What shaped the evolution of the Azure-crowned Hummingbird: geographic barriers or climate?

Genetic study reconstructs the evolutionary history of this Central American species.

In the classic verbal model of allopatric speciation, a geographic barrier arises and splits a population in two. Over time, these isolated population diverge genetically and slowly evolve into distinct species. So, when you observe a geographic barrier, such as a river or mountain chain, between two species, it is tempting to infer that this barrier played a role in the speciation process. But this does not have to be the case. The barrier might have formed millions of years before the divergence of the species. Other evolutionary, ecological or geological processes might explain the origin of these species. This situation is nicely illustrated by a recent study on the Azure-crowned Hummingbird (Amazilia cyanocephala) which appeared in the Journal of Ornithology.

Genetic Lineages

The Azure-crowned Hummingbird can be found from the north of Mexico to Nicaragua and has been divided into three subspecies: cyanocephala, guatemalensis and chlorostephana. Using a mitochondrial marker, Flor Rodriguez-Gómez and her colleagues detected four genetic lineages within this hummingbird species. Within the subspecies cyanocephala, there is a clear break between populations east and west of the Isthmus of Tehuantepec. A third genetic lineage corresponds to the subspecies guatemalensis and is separated from the other subspecies by the Motagua-Polochic-Jocotán fault system. Finally, the fourth genetic lineage (represented by the subspecies chlorostephana) can be found in the savannah pine forests of the Moskitia region in Honduras.

These patterns suggest that the geographic barriers across Central America – the Isthmus of Tehuantepec and the Motagua-Polochic-Jocotán fault system – drove the evolution of the Azure-crowned Hummingbird. But this scenario is not supported by the divergence times between the four genetic lineages. They originated within the last million years, whereas these geographic barriers arose much earlier (the current geological shape of the Isthmus of Tehuantepec probably formed about 6 million years ago). What factors did influence the evolution of the Azure-crowned Hummingbird?

Genetic analyses of the mitochondrial control region uncovered four genetic lineages (figure a). The distribution of these lineages is depicted in figure c. From: Rodríguez-Gómez et al. (2021) Journal of Ornithology.

Glacial Cycles

Additional genetic analyses detected signatures of a population expansion before the last glacial maximum (about 20,000 years ago). This finding was corroborated by ecological niche modelling which indicated that the suitable habitat for the Azure-crowned Hummingbird was larger during this period. However, during the last inter-glacial (about 120,000 to 140,000 years ago), the distribution of suitable habitat was more fragmented, with major regions separated by the Isthmus of Tehuantepec and other smaller regions isolated in Honduras and Nicaragua. Together, these patterns point to cycles of contraction and expansion driven by climatic changes. Indeed, the authors conclude that:

Likely, phylogeographic structure in Azure-crowned Hummingbird was promoted by habitat fragmentation during interglacials and, if so, competitive exclusion in zones of contact and habitat specialization played a greater role on differentiation among the three subspecies than isolation by geographic barriers.

References

Rodríguez-Gómez, F., Licona-Vera, Y., Silva-Cárdenas, L., & Ornelas, J. F. (2021). Phylogeography, morphology and ecological niche modelling to explore the evolutionary history of Azure-crowned Hummingbird (Amazilia cyanocephala, Trochilidae) in Mesoamerica. Journal of Ornithology162(2), 529-547.

Featured image: Azure-crowned Hummingbird (Amazilia cyanocephala) © Joseph C. Boone | Wikimedia Commons

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