Comparative phylogeographic analysis suggests a shared history among eastern North American boreal forest birds

Phylogeographic structure within high-latitude North American birds is likely shaped by a history of isolation in refugia during Pleistocene glaciations. Previous studies of individual species have come to diverse conclusions regarding the number and location of likely refugia, but no studies have explicitly tested for biogeographic concordance in a comparative phylogeographic framework. Here we use a hierarchical approximate Bayesian computation analysis of mitochondrial DNA sequences from 653 individuals of 6 bird species that are currently co-distributed in the boreal forest of North America to test for biogeographic congruence. We find support for congruent phylogeographic patterns across species, with shallow divergence dating to the Holocene within each species. Combining genetic results with paleodistribution modeling, we propose that these species shared a single Pleistocene refugium south of the ice sheets in eastern North America. Additionally, we assess modern geographic genetic structure within species, focusing on Newfoundland and disjunct high-elevation populations at the southern periphery of ranges. We find evidence for a “periphery effect” in some species with significant genetic structure among peripheral populations and between peripheral and central populations. Our results suggest that reduced gene flow among peripheral populations, rather than discordant biogeographic histories, can explain the small differences in genetic structure and levels of genetic diversity among co-distributed boreal forest birds.

The psychophysical periphery effect crosses the vertical meridian

This study measured the periphery effect and compared its magnitude when the peripheral stimulation was on the same or opposite side of the vertical meridian as the test spot. Test thresholds for a 1.5-deg diameter, 8-ms spot located 1.75 deg to one side of the vertical meridian were elevated by approximately 0.125 log units when a 0.25 cycles/deg (cpd) counterphased grating was presented at a similar eccentric offset on the other side of the vertical meridian. The periphery effect disappeared when the test spot was moved outward to 8-deg eccentricity. When the grating and test were presented on the same side of the vertical meridian, test thresholds at both retinal locations were elevated by the same amount, 0.2 log units. Consistent with the physiology in cat retina, the periphery effect in humans also crosses over the vertical meridian. However, the effect is small and the test spot must be in close proximity to the vertical meridian for it to be observed. Also, the crossover periphery effect is reduced in magnitude by 37.5% compared to when the grating and test are presented on the same side of the vertical meridian. This suggests there may be a difference in how the underlying neural mechanism that transmits the periphery effect signal laterally is organized for sending the periphery effect signal across the vertical meridian as compared to within a retinal hemifield.