Rocking Charles Darwin’s tree of life

Published in The Hindu on February 12, 2009

Scientists have proposed a web of life as lateral transfer of genes is seen in bacteria, eukaryotes, plants and animals

Charles Darwin’s oak tree with many branches to depicting the way in which one species evolves into many has been the simplest, yet the most effective way to describe how species evolve.

But the tree of life, long considered the very foundation of evolution, is now being rocked by scientists.

Evolutionary biologists with the most modern gadgets at their disposal are using the numerous single and multicellular organisms to question the validity of the tree of life.

It is a different story that 150 years ago Darwin knew nothing of bacteria and other primitive life forms. His theory was based on plants and animals that he could see with his naked eye.

The beginning

The discovery of the DNA structure in 1953 threw open the doors for evolutionary biologists. Using just the RNAs, these scientists made a startling discovery in the 1970s — the archaea — the unicellular organism that was previously thought to be bacteria. Their ability to sequence bacterial and archaea genes since 1990s should have been a scientific way of proving Darwin’s tree of life. But that was not to be. Unexpectedly, this new found ability proved counterproductive for the tree of life.

It, for the first time, challenged the well accepted scientific notion, which was also what Darwin assumed, that organisms passed their genes to their offspring. Instead, what was seen was that bacteria and archaea swapped genes across huge taxonomic distances (New Scientist, January 21, 2009).

Lateral transfer of genes was not the way the tree of life was to be.

The web of life

The idea of a tree came under scrutiny. A web of life that allows for lateral gene transfer was instead proposed.

Though staunch supporters of the tree of life dismissed this observation as some kind of an aberration, the result of a study published last year in the Proceedings of the National Academy of Sciences showed that 80 per cent of bacteria and archaea showed lateral gene transfer.

Evolutionary biologists had the axe ready to cut down the tree of life further. They showed that lateral gene transfer was seen in eukaryotes as well. Eukaryotes — amoeba and algae — are themselves a product of fusion of bacteria and archaea.

The web of life grew bigger in size and began to dwarf the tree of life — unicellular organisms make for at least 90 per cent of all species on earth.

Hybrid genomes

As if eukaryotes, made of one bacteria and one archaea, was not sufficient proof of the lateral transfer of genes, scientists observed that eukaryotes passed some chunks of their genome to their hosts. The result was hybrid genomes.

However, the tree of life was intact as long as the vertical transfer of genes from the parent to the progeny was the only route in plants and animals.

So evolutionary biologists next targeted plants and animals. To some scientists’ dismay, lateral transfers were seen in plants and animals too.

Hybridisation in plants

“Hybridisation clearly plays an important role in the evolution of plants as well,” (New Scientist, January 21, 2009). About 14 per cent of all living plant species, according to Loren Riesberg from the University of British Columbia, “are the products of fusion of two separate lineages.”

If lateral gene transfer was seen in plants, can animals be far behind? Latest studies show that hybridization “has been a major driving force in animal evolution,” (New Scientist). About 10 per cent of all animals have been found to regularly hybridise with other species.

Not much in number

Scientists are beginning to see the lateral transfer of genes in many animal species such as fish and insects, and many more plants. However, unlike microbes, the number of lateral transfer is not so high in plants and animals.

So what does all this mean? “The tree isn’t the only pattern,” biologists W. Ford Doolittle of Dalhousie University in Halifax, Canada, was quoted as saying in New Scientist. “Downgrading the tree of life doesn’t mean that the theory of evolution is wrong — just that evolution is not as tidy as we would like to believe.”

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