What does the ancient divergence of migration genes suggest about Willow Warbler evolution?
That genetic tools evolved long ago but were actively selected upon by recent ecological pressures.
The evolutionary timeline for the Willow Warbler presents a paradox: the highly differentiated genes controlling the essential compass setting for migration appear to be ancient variants, possibly maintained in their linked blocks for over a million years due to chromosomal inversions. However, the sharp phenotypic cline—the clear separation of migratory routes—is currently only observable across the Scandinavian hybrid zone, suggesting a relatively recent manifestation. This leads to the conclusion that while the underlying genetic variation and structure (the 'tools') predate major recent climatic events like the last glaciation, the specific ecological conditions or geographical realities (such as post-glacial range expansion patterns) that make these specific directions adaptive are relatively recent. This recent selection pressure acts strongly upon this pre-existing, tightly linked adaptive suite of genes, causing the rapid, sharp divergence observed today.
