Integrative tracking methods elucidate the evolutionary dynamics of a migratory divide.

Allison H Alvarado,Thomas B Smith,Trevon L Fuller

doi:10.1002/ece3.1205

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.

Highlights

Why, when, and where birds migrate have been central questions in biology since the time of Aristotle (Alerstam 1990)
Because eastern British Columbia (BC) birds are larger in many traits, we investigated whether populations differed in wing length and Kipps’ index after adjusting for differences in overall body size using a generalized linear model (GLM) with PC1 as a covariate (Supporting Information and Fig. S1)
Consistent with the microsatellites, we found high sequence divergence in mitochondrial DNA between the two groupings of populations identified by Bayesian clustering

Summary

Introduction

Why, when, and where birds migrate have been central questions in biology since the time of Aristotle (Alerstam 1990). Investigating divergent migratory behavior within a single species has been of particular interest because of its relevance to speciation (Irwin and Irwin 2005; Rohwer and Irwin 2011). Migratory divides represent the boundary between adjacent breeding populations whose migration pathways diverge, with individuals from each side traveling to distinct overwintering locations. Often falling along hybrid zones, migratory divides have been described as natural genetic laboratories (Bensch et al 1999). Reconstructing a species’ phylogenetic history can identify the origin of a migratory divide; a species’. Future evolutionary trajectory will be determined by present-day interactions between the two groups at the migratory divide. Understanding the evolutionary processes responsible for promoting divergence both in the past and present is fundamental to studying migration’s potential role in speciation

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