Every week we open this section with a remarkable picture. Now we offer words just as appealing, intriguing or provocative. Today, an extract from a speech to the Royal Society by Steve Jones, Professor of Genetics at University College London. The subjects: sex, taxes, and slugs ...
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Everyone thinks they know what sex means, but most of us have no idea. We are all obsessed with the erotic (or the engaging) details and neglect what it really means. Sex is 3 billion years old so there hasn't been much chance of seeing it evolve. However, plenty of creatures have given it up, and they may be the clue as to how it began.

I like to think of sex as evolution's attempt to destroy monopolies. It is, in many ways, the same as tax. It allows the capital (biological or financial) of one lineage to be diluted among all members of the population.

When it comes to cash, everyone is in favour of asexual reproduction because it keeps money in the family line. Tax is the sexual reproduction of money. Its counterpart in biology is called recombination.

Perhaps, then, understanding taxes will help us understand sex. There are two views of sex: the capitalist and the socialist - views based on conflict and cooperation, and real parallels between reproduction of money and of genes.

Take 1815, and Meyer Amstiel Rothschild. He became enormously rich by accurately forecasting the outcome of the battle of Waterloo. Some of his children had already married non-relatives. All the rest married their cousins and in the next 100 years 26 out of 30 Rothschilds chose mates from within the family. By marrying relatives who shared the same ancestors they reduced the recombination of their genes with those of other families and (no doubt coincidentally) the distribution of their family.

"Cousin marriage" is common, and getting commoner worldwide - particularly in Islamic countries. It means that children are more likely to have two copies of the same damaged gene descending through two paths of history from a single ancestor, and hence to suffer from genetic illness.

Take Finland, which is made up of small islands of people in a sea of trees. It is a natural laboratory of inbreeding with its own set of inherited diseases, many of which are confined to single villages. Tracing back the parents of those affected shows that all are relatives - perhaps not cousins, but certainly sharing a common ancestor in the recent past. No wonder it has high local frequencies of genetic disease. There is one, of which there are only 50 cases worldwide and all of them are in Finland.

Some creatures are much more incestuous than humans. Slugs are hermaphrodite. Many come in two different forms. A sexual one: boy-girl meets girl-boy; and one in which there is self-fertilisation: the male part of the slug fertilises the female part of itself.

This leads to a complete loss of genetic variation, as all genes become identical by virtue of shared descent. Just like the Rothschilds, the slugs pass on their biological capital unchanged to its offspring.

But why? When does it pay to give up sex to allow a sexual monopoly? Is there a pattern? Giving up sex is much more common in the north. If you draw a line from Liverpool across to The Wash you find slugs are asexual, whereas they are sexual in the south. There is no sex at all when you get as far north as Scandinavia, in spite of rumours to the contrary.

The reason for this is basically economic. In the north the enemies are, predictably, cold weather and a shortage of food. One genetic type evolves which is best at coping with this easily predicted challenge each year.

In the south, the enemies are other creatures - perhaps predators, competitors or disease organisms. These are themselves sexual and can constantly produce new mixtures of genes which threaten the slugs. Just as with capitalism, once competition and innovation get started, you have to join in or go extinct.

There is a beautiful example of the dangers of monopoly in a freshwater snail, Potamopyrgyus jenkinsii, which was imported by accident in a load of ballast in the 1880s from New Zealand to where Thamesmead now stands. Now there are billions throughout Europe, small, black and all female. Back in its home town in New Zealand there are plenty of males in some places, and here they are sexual, but only in ponds, and only because there are parasites that infect them. They have to evolve faster than the parasites by reshuffling their genetic hand of cards through sex - or become extinct.

It serves as a perfect natural experiment on the origin of sex - as a capitalist tool allowing conflict and competition.

All this has dealt with what happens once sex has got started, and that once it has, it cannot be given up in the face of others who are still indulging in it.

There is a new idea, based on DNA repair, on how sex began in the first place. Every time a man has sex he makes enough sperm to fertilise the entire female population of Europe and enough length of DNA to go twice around the world. Chemistry tells us that DNA would quickly be destroyed unless it was carefully repaired every time a cell divides. The DNA repair is far more active in sperm and egg than in body cells, and much more active in females than males: males do the damage; females repair it.

Perhaps this is how it began in the first place - two single strands of DNA got together and compared notes. One was damaged by mutation and could check what repair to make by looking at the DNA of the other one - a sort of genetic proof-reading process. So that is perhaps what sex really means: death for those who indulge in it, as their own genes are destroyed by mutation in their asexually-reproducing body cells, but immortal life for their genes.

Next Wednesday is the deadline for those being taxed under the new self- assessment system. Given what I have said about sex and taxes, think of it as a second honeymoon.