MORPHOLOGICAL AND MOLECULAR VARIATION ACROSS A MIGRATORY DIVIDE IN WILLOW WARBLERS, PHYLLOSCOPUS TROCHILUS

Area of Science:

• Ornithology
• Evolutionary Biology
• Population Genetics

Background:

• A migratory divide is a zone where populations with different migratory routes meet, potentially interbreeding.
• Banding studies in willow warblers (Phylloscopus trochilus) reveal a migratory divide across central Scandinavia.
• Southern populations migrate southwest to West Africa; northern populations migrate southeast to East and South Africa.

Purpose of the Study:

• To investigate the characteristics of the migratory divide in willow warblers.
• To determine the width of the hybrid zone and assess genetic differentiation.
• To understand the evolutionary processes maintaining morphological and behavioral divergence.

Main Methods:

• Analysis of banding data to identify migratory routes.
• Morphological measurements (size, plumage coloration) to characterize populations.
• Genetic analysis using mitochondrial DNA and microsatellite loci to assess gene flow and differentiation.

Main Results:

• A migratory divide exists latitudinally across central Scandinavia, with distinct migratory directions.
• Morphological differences in size and plumage are concordant with migratory directions.
• The transition zone is estimated to be less than 350 km wide.
• Linkage disequilibria were found between size and plumage traits within the contact zone.
• Despite morphological and behavioral divergence, genetic analysis showed near panmixia at neutral loci.

Conclusions:

• The hybrid zone likely resulted from secondary contact after allopatric differentiation during the last glaciation.
• The observed pattern of morphological divergence without genetic differentiation suggests strong selection on expressed genes and/or recent divergence.
• High gene flow or recent divergence may maintain the distinct traits despite interbreeding.