by Greg Mayer
[JAC note: read this! It's good biology!]
One of the key sources of evidence for evolution is biogeography– the geographic distribution of plants and animals. The great 20th-century biogeographer P.J. Darlington famously said that biogeography showed Darwinian evolution, and Jerry has long said that the facts of biogeography are among the most compelling evidences of descent with modification. Carl Zimmer in the New York Times has called attention to a paper published yesterday in Science by Kieren Mitchell of the University of Adelaide and colleagues which addresses an old problem in biogeography with new data. Another paper, in press in Molecular Biology and Evolution, by Allan Baker of the Royal Ontario Museum and colleagues, addresses the same problem– and specific example– in a similar way.
The problem is that of disjunction– related forms being found in widely separated places. The problem posed by disjunction is this: how did the related forms come to be in the disparate locations.? For land animals, a number of answers have been suggested over the years, all of which seem to be true in some cases:
The intervening space was once inhabited, but the creatures have since become extinct there. This is a well-known phenomenon: for example, species expand their range during climatically favorable times, and then contract during climatic deterioration, leaving relict disjuncts behind in locations that remain suitable. The wood frog (Rana sylvatica), a North American species found in colder areas and high latitudes, has disjunct populations in the Rockies, left behind as the general range shifted north as the climate warmed in interglacial periods (which for the wood frogs was a climatic deterioration).
Disjunction by extinction of intervening populations, while applicable to terrestrial animals separated by land, cannot account for disjunctions across water. This led to a once very popular explanation:
A land bridge connected the disjunct areas, allowing organisms to spread between them, but the bridge has now foundered and sunk beneath the sea. Speculative biogeographers once criss-crossed the oceans with hypothetical land bridges, but most of these land bridges are now seen as geologically improbable and biogeographically unnecessary. Nonetheless, land bridges definitely have existed and subsequently disappeared, and organisms crossed them when they were above water. Mammoths, for example, crossed between Asia and America during times of lower sea level when the Bering Strait became the Bering Land Bridge.
If there never was a land connection, then how did the organisms get from one place to the other?
The organisms dispersed across the water– flying, floating, rafting, swimming, etc.– by what Darwin called “occasional means of transport”. For land masses never connected, this is the only way a disjunction could have arisen, and numerous instances of over water dispersal have been observed, most frequently by flying, but even the rarer forms (e.g., lizards moving from island to island in the West Indies, an Aldabra tortoise coming ashore in Tanzania) are occasionally directly observed.
And finally, there’s a possibility that became popular after the establishment of plate tectonics in geology:
The disjunction was caused by the separation of an initially continuous landmass into two or more parts, so that the geographic distribution of organisms dispersed throughout the landmass became separated as well. In other words, the organisms didn’t move—their continents did. Once geology demonstrated the reality of continental drift, it became clear that a disjunction could occur not by the animals moving around (although they would have to have initially moved around throughout the contiguous primeval landmass), but by pieces of the earth moving around. So, to use a fossil example, the mammal-like reptile Cynognathus of the Triassic was found in both Africa and South America. This stout, meter-long animal was not a good candidate for trans-oceanic travel; but when it was realized that at that time the two continents were joined, the problem disappeared: continental drift accounted for the apparent disjunction.
The problem studied by both Mitchell and Baker is that of the distribution and relationships of the ratites—the group of large flightless birds that includes ostriches, rheas, cassowaries, kiwis, emus, and the very recently extinct moas and elephant birds. All of these birds are primarily found on the southern continents that were once joined together into the super-continent of Gondwana. Here is their present distribution:
Ratites have long been known to be related to one another and to the tinamous, a group of poorly-flying birds from South America. How the various flightless forms got their present distributions has long been a puzzle. Overwater dispersal seemed out (they’re flightless, after all), so perhaps they were once spread across the connecting northern continents, but have since gone extinct there. With the emergence of plate tectonics, a favored hypothesis was that the ancestral ratite was widespread on Gondwana, and, as the super-continent split up during the Cretaceous, the flightless birds on their respective chunks of land each went their own way and evolved into the modern forms. This view was championed by Joel Cracraft, now of the American Museum.
This view, however has come under criticism, with various authors suggesting that the birds actually had dispersed over water (see, for example, this precis of a talk by Mike Dickison from 2008). Both of the new papers, by Mitchell and Baker, support the over-water dispersal view. Baker, using ancient DNA techniques to study the DNA of the extinct New Zealand moas, confirms earlier work by him and his collaborators showing that the closest relatives of moas are the flighted tinamous, thus showing that flightlessness has evolved more than once within the ratites. Mitchell, also using ancient DNA techniques, shows that the Madagascan elephant birds’ closest relatives are the kiwis of New Zealand, and furthermore estimates the time of divergence at 50 million years ago—long after Gondwana had broken up to the point that wide seas separated Madagascar from New Zealand, thus implying that the common ancestor of the two was flighted.
Both research groups conclude that most or all of the distributional pattern of the ratites is due to over water dispersal by flying ancestors, occurring long after Gondwana had broken up into its modern parts, and that flightlessness evolved convergently within the ratites. According to the Times, Cracraft is not quite convinced yet, but thinks further work will soon reveal a fuller story of ratite evolutionary history.
Baker, A.J., Haddrath, O., McPherson, J.D., and A. Cloutier.2014. Genomic support for a moa-tinamou clade and adaptive morphological convergence in flightless ratites. Molecular Biology and Evolution in press.
Cracraft, J. 1973. Phylogeny and evolution of the ratite birds. Ibis 116:494-521.
Mitchell, K.J., Llamas, B. Soubrier, J, Rawlence, N.J., Worthy, T.H., Wood, J., Lee, M.S.Y., and A. Cooper. 2014. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science 344:898-900.