Charles Darwin’s “Tree of Life” metaphor for evolution is wrong.
So says NEW SCIENTIST this month in a clearly written, startling article that seems to undermine one of Darwin’s central insights.
The problem is HGT (horizontal gene transfer). Instead of a tree of evolution, it appears that we are living in an evolutionary WEB where genetic information does not just pass, in a linear fashion, from parents to offspring, but drifts across species by such means as viruses and interbreeding:
The problems began in the early 1990s when it became possible to sequence actual bacterial and archaeal genes rather than just RNA. Everybody expected these DNA sequences to confirm the RNA tree, and sometimes they did but, crucially, sometimes they did not. RNA, for example, might suggest that species A was more closely related to species B than species C, but a tree made from DNA would suggest the reverse.
Which was correct? Paradoxically, both – but only if the main premise underpinning Darwin’s tree was incorrect. Darwin assumed that descent was exclusively “vertical”, with organisms passing traits down to their offspring. But what if species also routinely swapped genetic material with other species, or hybridised with them? Then that neat branching pattern would quickly degenerate into an impenetrable thicket of interrelatedness, with species being closely related in some respects but not others.
We now know that this is exactly what happens. As more and more genes were sequenced, it became clear that the patterns of relatedness could only be explained if bacteria and archaea were routinely swapping genetic material with other species – often across huge taxonomic distances – in a process called horizontal gene transfer (HGT).
At first HGT was assumed to be a minor player, transferring only “optional extra” functions such as antibiotic resistance. Core biological functions such as DNA replication and protein synthesis were still thought to be passed on vertically. For a while, this allowed evolutionary biologists to accept HGT without jeopardising their precious tree of life; HGT was merely noise blurring its edges. We now know that view is wrong. “There’s promiscuous exchange of genetic information across diverse groups,” says Michael Rose, an evolutionary biologist at the University of California, Irvine.
This is all well and good for single-celled organisms, but Darwin was looking at multicellular organisms when he was coming up with his Tree of Life metaphor.
Does the tree still work for multicellular organisms?
NEW SCIENTIST says that even there the situation is more complicated:
[C]ases 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 (New Scientist, 27 August 2008, p 38). The same is probably true of the genomes of other big animals. “The number of horizontal transfers in animals is not as high as in microbes, but it can be evolutionarily significant,” says Bapteste.
Nobody is arguing – yet – that the tree concept has outlived its usefulness in animals and plants. While vertical descent is no longer the only game in town, it is still the best way of explaining how multicellular organisms are related to one another – a tree of 51 per cent, maybe. In that respect, Darwin’s vision has triumphed: he knew nothing of micro-organisms and built his theory on the plants and animals he could see around him.
Even so, it is clear that the Darwinian tree is no longer an adequate description of how evolution in general works. “If you don’t have a tree of life, what does it mean for evolutionary biology?” asks Bapteste. “At first it’s very scary… but in the past couple of years people have begun to free their minds.” Both he and Doolittle are at pains to stress that 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. Some evolutionary relationships are tree-like; many others are not. “We should relax a bit on this,” says Doolittle. “We understand evolution pretty well – it’s just that it is more complex than Darwin imagined. The tree isn’t the only pattern.”
Others, however, don’t think it is time to relax. Instead, they see the uprooting of the tree of life as the start of something bigger. “It’s part of a revolutionary change in biology,” says Dupré. “Our standard model of evolution is under enormous pressure. We’re clearly going to see evolution as much more about mergers and collaboration than change within isolated lineages.”
Mergers and collaboration.
In other words, everybody is fucking everybody.
Life is one big “hippy love” promiscuous orgy web of genetic exchange.
I like it.
It feels deeply ecological, and might give humans an even more deeply embedded psychological connection to all of life on earth.
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