Long-distance seasonal migration to the tropics promotes genetic diversity but not gene flow in boreal birds.

Differences in life history can cause co-distributed species to evolve contrasting population genetic patterns, even as they occupy the same landscape. In high-latitude animals, evolutionary processes may be especially influenced by long-distance seasonal migration, a widespread adaptation to seasonality. Although migratory movements are intuitively linked to dispersal and therefore promotion of gene flow, their evolutionary genetic consequences remain poorly understood. Using ~1,700 genomes from 35 co-distributed boreal-breeding bird species that differ in non-breeding latitude and thus migration distance, we find that most long-distance migrants unexpectedly exhibit spatial genetic structure, despite their strong movement propensity. This result suggests evolutionary effects of philopatry-the tendency of many migrants to return to the same breeding site year after year, resulting in restricted dispersal. We further demonstrate that migration distance and genetic diversity are strongly positively correlated in our study species. This striking relationship suggests that the adaptive seasonal shifts in biogeography inherent to long-distance migration may enhance population stability, preserving genetic diversity in long-distance migrants relative to shorter-distance migrants that winter in harsher conditions at higher latitudes. Our results suggest that the major impact of long-distance seasonal migration on population genetic evolution occurs through promotion of demographic stability, rather than facilitation of dispersal.