Genomic analyses suggest an important role for past climate changes.
The Amazonian floodplains are among the most diverse places on our planet. This collection of habitats covers over 300,000 square kilometres and houses about 10% of endemic tree species and 15% non-aquatic bird species. But where did all this diversity come from? What factors drive the origin of new species on these flooded plains? Numerous studies have attempted to answer these questions. Some researchers pointed to the role of rivers (as you can read in this blog post), while others suggested that climatic events are more important (such as in this blog post). Clearly, the jury is still out on the drivers of avian speciation on the Amazonian floodplains. A recent study in the journal Science Advances revisited this age-old conundrum.
The researchers focused on three species complexes:
- Ash-breasted antbird (Myrmoborus lugubris)
- Blackish-grey antshrike (Thamnophilus nigrocinereus) and Castelnau’s antshrike (T. cryptoleucus)
- Leaden antwren (Myrmotherula assimilis)
They obtained genetic material from all known subspecies in these species complexes and meticulously mapped the genetic patterns across the Amazonian floodplains. These analyses revealed that the three species showed similar genetic population structure across different sections of the floodplains. For example, all species exhibited higher genetic diversity in central portion of the Amazon River, suggesting the existence of hybrid zones. This finding already indicates that these populations have been geographically isolated in the past. Let’s have a closer look at the results.
The uncovered genetic population structure could be explained by three non-mutually exclusive effects: (1) isolation-by-distance in which populations become more genetically divergent the farther they are apart, (2) ecological gradients where different populations adapt to different environmental conditions, and (3) long-term geographic changes to the Amazon River basin.
Although the first two processes – isolation-by-distance and ecological gradients – explain some part of the genetic patterns, the main driver appears to be previous geographic changes to the Amazon Basin. Indeed, the diversification of the three species complexes coincides with the Mid- and Late Pleistocene when major reorganization of Amazonian tributaries occurred and when the current transcontinental Amazon River arose. Specifically, climatic changes affected the sediment dynamics and floodplain structure, isolating bird species adapted to life on the river edges. During the Holocene (less than 11,000 years ago), the floodplains expanded, resulting in the establishment of secondary contact zones in the central part of the distribution.
This scenario is reminiscent of the “forest refugia hypothesis” that explains the origin of numerous forest species. But instead of forest habitat contracting and expanding, it is now “the contraction or interruption of river edge forest resulting in isolated blocks restricted to the main Amazonian rivers.” This parallel between forest and river habitats suggests that a South American synthesis on the role of climate changes in avian speciation is getting closer.
Thom, G., Xue, A. T., Sawakuchi, A. O., Ribas, C. C., Hickerson, M. J., Aleixo, A., & Miyaki, C. (2020). Quaternary climate changes as speciation drivers in the Amazon floodplains. Science Advances, 6(11), eaax4718.