BOOK REVIEW / The pecking order: 'The Beak of the Finch' - Jonathan Weiner: Cape, 18.99

EVOLUTIONISTS face an insurmountable logical problem when trying to persuade sceptics of the scientific truth of their theories. Darwin's 'Tree of Life', with its 20 or 30 million branches, grew up in the past. There is no way any of its experiments - not even the development of blood corpuscles, let alone the emergence of the dinosaurs - is ever likely to be repeatable, according to classic scientific method. As an explanation of the past, evolutionary theory is closer to the proceedings of a judicial trial than to science, and has to rely on weight of circumstantial evidence rather than outright 'proof'.

But evolution as an on-going process is another matter. Jonathan Weiner's powerful and elegant book is subtitled 'Evolution in Real Time'. It is a meditation on Darwinism, from its beginnings to our current planetary crisis, when mutations and extinctions are happening at a furious rate. But at its core is a study of the changes that are still happening to the 13 finch species that inhabit the Galapagos Islands. They are famous (and fabled) birds, whose eccentric adaptations to the raw, unformed habitats of these young volcanic islands gave Darwin one of the crucial clues in the development of his theory of 'the Origin of Species by means of Natural Selection'. Some are cactus finches. They nest in cactus, drink cactus nectar, eat cactus flowers and seeds. Others strip bark, chew leaves, feed on ticks from the backs of iguanas. Their chief visible difference, understandably, is in the size and shapes of their feeding tools - their beaks.

Weiner's heroes are two doughty English-born scientists, Peter and Rosemary Grant, who have been affectionately logging the finches' births, deaths and individual quirks for more than two decades. In that time they have seen massive swings in the islands' weather and environmental conditions, with corresponding effects on the birds. During a relentless drought in 1976-77, for instance, only one in seven of the finches on the island of Daphne Major survived. And in that short time the average beak size of the larger seed-eating finches increased by half a millimetre. Variations too small to see with the naked eye had given those birds the edge in breaking into the last, forbiddingly hard, sun-baked caltrops seeds.

It was a predictable enough example of natural selection, of the 'Generation of Diversity (GOD)' in action, but hardly the Origin of Species. Yet Weiner writes persuasively, using language that is itself infused with evolutionary ideas and structures (he talks of evolution as 'an editor of beaks and bodies'). He shows how these tiny acts of selection under environmental stress mark the first steps in divergence, in the forking of the road that leads to species, as favoured strains move apart to new habitats and niches.

All this is easy to argue for minute changes in beak shape. But behind every beak are complex muscles, groups of neurotransmitter chemicals to give them grinding messages, the instinct for beak-use encoded in the brain. And just a few millimetres away is the eye of the finch . . . There is no reason why complex organs like this shouldn't also be perfected by natural selection. But those who have tried to do the sums have found it hard to marry the astronomical number of selections necessary with the few billion years available. Other processes must have been going on in the business of generating the variation: short-cuts, symbioses turning into amalgamation, periods of accelerated mutation.

This enthralling process occupies rather less of the book, unfortunately, than the more obvious material on selection. It is a beautiful but unsettling story, of hybridisation (the Galapagos finches are trying this out the whole time), and of what Lewis Thomas called 'The Wonderful Mistake', DNA's fortunate habit of making self-copying slip-ups. But it is also about how environmental pressure can cause mutations as well as urging selection from them. Bacteria mutate even when the temperature changes. When their development is driven by the challenge of antibiotics they can produce resistant strains in a matter of days. And insect species forever being drenched with agricultural poisons are not just developing resistance but incorporating and recycling the pesticides as boosters to their own predatory systems. One possible end of this road is the mutation that dare not speak its name - an airborne form of Aids.

Evolutionary biologists can be the most dogmatic of scientists (adapting themselves, perhaps, to environmental pressure from wild Creationists). And this has tended to overemphasise the darker side of Darwinism, its determinism and competitiveness and roots in the Victorian mercantile ethos. Weiner is a gentler species of naturalist - open-minded, lyrical, funny. In his eloquent last chapter he looks into the future, and at the way we are now the overriding selection pressure on the living world, driving evolution - and mainly in a downward direction: 'we hold all the fruits of the planet in our beaks'. But he never ignores real creatures, or those other great engines of creative change, learning and cooperation, cultural evolution. He talks of the African chimpanzees that are teaching their young to crack nuts, and which might one day evolve towards the thinking niche, if we didn't occupy it already; and about the finches that flew to Rosemary Grant as a refuge when an owl wafted past. The watchmaker may be blind, as Richard Dawkins has put it, but the parts of the watch are blessed with individuality and inventiveness.