Wednesday, 28 June 2006

Aussie! Aussie! Aussie! Tweet! Tweet! Tweet!

For a ridiculously long time, biogeographers viewed Australia as a benign Jurassic Park. A place in which the oldest, most clapped-out animals survived long after their use-by date because they were isolated by the ocean from the latest models in the Northern Hemisphere. No velociraptors, just emus. No Triceratops but platypus and echidna. And wombats.

(Now there’s an antipodean oddball—the wombat. Pre-Raphaelite artist Dante Gabriel Rossetti kept a wombat at his house in Cheyne Walk, Chelsea. He opined they were ‘the most beautiful of God’s creatures’. But I digress.)

After breaking free from the remnants of Gondwana, Australia slid inexorably northwards. As it neared the edge of Asia, the flash, new, improved flora and fauna crossed the moat. A sort of gravity-fed colonisation as species first dripped and then cascaded down the map.

That was the story.

The perching birds tell a completely different tale.

Almost 60% of the world’s bird species are perching birds—passerines. Swallows, warblers, thrushes, starlings, sparrows, finches, wagtails, crows ... That’s 5800 species, give or take a few. Among them, the oscines (the largest group at almost 4600 species) originated in Australia and spread to the rest of the world.

The oscines arose more than 66 million years ago, when Australia was still a long way from the northern land masses. The birds diversified in isolation for at least 20 million years. (There was some dispersal to New Zealand during that time.) Then they made a break for it, spreading north and west.

We know that crows and their allies started flapping across the islands squeezed into archipelagos by the converging Australian and Asia boundaries. They dispersed across the rest of the world from South East Asia.

But this is the odd bit. Another group seems to have spread from Australia to Africa without travelling through Asia. Do not pass Go. Do not collect $200.

Some of the oldest non-Australian genera (e.g. Hyliota) occur in southern Africa. Many others are widespread but are curiously absent from Wallacea (the biogeographical area between Asia and Australia). Surely, if they had dispersed through South East Asia they would still occur there—unless they had all become locally extinct. And what are the odds of that?

The evidence for a trans-Indian Ocean dispersal event is not conclusive. The researchers (Knud Jønsson and Jon Fjeldså of the University of Copenhagen) aren’t yet convinced but it’s an interesting idea. I’ll keep an eye on this one, the In to Africa hypothesis.

Read more
Jønsson, K.A. & Fjeldså, J. (2005). A phylogenetic supertree of oscine passerine birds (Aves: Passeri). Zoologica Scripta 33: 149–186.

Jønsson, K.A. & Fjeldså, J. (2006). Determining biogeographical patterns of dispersal and diversification in oscine passerine birds in Australia, Southeast Asia and Africa. Journal of Biogeography 33: 1155–1165.

Sibley, C.G. & Ahlquist, J.E. (1990). Phylogeny and classification of birds. Yale University Press, New Haven, CT.


lefty said...

This is wonderful stuff - when's the book coming out?

Can we call it Wallace's Washing Line?

Snail said...

I like it! May I use that? :)