The establishment of sedentary populations has several consequences for genetics, morphology and behavior.
Numerous bird species migrate. Occasionally, a migrating population “decides” to stop their annual trips and become sedentary. These migratory drop-offs have been documented in several taxa and often result in morphological and physiological changes. But can they also drive speciation? Theoretically, it is certainly possible that differences in morphology and genetics start to accumulate between migratory and sedentary populations, ultimately resulting reproductive isolation and the origin of new species. A recent study in the journal Current Biology tested this idea in the Fork-Tailed Flycatcher (Tyrannus savana).
The Fork-Tailed Flycatcher contains four subspecies. One subspecies (savanna) is a long-distance migrant that breeds from central Brazil to Argentina and spends the non-breeding season in northern South America. The other three subspecies (monachus, sanctaemartae, and circumdatus) are sedentary in Central and South America. This system provides the ideal circumstances to test the idea that loss of migration can result in speciation. If this idea holds true, you would expect clear differences in genetics, morphology and behavior between the migratory and sedentary subspecies. And that is exactly what Valentina Gómez-Bahamón and her colleagues found. Time for a quick overview!
Demographic analyses using genomic data indicated that the migratory and sedentary populations diverged about 1.08 million years ago. There was no sign of gene flow between these populations after the initial split, suggesting that the loss of migration happened only once. This finding is supported by a phylogenetic analyses that shows the sedentary populations nested within the migratory subspecies. The researchers conclude that “[the genomic] patterns are consistent with the hypothesis that migratory and sedentary fork-tailed flycatchers are on separate evolutionary trajectories.”
The genetic patterns are corroborated by morphological and behavioral data from the field. The researchers reported morphometric divergence between migratory and sedentary birds in traits associated with flight performance, such as the shape of the wings and tails. In line with previous studies, migratory birds had longer, more pointed wings for powered flight, while sedentary birds had shorter and more rounded wings to enhance maneuverability. The populations also exhibited differences in the timing of breeding (see figure below), contributing to reproductive isolation.
Taken together, these findings indicate that “migratory and tropical sedentary fork-tailed flycatchers are reproductively isolated due to spatial and temporal separation in breeding activities as a result of changes in migratory behavior leading to alternative strategies.”
The genetic data show that a small section of the migrating population became sedentary: both the demographic modelling and several measure of genetic diversity (e.g., heterozygosity and Tajima’s D) point to a reduction in effective population size. In a sense, this event can be seen as a founder event speciation. But how did this event happen? Migration is a complex trait that is influenced by numerous genes. A single genetic mutation cannot simply overturn migratory behavior and lead to sedentary individuals.
The researchers speculate that the change in migration strategy was due to behavioral plasticity. Some individuals switched to a sedentary lifestyle and were exposed to new selection pressures, leading to the accumulation of genetic differences between the sedentary and the migratory populations. Later on, the plasticity was lost when the new sedentary behavior was “assimilated” into the genome. An interesting hypothesis that needs further investigation.
Regardless of the mechanism behind the establishment of sedentary populations. It seems that loss of migration might be a common road to speciation. A macroevolutionary analysis of the family Tyrannidae shows that several species have lost the ability to migrate. Interestingly, speciation rates were higher for migratory and partially migratory lineages than those of sedentary lineages. Migration is thus an important factor to take into account if we want to understand the origin of bird species.
Gómez-Bahamón, V., Márquez, R., Jahn, A. E., Miyaki, C. Y., Tuero, D. T., Laverde-R, O., Restrepo, S. & Cadena, C. D. (2020). Speciation associated with shifts in migratory behavior in an avian radiation. Current Biology.