What evolutionary mechanism maintains tightly linked adaptive genes within the differentiated regions?

Large chromosomal inversion polymorphisms.

The maintenance of blocks of locally adapted genes together, despite ongoing gene flow in the contact zones, is attributed to a specific type of structural variation within the genome. The genetic structure observed within the three highly differentiated regions exhibits a pattern suggestive of a lack of recombination between the northern and southern versions of the DNA segment. This phenomenon is typically caused by large chromosomal inversion polymorphisms. Inversions physically rearrange sections of the chromosome, often suppressing recombination in the inverted region, thereby tightly linking the genes contained within that block. This evolutionary strategy ensures that sets of genes conferring adaptive advantages, such as those governing complex migratory physiology, are inherited as a single unit across generations, even where different versions of the warbler meet and hybridize.