Despite their recent divergence, Solomons White-eye and Kolombangara White-eye don’t interbreed.
When closely related species come into secondary contact, they often hybridize. In 2002, Trevor Price and Michelle Bouvier estimated that after, on average, 5 million years of divergence birds tend to produce sterile hybrids. After about ten million years of independent evolution, the hybrids are not viable anymore. Hence, it would be surprising to find species pairs markedly younger than 5 million years to show complete reproductive isolation. But that is exactly what two researchers describe in a recent paper in the journal Evolution.
Sarah Cowles and Albert Uy investigated a contact zone between the Solomons White-eye (Zosterops kulambangrae) and the Kolombangara White-eye (Z. murphyi) on Kolombangara Island in the Solomon Archipelago. These species diverged about two million years ago and are thus expected to be able to hybridize. However, the genetic analyses (based on 20,000 SNPs) revealed no evidence for gene flow. This suggests that the reproductive isolation between these two passerines is complete.
The researchers searched the literature for other such cases. They found that these White-eyes represent the youngest known case of complete reproductive isolation in sympatric birds tested within a genomic context.
But what is preventing these birds from interbreeding? It could be that these species do not recognize each other as potential partners. They produce different songs and calls. And the size of the white eye-ring is significantly different. Whether the birds use these traits in species recognition remains to be investigated. Another possibility is that the hybrids are not viable due to genetic mismatches. A genomic study in 2015 suggested that White-eyes have elevated rates of genomic evolution, which could result in the rapid accumulation of such genetic mismatches. Clearly, more research is needed here.
The Paradox of the Great Speciator
This study might provide a solution to a paradox raised by Jared Diamond and his colleagues. They wondered how these White-eyes (the family Zosteropidae) can diversify so quickly while colonizing a vast geographical range. You would expect that incipient species come into secondary contact during their colonization. Then hybridization and consequent gene flow would prevent further divergence and effectively reverse the speciation process. However, in just two million years, the White-eyes diversified into more than 100 species while spreading to Africa, Asia, Australia and throughout the Indo-Pacific. How can this happen?
Solomons White-eye and Kolombangara White-eye might hold the answer to this paradox: lineages with high dispersal capacities can have high speciation rates if they evolve complete reproductive isolation more rapidly than other lineages. Testing this idea with other pairs of White-eyes will be necessary to see if the solution is correct.
I wanted to cover this story earlier. The paper was gathering dust at the top of my writing list for some time now. But I waited for the publication of another paper, namely my digest of this study. The journal Evolution provides the opportunity to write short news articles (so-called digests) about selected original research. I wrote one about the White-eye study, which you can read here (although it is a more technical story that covers the same ground as this blog post). Apart from my Avian Hybrids blog, these digests are a great way to spread the latest findings about hybridization in birds. Even if the species under investigation don’t hybridize…
Cowles, S. A., & Uy, J. A. C. (2019). Rapid, complete reproductive isolation in two closely‐related Zosterops White‐eye bird species despite broadly overlapping ranges. Evolution.
Ottenburghs, J. (2019) Digest: White‐eye birds provide possible answer to the paradox of the great speciator. Evolution
This paper has been added to the Zosteropidae page.