Uprooting Darwin's Tree of Life?


The bicentennial of evolutionary theorist Charles Darwin's birth will be celebrated on February 12th. My personal celebration got started early with a visit to the excellent Darwin exhibit at the American Museum of Natural History in New York City three years ago. One spine-tingling moment for me was viewing the famous page from Darwin's 1837 notebooks in which he first outlined his idea of the evolutionary tree of life.

In Darwin's schematic, ancestral species are at the bottom with later species diverging from the trunk into a branching tree of descendants. A 150 years after the publication of Darwin's On the Origin of Species, some biologists are questioning whether the tree of life really exists. A fascinating article with a bit of an overhyped title –"Darwin Was Wrong"—in the New Scientist looks at how advances in molecular biology are turning the tree of life into a web of life. 

As researchers began comparing genes between species, especially among microbial species such as bacteria, amoebas, and archaea, they found that these creatures promiscuously engage in horizontal gene transfer (HGT). As the New Scientist reports:

The true extent of HGT in bacteria and archaea (collectively known as prokaryotes) has now been firmly established. Last year, [researchers] examined more than half a million genes from 181 prokaryotes and found that 80 per cent of them showed signs of horizontal transfer.

So the bottom of the tree of life is a messy web of life. But HGT doesn't end with single cell microbes. As more and more plant and animal genomes have been sequenced, researchers have discovered that multicellular creatures also experience horizontal gene transfer. It turns out the hybridization, cross-breeding between species, is a major force driving evolution. For instance, wheat is the result of the combination of the genomes of three different ancestral grass species. And our species may be the result of interbreeding between the ancestors of chimpanzees and earlier human species.

Viruses also carry genes between multicellular species. As the New Scientist notes:

Other cases of HGT in multicellular organisms are coming in thick and fast. HGT has been documented in insects, fish and plants, and a few years ago a piece of snake DNA was found in cows. The most likely agents of this genetic shuffling are viruses, which constantly cut and paste DNA from one genome into another, often across great taxonomic distances. In fact, by some reckonings, 40 to 50 per cent of the human genome consists of DNA imported horizontally by viruses, some of which has taken on vital biological functions.

Nevertheless, Darwin's central insight regarding natural selection and descent with modification as being the origin of species remains firmly grounded. The New Scientist article concludes:

…the tree concept could become biology's equivalent of Newtonian mechanics: revolutionary and hugely successful in its time, but ultimately too simplistic to deal with the messy real world.

One final note: The fact of promiscuous and natural horizontal gene transfer between species should undermine the claims of anti-biotech activists. For example, the activist consortium, Consumers International's scientifically ignorant "Say No to GMO" campaign states: 

Genetic modification (GM) is a major change in food production; Genes are transferred between unrelated species, for example from animals to plants. This technology makes it possible to break species boundaries set up over millions of years….

Well, yes. But we now know that what biotech scientists are doing is what nature as been doing for millions of years, just doing it more precisely and with the aim of benefiting our species. We don't have to wait around for nature to randomly mix up the genomes of three grass species to produce a useful crop variety. 

In any case, it is definitely worth your while to read the whole New Scientist article here.