a general pattern. Comparative phylogeography therefore provides an indirect method of evaluating the long-term stability of faunas via assessment of levels of phylogeographic congruence, and can show whether particular species are likely to have had a long period of co-association.

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We compared mitochondrial DNA sequences of Three-toed Woodpeckers (Picoides tridactylus) within and between Eurasia and North America. Samples from the two continents are each reciprocally monophyletic, and differ by approximately 4% sequence divergence, suggesting that a separate species exists on each continent. We recommend formal taxonomic action. Within continents, no phylogeographic structuring was found, suggesting that both species have recently expanded their ranges following the end of the last ice age.Filogeografía Holártica y Límites de la Especie Picoides tridactylusResumen. Comparamos secuencias de ADN mitocondrial en Picoides tridactylus, dentro y entre Eurasia y América del Norte. Las muestras de cada uno de los dos continentes son recíprocamente monofiléticas y difieren aproximadamente en un 4% de divergencia secuencial, sugiriendo que existe una especie distinta en cada continente. Recomendamos acción taxonómica formal. No encontramos estructura filogeográfica dentro de los continentes, sugiriendo que ambas especies han expandido recientemente sus rangos luego del final de la última era glacial.

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We surveyed 1,115 base pairs of mitochondrial DNA sequence from three gene regions in 66 Curve-billed Thrashers (Toxostoma curvirostre) taken from throughout their range. We found that populations sorted unambiguously into three groups. Two of these groups, the Curvirostre and Palmeri groups, have been known for many years because of their distinctive phenotypic characteristics, and their status was recently confirmed with an intensive morphometric analysis. The third (southern) group consists of populations in Puebla and Oaxaca, Mexico. The strong morphological and mtDNA distinctiveness of the Curvirostre and Palmeri groups suggests that they warrant species status. We recommend that more specimens of the southern group be obtained prior to formal taxonomic recognition of this form. Coalescence analyses suggest that the Curvirostre group has undergone a recent population increase, whereas the Palmeri group seems to have been more stable in its Sonoran Desert range.