A study on Woodcreepers and Antbirds tries to answer this question.
A few years ago, Jason Weir and colleagues published a paper in the journal Evolution, comparing seven bird species pairs that meet at Amazonian headwaters (see a short blog post about it here). They clearly showed that these species pairs are hybridizing, but what prevents these pairs from merging into one? To figure this out, they focus on two specific cases in a new paper:
- Elegant (Xiphorhynchus elegans) and Spix’s (X. spixii) Woodcreepers
- Scale-backed (Willisornis poecilonotus) and Xingu (W. vidua) Antbirds
Reproductive Isolation Mechanisms
Before we continue our expedition into the Amazonian forest, I will need to introduce some reproductive isolation mechanisms. Biologists discriminate between prezygotic and postzygotic barriers. This distinction refers to processes that work before (prezygotic) or after (postzygotic) fertilization.
Prezygotic barriers can be behavioral. Members from different species don’t see each other as potential mates because they look or sound too different. When behavioral isolation is imperfect and copulation does occur, fertilization might still fail. Perhaps the key does not fit the lock. Or maybe the sperm and egg are incompatible.
Postzygotic isolation mechanisms act after fertilization and can be either intrinsic or extrinsic. Intrinsic mechanisms lead to sterility or inviability of the offspring, while extrinsic mechanisms encompass lower fitness of the offspring for ecological or behavioral reasons, not developmental defects.
For more information about these mechanisms you can check chapter 2 from my PhD dissertation (see here).
In birds, prezygotic and extrinsic postzygotic isolation are generally seen as the most important species barriers. Does this also hold for Amazonian species? To test this, Paola Pulido-Santacruz, Alexandre Aleixo and Jason Weir looked into the DNA of birds from the hybrid zones. Based on two statistics – heterozygosity and hybrid index – they constructed triangle plots. Below you can see the result. Pure individuals are located in the lower corners, while first generation hybrids are at the top. The sides of the triangles (D1 and D2) indicate backcrosses.
Next, the researchers simulated data under different scenarios of pre- and postzygotic isolation. Comparing the simulations with the actual data suggested that reproductive isolation in these hybrid zones is mainly maintained by postzygotic isolation mechanisms.
Amazonian Hybrid Zones
In contrast to hybrid zones in North America and Europe, it seems that postzygotic isolation is more important than prezygotic isolation in the Amazonian forest. However, the species pairs in this study are already quite old. Elegant and Spix’s Woodcreeper diverged about 2.5 million years ago. Scale-backed and Xingu Antbird are even older, having split ways around 4 million years ago. It remains to be tested if the same patterns hold for younger species pairs.
Pulido-Santacruz, P., Aleixo, A. & Weir, J.T. (2018) Morphologically cryptic Amazonian bird species pairs exhibit strong postzygotic reproductive isolation. Proceedings of the Royal Society B, 285:20172081.
Weir, J. T., Faccio, M. S., Pulido-Santacruz, P., Barrera-Guzman, A. O. & Aleixo, A. (2015). Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds. Evolution 69, 1823-1834.