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Tuesday, 26 August 2014

Tracking and genetics for the interpretation of Hermit Thrush migration and population structure. Alvarado et al; Ecology and Evolution, August 2014

Integrative tracking methods elucidate the evolutionary dynamics of a migratory divide.

Ecology and Evolution

LINK (website / open access)

Allison H. Alvarado 1,*, Trevon L. Fuller 1 and Thomas B. Smith 1,2

Author Information
1. Center for Tropical Research, Institute of the Environment and Sustainability, University of California, Los Angeles, California
2. Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California
* Correspondence 
Allison H. Alvarado, Center for Tropical Research, Institute of the Environment and Sustainability, University of California, Los Angeles, La Kretz Hall, Suite 300, 619 Charles E. Young Dr. East, Los Angeles, CA 90095. Tel: 310-206-6234; Fax: 310-825-5446; 

Article first published online: 20 AUG 2014

DOI: 10.1002/ece3.1205

© 2014 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.

Migratory divides, the boundary between adjacent bird populations that migrate in different directions, are of considerable interest to evolutionary biologists because of their alleged role in speciation of migratory birds. However, the small size of many passerines has traditionally limited the tools available to track populations and as a result, restricted our ability to study how reproductive isolation might occur across a divide. Here, we integrate multiple approaches by using genetic, geolocator, and morphological data to investigate a migratory divide in hermit thrushes (Catharus guttatus). First, high genetic divergence between migratory groups indicates the divide is a region of secondary contact between historically isolated populations. Second, despite low sample sizes, geolocators reveal dramatic differences in overwintering locations and migratory distance of individuals from either side of the divide. Third, a diagnostic genetic marker that proved useful for tracking a key population suggests a likely intermediate nonbreeding location of birds from the hybrid zone. This finding, combined with lower return rates from this region, is consistent with comparatively lower fitness of hybrids, which is possibly due to this intermediate migration pattern. We discuss our results in the context of reproductive isolating mechanisms associated with migration patterns that have long been hypothesized to promote divergence across migratory divides.

Example Data

Figure 2
Geolocators and range-wide genetic data reveal a migratory divide in hermit thrushes. Geolocators indicate western British Columbia (BC) birds (n = 3) overwinter in California and Oregon (W1–W3) while eastern BC birds (n = 2) overwinter in Arkansas (E1–E2). Solid lines represent spring migration routes of eastern BC birds. Dotted lines link breeding and overwintering locations of western BC birds. Black dots connecting lines represent geolocator deployment/retrieval sites on breeding grounds and the centroid of overwintering home range on nonbreeding grounds. For our range-wide sample of birds (n = 380), we genotyped a β-fibint7 SNP within breeding (hatched circles) and nonbreeding (nonhatched circled) populations. Circle diameter represents genotype frequency (small circles = 1–3 birds; medium circles = 4–10 birds; large circles =11–20 birds; also see Table S1 for sample sizes). On the breeding grounds, genotypes AA (green) and BB (blue) occur west and east of the migratory divide in BC, respectively. During the nonbreeding season, these genotypes do not mix, indicating birds from opposing sides of the divide migrate different directions and are geographically separated while overwintering. Heterozygote genotype AB (red) is only found in central BC during the breeding season and only in New Mexico during the nonbreeding season, suggesting a likely intermediate nonbreeding location for birds from the hybrid zone.

Our study uses a combination of very different and complementary approaches to address interesting questions on the past and present evolutionary mechanisms thought to generate and maintain migratory divides. Deep divergence in mtDNA and microsatellites reveals that the hermit thrush migratory divide is the result of secondary contact following Pleistocene divergence of two lineages. A combination of fine-scale tracking with geolocators and broad-scale tracking using a diagnostic genetic marker indicate the lineages correspond to distinct eastern and western migratory forms. Evidence of genetic admixture and the intermediate morphology of individuals reveal evidence of hybridization at the secondary contact zone in central BC. Birds from this site appear to migrate to an intermediate nonbreeding location (based on genetic data) and experience lower return rates (based on geolocator data), potentially suggesting lower fitness. Therefore, postzygotic isolating mechanisms related to migratory patterns are likely involved in maintaining high levels of genetic divergence and significant morphological differences among hermit thrush populations from either side of the migratory divide. It is unclear whether and at what stage the distinct migratory forms of hermit thrushes are in the speciation process (Coyne, J. A., and H. A. Orr. 2004. Speciation. Sinauer Associates, Sunderland, MA.; Irwin, D. E., and J. H. Irwin. 2005. Siberian migratory divides: the role of seasonal migration in speciation. Pp. 27–40 in R. Greenberg and P. P. Marra, eds. Birds of Two Worlds: The Ecology and Evolution of Migration. The Johns Hopkins Univ. Press, Baltimore, MD.). However, the biogeographic histories (Weir, J. T., and D. Schluter. 2004. Ice sheets promote speciation in boreal birds. Proc. R. Soc. Lond. B 271:1881–1887.) and patterns of divergent migratory behavior (Brewer, D. 2000. Canadian Atlas of Bird Banding. Canadian Wildlife Service, Ottawa, ON.) of numerous other parapatric sister taxa, many of which have recently been designated as separate species, are similar to those of hermit thrushes. The mechanisms that historically create, currently maintain, and further promote population differences across migratory divides may also shape patterns of biodiversity and speciation in avian taxa of North America that have not traditionally been studied within this context.

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