Integrative tracking methods elucidate the evolutionary dynamics of a migratory divide

Abstract

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.

The pull of the Central Flyway? Veeries breeding in western Canada migrate using an ancestral eastern route

“…This might be due to lower overall survivorship, and linkages between population demography and the nature of the migration route (e.g., distance flown, available stopover sites, timing, or carry‐over) for this and other species of long‐distance migratory songbirds represent the next frontier in understanding the costs of migration (Alvarado et al. , Delmore and Irwin ).…”

Disruptive selection without genome‐wide evolution across a migratory divide

“…; Alvarado et al . ). If intermediate or transgressive hybrid phenotypes face the risk of increased mortality or reduced condition, disruptive selection has the potential to promote reproductive isolation among parental populations (Rolshausen et al .…”

“…The emergence of migratory divides can have important implications for species' ecology and evolution. Sympatric breeding populations utilizing different migratory routes can be exposed to different selection pressures such as parasite communities (von R€ onn et al 2015), weather conditions (Newton 2007) or simply overall distance requiring morphological adaptation (Rolshausen et al 2009;Alvarado et al 2014). If intermediate or transgressive hybrid phenotypes face the risk of increased mortality or reduced condition, disruptive selection has the potential to promote reproductive isolation among parental populations (Rolshausen et al 2009) which eventually can result in generalized genome-wide differentiation ('ecological speciation ' Nosil 2012;Shafer & Wolf 2013).…”

Genetic structure among greater white‐fronted goose populations of the Pacific Flyway

“…Ecological dispersal (natal and breeding dispersal) can be measured directly through observation and marking studies, and evolutionary dispersal (gene flow) can be measured via genetic investigations. While preferably, both measures of within‐species connectivity should be used (Alvarado, Fuller, & Smith, ; Lecomte, Gauthier, Giroux, Milot, & Bernatchez, ), such measurements can be complex and logistically difficult. This is particularly true for migratory birds, given their often extensive population ranges and tremendous dispersal potential.…”