Abstract:
Spatial heterogeneity in selection pressures can lead to extensive morphological variation and differences at functional genes between populations across a species' range without corresponding genetic variation at neutral loci. Divergent selection among populations may thus lead to intraspecific variation and in many cases speciation. Phenotypic and genetic structure within and between Cinnamon Teal (Anas cyanoptera) and the closely related Blue-winged Teal (A. discors) was assessed using body size measurements and neutral genetic markers in conjunction with a functional locus, hemoglobin. Cinnamon Teal are composed of five subspecies corresponding to distinct ecogeographic regions in North and South America. Subspecies and geographic regions differed significantly in overall body size, with the largest subspecies and the largest individuals found at high elevations in the central Andes (A. c. orinomus) and at high latitudes in southern Patagonia (A. c. cyanoptera). South American populations showed strong positive correlations with latitude and elevation while the migratory subspecies in North America (A. c. septentrionalium) showed few significant correlations with elevation and no relationship between latitude and body size. In addition, plumage differences were restricted to between North and South America as there was extensive variation observed within continents. Cinnamon Teal highland and lowland populations showed strong divergence in body size (PST = 0.56) and exhibited frequency differences in one non-synonymous alpha-globin amino acid polymorphism (Asn/Ser-alpha9; FST = 0.60), despite considerable admixture of reference loci. Selection pressures imposed by the hypoxic highland environment have likely resulted in asymmetric gene flow from the highlands into the lowlands following a highland colonization event from the lowlands. Cinnamon Teal and Blue-winged Teal show distinct but paraphyletic mitochondrial DNA (phiST = 0.41) and broadly shared nuclear alleles. Unlike South American Cinnamon Teal, North American Cinnamon Teal and Blue-winged Teal are characterized by high genetic diversity, large effective population size, and recent population expansion. Haplotypic and allelic sharing across continents is likely because of incomplete lineage sorting rather than ongoing gene flow. Within-continent estimates yielded higher migration rates consistent with hybridization. However, Cinnamon Teal and Blue-winged Teal are similar in body size; differences in plumage coloration may reduce hybridization events.
