Mitochondrial DNA Suggests Recent Origins of Subspecies of the Sharp-shinned Hawk and Great Blue Heron Endemic to Coastal British Columbia and Southeast Alaska

Genetic studies of subspecies endemic to Haida Gwaii (Queen Charlotte Islands) in British Columbia and the Alexander Archipelago of southeast Alaska have frequently found patterns of genetic differentiation corresponding to these phenotypically based taxa. Divergence and speciation are common among island populations of birds, and evidence suggests this region has fostered such divergence during previous glacial maxima. We examined divergence in the mitochondrial gene for NADH dehydrogenase subunit 2 (ND2, a marker used in other studies of regional endemism) in two additional coastal subspecies endemic to this region, of the Sharp-shinned Hawk (Accipiter striatus perobscurus) and Great Blue Heron (Ardea herodias fannini). In both the hawk and heron genetic diversity in ND2 was remarkably low in contrast to that in mitochondrial genes in other species with regional endemics. In both Accipiter striatus perobscurus and Ardea herodias fannini we found only the haplotype most common in continental populations. We found low but significant divergence in frequencies of haplotypes of ND2 between A. s. perobscurus and continental populations of the Sharp-shinned Hawk and no significant population divergence in the Great Blue Heron. In contrast with other regional endemics that do show signals of having persisted through at least one past Ice Age in an unglaciated refugium, these subspecies may have arisen relatively recently, with their adaptation to the local environment leading to darker coloration paralleling that of the region’s older endemics. Alternatively, species-wide selective sweeps of mitochondrial DNA prior to divergence of these taxa may have rendered this genetic marker less useful for tracking divergence arising in a refugium. The Pleistocene epoch that began about 2.6 million years ago was characterized by dramatic fluctuations in Earth’s climate (Berger 1984). Global cooling on a 100,000-year cycle caused a series of glaciations that had a profound effect on the distribution of species (Avise and Walker 1998). Genetic evidence has suggested that isolation during Pleistocene glacial cycles promoted divergence and speciation in habitats fragmented by the advance and retreat of continental ice sheets (Weir and Schluter 2004). Many phylogenetic and population genetic studies have focused on Haida Gwaii (Queen Charlotte Islands), British Columbia, and the surrounding region because of the number of endemic taxa from this region that have been described for many classes of organisms, including birds (Topp and Winker 2008), plants (Ogilvie 1989), insects (Kavanaugh 1989), and mammals (Fleming and Cook 2002). Many of these studies have supported the Western Birds 49:47–61, 2018; doi 10.21199/WB49.1. 3 48 hypothesis of an unglaciated refugium in the Haida Gwaii area for mammals, insects, plants, reptiles, gastropods, fish, and birds (Schafer et al. 2010, table 2; Pruett et al. 2013, table 4; Klicka et al. 2011, Graham and Burg 2012, Lait et al. 2012, Burg et al. 2014, Withrow et al. 2014). Although Haida Gwaii has been a focal area for many of these studies, the evidence for the refugium is regional, as the ranges of some of the taxa centered on Haida Gwaii extend beyond these islands (possibly as a result of post-glacial expansion). It is also not possible at present to determine exactly where the putative refugium was located. Past shifts in spatial distribution and population size can leave distinct patterns in the genetic makeup of populations and species. Populations that became isolated in ice-free refugia during glacial cycles and remain reproductively segregated from immigrants should be genetically distinct via complete or nearly complete lineage sorting (Nei 1975, Pruett 2013). Rapid range expansion following glacial retreat is expected to reduce genetic diversity as alleles are lost and homogeneity increases (Ibrahim et al. 1996, Hewitt 1996, 2000), as seen in populations that occurred on the northern Figure 1. Ventral and dorsal views of adult males of the darker Accipiter striatus perobscurus (A, UAM 28334) and the paler A. s. striatus (B, UAM 34176). For scale, the museum labels are 19 mm wide. SUBSPECIES OF THE SHARP-SHINNED HAWK AND GREAT BLUE HERON