The Evolution of Everything: How Small Changes Transform Our World. Matt Ridley

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      I’m now going to tell you that you have not gone far enough. God is not the only skyhook. Even the most atheistic scientist, confronted with facts about the genome, is tempted into command-and-control thinking. Here’s one, right away: the idea that genes are recipes patiently waiting to be used by the cook that is the body. The collective needs of the whole organism are what the genes are there to serve, and they are willing slaves. You find this assumption behind almost any description of genetics – including ones by me – yet it is misleading. For it is just as truthful to turn the image upside down. The body is the plaything and battleground of genes at least as much as it is their purpose. Whenever somebody asks what a certain gene is for, they automatically assume that the question relates to the needs of the body: what is it for, in terms of the body’s needs? But there are plenty of times when the answer to that question is ‘The gene itself.’

      The scientist who first saw this is Richard Dawkins. Long before he became well known for his atheism, Dawkins was famous for the ideas set out in his book The Selfish Gene. ‘We are survival machines – robot vehicles blindly programmed to preserve the selfish molecules known as genes,’ he wrote. ‘This is a truth that still fills me with astonishment.’ He was saying that the only way to understand organisms was to see them as mortal and temporary vehicles used to perpetuate effectively immortal digital sequences written in DNA. A male deer risks its life in a battle with another stag, or a female deer exhausts her reserves of calcium producing milk for her young, not to help its own body’s survival but to pass the genes to the next generation. Far from preaching selfish behaviour, therefore, this theory explains why we are often altruistic: it is the selfishness of the genes that enables individuals to be selfless. A bee suicidally stinging an animal that threatens the hive is dying for its country (or hive) so that its genes may survive – only in this case the genes are passed on indirectly, through the stinger’s mother, the queen. It makes more sense to see the body as serving the needs of the genes than vice versa. Bottom–up.

      One paragraph of Dawkins’s book, little noticed at the time, deserves special attention. It has proved to be the founding text of an extremely important theory. He wrote:

      Sex is not the only apparent paradox that becomes less puzzling the moment we learn to think in selfish gene terms. For instance, it appears that the amount of DNA in organisms is more than is strictly necessary for building them: a large fraction of the DNA is never translated into protein. From the point of view of the individual this seems paradoxical. If the ‘purpose’ of DNA is to supervise the building of bodies it is surprising to find a large quantity of DNA which does no such thing. Biologists are racking their brains trying to think what useful task this apparently surplus DNA is doing. From the point of view of the selfish genes themselves, there is no paradox. The true ‘purpose’ of DNA is to survive, no more and no less. The simplest way to explain the surplus DNA is to suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines created by the other DNA.

      One of the people who read that paragraph and began thinking about it was Leslie Orgel, a chemist at the Salk Institute in California. He mentioned it to Francis Crick, who mentioned it in an article about the new and surprising discovery of ‘split genes’ – the fact that most animal and plant genes contain long sequences of DNA called ‘introns’ that are discarded after transcription. Crick and Orgel then wrote a paper expanding on Dawkins’s selfish DNA explanation for all the extra DNA. So, at the same time, did the Canadian molecular biologists Ford Doolittle and Carmen Sapienza. ‘Sequences whose only “function” is self-preservation will inevitably arise and be maintained,’ wrote the latter. The two papers were published simultaneously in 1980.

      It turns out that Dawkins was right. What would his theory predict? That the spare DNA would have features that made it good at getting itself duplicated and re-inserted into chromosomes. Bingo. The commonest gene in the human genome is the recipe for reverse transcriptase, an enzyme that the human body has little or no need for, and whose main function is usually to help the spread of retroviruses. Yet there are more copies and half-copies of this gene than of all other human genes combined. Why? Because reverse transcriptase is a key part of any DNA sequence that can copy itself and distribute the copies around the genome. It’s a sign of a digital parasite. Most of the copies are inert these days, and some are even put to good use, helping to regulate real genes or bind proteins. But they are there because they are good at being there.

      The skyhook here is a sort of cousin of Locke’s ‘mind-first’ thinking: the assumption that the human good is the only good pursued within our bodies. The alternative view, penetratingly articulated by Dawkins, takes the perspective of the gene itself: how DNA would behave if it could. Close to half of the human genome consists of so-called transposable elements designed to use reverse transcriptase. Some of the commonest are known by names like LINEs (17 per cent of the genome), SINEs (11 per cent) and LTR retrotransposons (8 per cent). Actual genes, by contrast, fill just 2 per cent of the genome. These transposons are sequences that are good at getting themselves copied, and there is no longer a smidgen of doubt that they are (mostly inert) digital parasites. They are not there for the needs of the body at all.

      Junk is not the same as garbage

      There is a close homology with computer viruses, which did not yet exist when Dawkins suggested the genetic version of the concept of digital parasitism. Some of the transposons, the SINEs, appear to be parasites of parasites, because they use the apparatus of longer, more complete sequences to get themselves disseminated. For all the heroic attempts to see their function in terms of providing variability that might one day lead to a brave new mutation, the truth is that their more immediate and frequent effect is occasionally to disrupt the reading of genes.

      Of course, these selfish DNA sequences can thrive only because a small percentage of the genome does something much more constructive – builds a body that grows, learns and adapts sufficiently to its physical and social environment that it can eventually thrive, attract a mate and have babies. At which point the selfish DNA says, ‘Thank you very much, we’ll be making up half the sequence in the children too.’

      It is currently impossible to explain the huge proportion of the human genome devoted to these transposons except by reference to the selfish DNA theory. There’s just no other theory that comes close to fitting the facts. Yet it is routinely rejected, vilified and ‘buried’ by commentators on the fringe of genetics. The phrase that really gets their goat is ‘junk DNA’. It’s almost impossible to read an article on the topic without coming across surprisingly passionate denunciations of the ‘discredited’ notion that some of the DNA in a genome is useless. ‘We have long felt that the current disrespectful (in a vernacular sense) terminology of junk DNA and pseudogenes,’ wrote Jürgen Brosius and Stephen Jay Gould in an early salvo in 1992, ‘has been masking the central evolutionary concept that features of no current utility may hold crucial evolutionary importance as recruitable sources of future change.’ Whenever I write about this topic, I am deluged with moralistic denunciations of the ‘arrogance’ of scientists for rejecting unknown functions of DNA sequences. To which I reply: functions for whom? The body or the sequences?

      This moral tone to the disapproval of ‘so-called’ junk DNA is common. People seem to be genuinely offended by the phrase. They sound awfully like the defenders of faith confronted with evolution – it’s the bottom–up nature of the story that they dislike. Yet as I shall show, selfish DNA and junk DNA are both about as accurate as metaphors ever can be. And junk is not the same as garbage.

      What’s the fuss about? In the 1960s, as I mentioned earlier, molecular biologists began to notice that there seemed to be far more DNA in a cell than was necessary to make all the proteins in the cell. Even with what turned out to be a vastly over-inflated estimate of the number of genes in the human genome – then thought to be more than 100,000, now known to be about 20,000 – genes and their control sequences could account for only a small percentage of the СКАЧАТЬ