The Single Helix, by Steve Jones

Easy pieces on science's hard laws

Humanity breaks a golden rule of science known as Zipf's law, named after a Harvard professor of linguistics. The textbooks define the law as the observation that the frequency of occurrence of some event (P), as a function of the rank (i) when the rank is determined by the above frequency of occurrence, is a power-law function Pi ~ 1/iª with the exponent "a" close to unity. There is by now a good chance that some of you may no longer be reading, because there is another rule of science writing. It states that you will lose about half your readers for every equation or mathematical explanation you include in a piece of text.

The difficulty with writing about science is that science itself is difficult. For most of us, equations typify the brutal hardness of scientific discovery, even though the purists would say that if you can't count it, it's not science.

Steve Jones's approach is to fill his explanations with anecdotes, fascinating asides and the odd witticism for which he is famous. In this anthology, drawn largely from his newspaper column, Jones manages to ruminate on some of the potentially more demanding aspects of science without appearing to require any great intellectual effort on the part of the reader. Learning about science is no chore when you read Steve Jones.

One way of explaining Zipf's law is to say that the probability of occurrence of words in a book, or many other items, starts high and tapers off. A few occur very often; many occur rarely. The law applies to many aspects of economics, and also in biology. There are more mice than elephants and more sparrows than albatrosses. Whatever group of animals you look at, the larger ones tend to be rarer.

This is a useful concept because if you then look at the total amount of any one animal - the total weight of mouse or elephant in the world - you get a good measure of where it sits in the overall scheme of things. Mice, rats and other small land animals are a hundred times more common than small birds of about the same size. Farm animals break the Zipfian guidelines because 95 per cent of animal flesh in Britain is not buzzard, bat or badger but sheep, pig and cow, says Jones. Pheasants - farmed in all but name - are a hundred times more common than they should be.

The reason the human population breaks the golden rule of Zipf is that, for an animal with our body weight, we are about 10,000 times more abundant than we should be. It is a stark reminder of just how odd we are in terms of the other species that share our living space.

There is no unifying theme running through the 100 miscellaneous items of The Single Helix - a name that salutes both the snail and the immortal coil of DNA that Jones studies - except the search for truth through the entertaining medium of the written word. Whether it is a digression on the p53 cancer gene, or how to measure the speed of light at home, Jones once again shows that, for all its difficulties, science can still be fun.

Steve Connor is science editor of 'The Independent'