Convergent Evolution

The Honeyeaters' Tree

While kicking The Honeyeaters’ Tree into some semblance of a decent second draft, I have revisited thoughts inspired by my 883rd lifer – a black-capped sittella. This is the most widespread race of varied sittella, whose close northern equivalent is a nuthatch. Except they could hardly be further away taxonomically. Our nuthatch sits between wrens and treecreepers, two-thirds of the way through the 6,000-strong passerines. At the one-third mark lies the sittella, between cuckoo-shrikes and whistlers.

Yet the two species look and behave so alike. That’s convergent evolution in action. 200 million years ago heavy volcanic eruptions may have caused the mass extinction that defines the boundary between the Triassic and Jurassic periods. They may also have split the ancient super-continent of Pangaea but the drifting landmasses certainly isolated the survivors. Among them were the ancestors of early birds, which would not appear for another 50 million years.

We can probably assume that these were pretty crappy fliers but the distance between Australia and Africa was not so great for another 20 million years that improving species could not cross it. Thereon smaller birds, like passerines, wouldn’t have made it so easily and would have diverged just by genetic drift.

But similar environments, like forests, would have kept the divergence within certain limits. They would have imposed certain patterns that worked. It was just a question of which branch of the burgeoning bird class would occupy which niche. And on different continents different branches diverged into the same niche – in this case small bird that forages down tree trunks in search of insects. Only the genetic code can now unravel this, give some clue to that Jurassic speciation.

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