Thursday, May 16, 2013

Again! Charles Darwin Was Wrong.. About The Tree Of Life

Well.. I guess the headline "Darwin Was Wrong" always sells!

But The Guardian should have thought a little before publishing this. It is an article which appeared sometime back but is the most viewed in the science section and is about how the recognition of lateral gene transfer i.e. the transfer of genes aross taxonomic groups is proving Darwin's idea of a tree of life wrong.

Read this passage:

Evolutionary biologists say crossbreeding between species is far more common than previously thought, making a nonsense of the idea of discrete evolutionary branches.

I don't think their science correspondence Ian Sample thought enough to see the contradiction in this passage.

Cross breeding between species implies that species exist in the first place which in turn means that there has been in existence discrete evolutionary branches i.e. long periods of independent evolution of different populations so as to have accumulated unique features recognizably different from other populations. Its only when there are discrete unique branches is it possible to recognize lateral gene transfer between the two!

For example, coyotes and wolves are closely related animals that occasionally interbreed in the wild. But the fact that we can recognize coyotes from wolves means that these are two discrete lineages having diverged from a common ancestor at some point in the past and have since followed separate evolutionary trajectories accumulating unique traits along the way. Occasional interbreeding between the two does not wipe away all these differences. They remain two distinct branches on the canid family tree.

Gene trees are not the same as species trees. Gene trees reflect the evolutionary history of a gene which may be transferred across taxonomic groups by a variety of processes. Early in the divergent history of populations genes may be exchanged through interbreeding. Or, for example a not so insignificant portion of the genome of mammals is made up of fragments of genes from other species transferred into us long ago by viral infections. However, that does not change the historical fact that the coyote and the wolf, or for that  matter humans and chimps diverged from a common ancestor. Species trees or the tree of life reflects this history of speciation. So, Darwin's tree of life idea is about diverging populations from a common ancestor. These populations despite occasional transfer of genes maintain reproductive integrity and become different enough over time. Life has become diverse over time by such branching. Darwin was not wrong about that.

And what does one make of this confused para- Last year, scientists at the University of Texas at Arlington found a strange chunk of DNA in the genetic make-up of eight animals, including the mouse, rat and the African clawed frog. The same chunk is missing from chickens, elephants and humans, suggesting it must have become wedged into the genomes of some animals by crossbreeding.

Does that mean there was interbreeding between mouse, rats and the African clawed frog? Ridiculous!

Update May 16 2013: There is another potentially misleading passage in the article:

But modern genetics has revealed that representing evolutionary history as a tree is misleading, with scientists saying a more realistic way to represent the origins and inter-relatedness of species would be an impenetrable thicket. Darwin himself also wrote about evolution and ecosystems as a "tangled bank".

Now, the "tangled bank" has nothing to do with lateral gene transfer and interbreeding at all. It should not be conflated with evolutionary relationships being described as a web or a thicket. Darwin was describing the complex inter-dependencies in an ecosystem as organisms compete and co-operate with each other for resources.

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