The origin and diversification of Galapagos mockingbirds.

Evolutionary radiations of colonists on archipelagos provide valuable insight into mechanisms and modes of speciation. The apparent diversification of Galapagos mockingbirds (Nesomimus) provoked Darwin's initial conception of adaptive radiation, but the monophyly of this historically important exemplar has not been evaluated with molecular data. Additionally, as with most Galapagos organisms, we have a poor understanding of the temporal pattern of diversification of the mockingbirds following colonization(s) from source populations. Here we present a molecular phylogeny of Galapagos and other mockingbird populations based on mitochondrial sequence data. Monophyly of Galapagos mockingbirds was supported, suggesting a single colonization of the archipelago followed by diversification. Our analyses also indicate that Nesomimus is nested within the traditional genus Mimus, making the latter paraphyletic, and that the closest living relatives of Galapagos mockingbirds appear to be those currently found in North America, northern South America, and the Caribbean, rather than the geographically nearest species in continental Ecuador. Thus, propensity for over-water dispersal may have played a more important role than geographic proximity in the colonization of Galapagos by mockingbirds. Within Galapagos, four distinct mitochondrial DNA clades were identified. These four clades differ from current taxonomy in several important respects. In particular, mockingbirds in the eastern islands of the archipelago (Española, San Cristóbal, and Genovesa) have very similar mitochondrial DNA sequences, despite belonging to three different nominal species, and mockingbirds from Isabela, in the west of the archipelago, are more phylogenetically divergent than previously recognized. Consistent with current taxonomy is the phylogenetic distinctiveness of the Floreana mockingbird (N. trifasciatus) and close relationships among most mockingbirds from the central and northern region of the archipelago (currently considered conspecific populations of N. parvulus). Overall, phylogeographic patterns are consistent with a model of wind-based dispersal within Galapagos, with colonization of more northerly islands by birds from more southern populations, but not the reverse. Further radiation in Galapagos would require coexistence of multiple species on individual islands, but this may be prevented by relatively limited morphological divergence among mockingbirds and by lack of sufficient habitat diversity in the archipelago to support more than one omnivorous mimid.

Appraisal of the consequences of the DDT-induced bottleneck on the level and geographic distribution of neutral genetic variation in Canadian peregrine falcons, Falco peregrinus.

Peregrine falcon populations underwent devastating declines in the mid-20th century due to the bioaccumulation of organochlorine contaminants, becoming essentially extirpated east of the Great Plains and significantly reduced elsewhere in North America. Extensive re-introduction programs and restrictions on pesticide use in Canada and the United States have returned many populations to predecline sizes. A proper population genetic appraisal of the consequences of this decline requires an appropriate context defined by (i) meaningful demographic entities; and (ii) suitable reference populations. Here we explore the validity of currently recognized subspecies designations using data from the mitochondrial control region and 11 polymorphic microsatellite loci taken from 184 contemporary individuals from across the breeding range, and compare patterns of population genetic structure with historical patterns inferred from 95 museum specimens. Of the three North American subspecies, the west coast marine subspecies Falco peregrinus pealei is well differentiated genetically in both time periods using nuclear loci. In contrast, the partitioning of continental Falco peregrinus anatum and arctic Falco peregrinus tundrius subspecies is not substantiated, as individuals from these subspecies are historically indistinguishable genetically. Bayesian clustering analyses demonstrate that contemporary genetic differentiation between these two subspecies is mainly due to changes within F. p. anatum (specifically the southern F. p. anatum populations). Despite expectations and a variety of tests, no genetic bottleneck signature is found in the identified populations; in fact, many contemporary indices of diversity are higher than historical values. These results are rationalized by the promptness of the recovery and the possible introduction of new genetic material.

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

Breeding redpoll finches (Aves: Carduelinae) show extensive plumage and size variability and, in many cases, a plumage polymorphism that is not related to age or sex. This has been ascribed to extreme phenotypic variation within a single taxon or to moderate variability within distinct taxa coupled with hybridization. The predominant view favors the recognition of two largely sympatric species: Carduelis flammea, comprised of four well-marked subspecies-flammea, cabaret, islandica, and rostrata; and C. hornemanni, comprised of two subspecies-hornemanni and exilipes. We studied representative samples of these putative subspecies (except islandica) for variation in mitochondrial DNA (mtDNA). Using 20 informative restriction enzymes that recognized 124 sites (642 base pairs [bp] of sequence or ≈ 3.7% of the molecule), we identified 17 RFLP haplotypes in the 31 individuals surveyed. The haplotypes formed a simple phylogenetic network with most clones diverging by a single site difference from a common haplotype found in almost half of the individuals. Within populations and taxa, levels of mtDNA diversity were similar to those observed in other avian species. The pattern of mtDNA divergence among populations was statistically unrelated to their geographic or traditional taxonomic relationships, and the estimated distance between the two traditionally recognized species was very small relative to those typically observed among avian sister species.

HISTORICAL BIOGEOGRAPHY OF THE BANANAQUIT (COEREBA FLAVEOLA) IN THE CARIBBEAN REGION: A MITOCHONDRIAL DNA ASSESSMENT.

We analyzed mitochondrial DNA (mtDNA) restriction-site variation in bananaquit (Coereba flaveola; Aves, Coerebinae) populations sampled on 12 Caribbean islands and at 5 continental localities in Central America and northern South America. Multiple fixed restriction-site differences genetically defined several regional bananaquit populations. An mtDNA clade representing all Jamaican bananaquits was the most divergent; the estimated average sequence divergence (dxy ) between Jamaican and all other mtDNA haplotypes surveyed was 0.027. Three groups of populations, representing Central America, northern South America, and the eastern Antilles (Puerto Rico to Grenada) were nearly equally differentiated among themselves (average dxy = 0.014), and may represent a single, recent range expansion. Within the eastern Antilles, three geographically restricted haplotype groups were identified: Puerto Rico, north-central Lesser Antilles (U.S. Virgin Islands to St. Lucia), and Grenada-St. Vincent. The evolutionary relationships of these groups were not clear. Genetic homogeneity of the island populations from the U.S. Virgin Islands to St. Lucia suggested a recent spread of a specific north-central Lesser Antillean haplotype through most of those islands. Haplotype variation across this region indicated that this spread may have occurred in two waves, first through the southernmost islands of St. Lucia, Martinique, and Dominica, and more recently from Guadeloupe to the north. The geographic distribution of mtDNA haplotypes, and of bananaquit populations, suggests periods of invasiveness followed by relative geographic quiescence. Although most genetic studies of bird populations have revealed homogeneity over large geographic areas, our findings provide a remarkable counterexample of strong geographic structuring of mtDNA variation over relatively small distances. Furthermore, although the mtDNA data were consistent with several subspecific distinctions, it was clear that named subspecies do not define equally differentiated evolutionary entities.