A Reason for Everything by Marek Kohn

One night on the Atlantic in the 1830s, when he still believed in the Almighty, Charles Darwin wrote in his diary, "I have worked all day at the produce of my plankton net. Many of these creatures so low in the scale of nature are most exquisite in their forms and rich colours. It creates a feeling of wonder that so much beauty should be apparently created for so little purpose."

Beauty and purpose: their relationship in living organisms is this book's key question. A Reason for Everything focuses on the fortunes of "Darwin's dangerous idea": adaptation by natural selection. Individual organisms inherit features that tend to increase their descendants: they are "designed" by adaptation. No internal force produces adaptation; everything happens by selection. There is no divine designer either, yet adaptation by selection is an ideal creator-substitute.

When hardline adaptationists look at a greenfinch, they see design in everything the bird is and does, "a reason for everything". Others, led by Stephen Jay Gould, say some characteristics have no adaptive value, and therefore did not come about through selection. They are by-products of change. Evolution is untidy, random. Some characteristics have reasons, but not all.

Marek Kohn illuminates the background of this debate through a dancing blend of biography and science. To explain why British scientists have taken the adaptationist side, he focuses on six who have influenced evolutionary thought: Alfred Russel Wallace, RA Fisher, JBS Haldane, WD Hamilton, "presiding genius of modern Darwinism", John Maynard Smith - Kohn's mentor at Sussex, where he studied biology - and Richard Dawkins, the only one whose inspiration did not come from going into the field. Their minds ranged from the intuitive to the mathematical: an important sub-motif here is the critical and increasing contribution that maths makes to biology.

Kohn expounds the science - evolutionary thinking about the ageing process, about the spare energy a gazelle demonstrates, bounding away from a predator - with lively examples. What, for instance, is the evolutionary purpose of sex? Northern European dandelions reproduce asexually. Their flaunting blossoms are purposeless. A few southern ones linger whose flowers are sexually active, but asexual reproducers make great colonisers. So why bother with sex, with flowers?

He argues, to put it crudely, that British biology came from middle-class boyhoods spent in the pre-1914 countryside. Darwin and his plankton, Wallace and his beetles, were basically boys on bicycles with butterfly nets. Continental scientists, more philosophical and abstract, emigrated to America and engendered speculative thinkers like Gould.

But some of today's greatest field biologists, such as George Schaller, are American. Kohn's thesis is oddly ungrounded in the wider history and philosophy of science. Why particular minds turn to particular shapes of theory at particular moments in a particular culture is a fascinating subject - and it is a subject. Thomas Kuhn is not in Kohn's bibliography.

Science has been hot for a "reason for everything" from the very beginning: it is the desire behind all science. Pre-Socratics found their reason for everything in water, fire, or war between love and strife; Hippocratics in the balance of humours or "coming-in things". Gould's banner-wavings about by-product, randomness and untidiness are his own ultra-sophisticated version of the same thing.

The war between the abstract and the empirical long pre-dates evolutionary theory, and Brits did not always back empiricism. When a Dutchman, the 17th-century lens-grinder Leeuwenhoek, wrote to the British Royal Society to say he had seen tiny live animals scraped off his teeth, most of them thought him mad. He showed them his microscope in 1677; he later wrote to say that hot coffee banished these animals from his teeth.

They elected him; but how could they know they had been told that heat kills germs? No one connected these silly little animals with disease until Pasteur.

There is more to British imagination than butterfly nets. "From 1836 to 1839 I took much delight in Wordsworth and Coleridge," wrote Darwin. "But in my excursions from the Beagle, when I could take only a single small volume, I always chose Milton." Kohn does not mention Gillian Beer's revelations: how structures of fiction, too, with powerful underlying assumptions about beauty and purpose, shaped 19th-century evolutionary theory.

Still, it makes a nice story; the biographies are written engagingly, and the science beautifully, with enormous love for biology. "It is an impassioned science," says Kohn. "There is a grace and art to it." So there is to his own book.

Ruth Padel's latest book of poems, 'The Soho Leopard', is short-listed for the T S Eliot Prize