Genetic Divergence of an Avian Endemic on the Californian Channel Islands

Abstract

The Californian Channel Islands are near–shore islands with high levels of endemism, but extensive habitat loss has contributed to the decline or extinction of several endemic taxa. A key parameter for understanding patterns of endemism and demography in island populations is the magnitude of inter–island dispersal. This paper estimates the extent of migration and genetic differentiation in three extant and two extinct populations of Channel Island song sparrows (Melospiza melodia graminea). Inter–island differentiation was substantial (G''ST: 0.14–0.37), with San Miguel Island having the highest genetic divergence and lowest migration rates. Santa Rosa and Santa Cruz Island populations were less diverged with higher migration rates. Genetic signals of past population declines were detected in all of the extant populations. The Channel Island populations were significantly diverged from mainland populations of M. m. heermanni (G''ST: 0.30–0.64). Ten mtDNA haplotypes were recovered across the extant and extinct Channel Island population samples. Two of the ten haplotypes were shared between the Northern and Southern Channel Islands, with one of these haplotypes being detected on the Californian mainland. Our results suggest that there is little contemporary migration between islands, consistent with early explanations of avian biogeography in the Channel Islands, and that song sparrow populations on the northern Channel Islands are demographically independent.

The population genetics of nonmigratory Allen’s Hummingbird (Selasphorus sasin sedentarius) following a recent mainland colonization

“…Some species on the mainland are weakly differentiated and show high levels of gene flow between the mainland and Channel Islands (Horned Lark, Mason et al., 2014; Orange‐crowned Warbler, Oreothlypis celata , Sofaer et al., 2012; Hanna et al., 2019; nonmigratory Allen's Hummingbird). In contrast, others species have island populations that are strongly differentiated from their mainland counterparts, indicating little to no gene flow between the mainland and the Channel Islands (Island Scrub‐Jay, Aphelcoma insularis , Delaney & Wayne, 2005; Loggerhead Shrike, Caballero & Ashley, 2011; Spotted Towhee, Walsh, 2015; Song Sparrow, Wilson et al., 2015). Further, populations on the northern Channel Islands tend to show high differentiation relative to other populations (Loggerhead Shrike, Caballero & Ashley, 2011; Song Sparrow, Wilson et al., 2015; Spotted Towhee, Walsh, 2015), as we show here for nonmigratory Allen's Hummingbird (Figure 6).…”

“…Genetic drift is one potential explanation for the relative isolation of nonmigratory Allen's Hummingbird on Santa Cruz Island. For example, drift is documented to have contributed to elevated genetic divergence of the Loggerhead Shrike and Song Sparrow on the northern islands (Caballero & Ashley, 2011; Wilson et al., 2015). Recent management effort on Santa Cruz Island to enhance the habitat for avifauna was followed by a colonization of the island by the Song Sparrow, and possibly the subsequent drift effects observed.…”

“…There are 41 species of terrestrial birds that breed on the Channel Islands, including 1 endemic species and 15 endemic subspecies (Johnson, 1972; Schoenherr et al., 2003). Seven of these endemics have been studied and compared to the mainland within a population genetic context (Caballero & Ashley, 2011; Delaney & Wayne, 2005; Hanna et al., 2019; Karin et al., 2018; Mason et al., 2014; Rutledge et al., 2017; Wilson et al., 2015). Because genetic connectivity between the Channel Islands and California has only been studied for some breeding bird species, we lack a general consensus on island–mainland relationships.…”

“…(2020) reported elevated levels of nucleotide diversity on the southern California mainland. However, their main focus was to evaluate the genetic relationships between nonmigratory and migratory Allen's Hummingbird, and sampling of nonmigratory Allen's Hummingbird was restricted to the coast on the mainland and the southern Channel Islands and did not include the northern Channel Islands, where bird species such as the Loggerhead Shrike ( Lanius ludovicianus , Caballero & Ashley, 2011), Song Sparrow ( Melospiza melodia , Wilson et al., 2015), and Spotted Towhee ( Pipilo maculatus , Walsh, 2015) have exhibited high differentiation. While Godwin et al.…”

The Mouse-colored Tyrannulet (Phaeomyias murina) is a species complex that includes the Cocos Flycatcher (Nesotriccus ridgwayi), an island form that underwent a population bottleneck

“…Island forms can serve as evidence of historical diversity and distributions for closely related populations or taxa on the mainland (Gotelli and Graves, 1990;Miller et al in review;Olson, 1997Olson, , 1993Snow, 1985). Genetic information from island populations can be used to assess the monophyly of mainland populations (Crews et al, 2010;Fernández-Mazuecos and Vargas, 2011;Phillimore et al, 2008;Wilson et al, 2015). In cases where the geological history of the island is well-known, island populations can be used to calibrate divergence time estimates among mainland populations (Almeida et al, 2005;Runemark et al, 2012;Smith and Klicka, 2013; Tollis and Boissinot, 2014), and infer histories of selection (Blondel et al, 1999;Clegg et al, 2002b;Edwards, 1993;Griffith et al, 1999) or demographic or distributional changes in the mainland populations.…”