Chameleon's ocean crossing baffles biologists
ABC Online 15/02/2002
The chameleon, a creature better known for changing its colour than for swimming, crossed
the ocean millions of years ago from Madagascar, leaving biologists baffled as to how the
little lizard did the trick.
The "Out of Madagascar" exploit is suggested in a piece of DNA detective work by
scientists led by Christopher Raxworthy at the American Museum of Natural History in New
York.
A major evolutionary theory, called vicariance biogeography, says that when a species is
split up into two or more groups by a natural catastrophe - such as being separated by a
mountain range thrown up by seismic activity - each group tends to evolve differently.
Successive generations select the genes that give them the best chance of survival and
reproduction in the local environment.
Over tens of millions of years, the divergence can be enormous, in some cases giving rise
to species that hardly resemble each other at all.
Raxworthy's team puzzled over how this theory could apply to the chameleon, an ancient
species whose origins remain mysterious.
Almost all chameleons live in Madagascar, but they have cousins in the Seychelles and the
Comoros, in India, Sri Lanka, the Middle East and even southern Europe.
Under the vicariance theory, the ancestors of all chameleons today would once have lived
on Gondwana - the name given to a "supercontinent" that existed more than 140
million years ago and consisted of the major continents, all glommed together.
As the continents were pulled apart, gradually forming the entities that would later
become Asia, Europe, Africa, Australia and so on, the chameleon community too would be
separated, and each group would head its separate evolutionary way.
But Raxworthy carried out a genetic ID check of 52 chameleon clans - 40 per cent of all of
this lizard's species - and took a detailed look at their anatomy.
As DNA tends to evolve at a steady rate, a phenomenon called the molecular clock, it was
clear that the 52 chameleon species could not have been split up so long ago, the
researchers write in the latest edition of Nature, the British science weekly.
In addition, they have must have dispersed from their home several times, rather than just
once, to achieve their present distribution.
The big question, then, is how the chameleon managed to get out of the island of
Madagascar, given that it is not a terrific swimmer.
Olivier Rieppel, a geologist at the Field Museum in Chicago, suggests there could have a
"land bridge" across the Mozambique Channel between 45 million and 26 million
years ago that could have enabled some plucky lizard pioneers to cross into Africa.
Even so, "how chameleons managed to disperse across the ocean must remain a matter of
speculation", he says.
Some chameleons could have hitched a ride on trees uprooted by storms, or perhaps migrated
on sandbars or other pieces of land that were detached by coastal erosion, he suggests
END
OF REPORT
Back |
|
