The ebb and flow of the Taiwan Strait shaped patterns of gene flow between two partridge species

Genetic analyses point to several bouts of gene flow.

The Strait of Taiwan separates the Chinese Bamboo Partridge (Bambusicola thoracicus) from the Taiwan Bamboo Partridge (B. sonorivox). It is easy to imagine that these bird species have been in contact during periods of low sea levels. And indeed, a taxonomic study from 2014 provided evidence for gene flow after their divergence, roughly 1.8 million years ago. However, these genetic analyses – using an isolation-with-migration model – only indicated that gene flow occurred, but not when. A recent paper in the journal Avian Research addressed this knowledge gap using a set of 31 nuclear loci. When did the Chinese and Taiwan Bamboo Partridge exchange genetic material?

Comparing models

The researchers compared several demographic models with different timing of gene flow. The most likely model (with a posterior probability of 0.53) pointed to early gene flow during the first 20 percent of divergence. However, a second model with late gene flow could not be rejected (posterior probability of 0.30). Together, these patterns suggest that the partridges experienced multiple bouts of gene flow. The researchers speculate that “fluctuations in the sea level of the Taiwan Strait during the early late Pleistocene may have led to changes in their distribution alternating between sympatry and allopatry.” This scenario was supported by ecological niche modelling, showing that the ranges of ancestral populations overlapped during the Last Glacial Maximum.

Three different demographic models that could explain the evolutionary history of these partridges. From: Wang et al. (2021).

Merging and diverging

The evolutionary history of the Chinese and the Taiwan Bamboo Partridge was thus shaped by multiple bouts of gene flow. As methods to detect and date gene flow events improve, we can expect to find similar scenarios in other bird species. The glacial cycles of the Pleistocene impacted the distribution of numerous species, regularly giving rise to zones of secondary contact. Many species pairs were probably subjected to cycles of merging and diverging.

These insights can help us to assess the consequences of current climate change. As species distributions change, some previously isolated populations might establish secondary contact and enter a phase of merging. These human-induced hybridization events are both a curse and a blessing. As I wrote in my review on hybridization the Anthropocene: “As humans continue to change the environment and alter species distributions, more anthropogenic hybridization events will definitely occur. This will pose challenges for the conservation of endangered species, but also provide unique opportunities for evolutionary biologists.”

Ecological niche modelling indicated overlap between both partridge species (in green) during the Last Glacial Maximum. From: Wang et al. (2021).

References

Hung, C. M., Hung, H. Y., Yeh, C. F., Fu, Y. Q., Chen, D., Lei, F., … & Li, S. H. (2014). Species delimitation in the Chinese bamboo partridge Bambusicola thoracica (Phasianidae; Aves). Zoologica Scripta, 43(6), 562-575.

Wang, P., Yeh, C., Chang, J., Yao, H., Fu, Y., Yao, C., … & Zhang, Z. (2021). Multilocus phylogeography and ecological niche modeling suggest speciation with gene flow between the two Bamboo Partridges. Avian Research, 12(1), 1-10.

Featured image: Chinese Bamboo Partridge (Bambusicola thoracicus) © Sun Jiao | Wikimedia Commons