NO NEWSPAPER editor, surely, would publish such an obviously ignorant and absurd letter. If he received an article announcing that the Second Law of Thermodynamics was wrong and that the author had built a working perpetual motion machine, the editor would surely consult an expert (or at least somebody with GCSE physics) before publishing it. If a contributor claimed the Romans, Saxons, Danes and Normans never existed - the British being the Lost Tribe of Israel - the editor would reject it from his or her own knowledge of history.
Why then are attacks on evolution regularly published without anyone ascertaining whether they embody elementary misunderstandings? I can only fall back on the great French molecular biologist Jacques Monod: 'Another curious aspect of the theory of evolution is that everybody thinks he understands it]'
This month the Independent published three replies to an article by Jonathan Weiner trailing his book, The Beak of the Finch. The book is an account of the painstaking researches of Peter and Rosemary Grant on 'Darwin's Finches' of the Galapagos Islands. These birds appear to be all descended from a single lineage, perhaps a migrating flock blown off course, which reached the remote archipelago a few million years ago. They have since diverged into 13 species, each one specialised for a particular way of life. The Grants are responsible for what is probably the most thorough documentation of Darwinian natural selection that we have. Their achievement gains added interest from the fact that their birds are descended from the very ones that inspired Darwin, and in the very same place. Their work is important, theoretically sophisticated, long-term and still continuing. It deserves, and receives, admiration from educated professionals.
Two of the replies to Weiner's article reminded me of the almost unique status of evolution as a favoured target of ignorant philistinism. Let me explain the errors patiently and constructively, for they are widespread and recurrent.
'Sir: I have no time for creationists, but . . .' begins the first letter - a sure sign that creationists are precisely what he does have time for. He goes on:
'So the number of finches diminished during a drought, and the birds which survived had, on average, larger beaks. This is hardly surprising, as I assume that the birds with smaller beaks starved.'
I feel like congratulating the writer. You have, in spite of your best efforts, grasped the essence of Darwinism, but please have the courage to follow through. Small, step-by-step changes, as subtle as a beak becoming a tiny bit longer, or a toe becoming imperceptibly shorter, eventually, after thousands of generations, add up to substantial changes. Darwin's mills grind slowly and they grind exceeding small. Natural selection is a relentless slight bias in death rates, or a slight bias in reproductive rates, among the individuals within a species. Given enough time, small changes in lineages accumulate into large differences between species. Given even more time, lineages diverge yet further until we have descendants as different as elephants from moles, or squid from warthogs.
The author of the letter, however, has other ideas on what constitutes Darwinism:
'A main tenet of Darwinist theory is genetic mutations brought on by blind chance. If Mr Weiner is suggesting that blind chance brought about the right kind of beak in 1977 and then in 1983, a trip to the roulette table would be a surprising experience for him.'
I should explain 1977 was a drought year in the Galapagos, and 1983 a year of unprecedented floods. After the drought, a higher proportion of large-beaked finches were found to have survived. After the floods, small-beaked finches predominated, presumably because they were better equipped to pick up the smaller seeds that wet weather favours. Reversals of this kind are common. Long-term evolution will result only if there is a sufficiently drawn-out trend in a particular direction - as there often is, especially when evolving enemies such as predators or parasites are involved rather than just the caprices of the weather.
But let me take up the matter of 'blind chance' and try, yet again, to clear up this ubiquitous misunderstanding of mutation. Mutations are spontaneous, random errors in genes transmitted from one generation to the next. You can call mutation 'blind chance', but all this means is that new mutations are not directed preferentially towards the improvements that are needed. The drought of 1977 did not induce beak-enlarging mutations, nor did the floods of 1983 provoke opposite mutations. The mutations had already happened, regardless of the weather. Droughts and floods enter the story only afterwards. In the role of non-random selecting agents.
Moreover, the mutations involved in natural selection usually occur many generations earlier than the selection itself. There is no suggestion that the large-beaked mutations that were favoured in 1977 actually arose in, or shortly before, 1977. On the contrary, there is a constant pool of variation in the population, fed by continually recurring small mutations. This trickle of mutations, followed by generations of sexual shuffling, ensures that there is always plenty of variation available. Natural selection pressures - the pressures of drought and flood, predators and parasites, rivals and sexual partners - find evolutionary release in the variation already waiting for them. Mutation is only the ultimate provider of variation, just as rain is the ultimate provider of the water in a hydroelectric dam. You don't have to wait for a rainstorm before you can switch on a light.
The second letter is similar, but its author doesn't even pretend to be anything but a creationist.
'When Jonathan Weiner has evidence that a finch has evolved into, say, a parrot (or indeed, an elephant as Darwin expected the birds to have preceded the mammals), let us have an informed article about Darwin's theory of evolution.'
Not that it matters, but the fossils tell us that the first recognisable mammals preceded the first birds by some 70 million years. This fact wouldn't have surprised Darwin for a moment, for he never suggested that birds and mammals were anything but cousins. Elephants are cousins to finches, with a common ancestor who lived more than 300 million years ago. Chimpanzees are not our ancestors, they are our closest cousins, with a common ancestor who lived maybe 7 million years ago. Parrots are not descended from finches, they are cousins of finches. Surely it isn't difficult to understand the difference between an ancestor and a cousin, is it? The same author asserts that there is no evidence that guppies, grass, flies, moths, mice and elephants have a common ancestor, and he attributes this 'error' to 'amateur evolutionists'. Well, I am a professional evolutionist, not without qualifications in the difficult science of biology, and my judgement is that the evidence for this common ancestor is strong. More, it is utterly, swingeingly, overwhelmingly strong: not just beyond reasonable doubt but beyond all sane doubt. Briefly (you might as well try to summarise Gibbon's Decline and Fall in a paragraph), the most telling evidence is that if you examine resemblances among the creatures mentioned, you will find that they all fall into the same branching hierarchical pattern. A natural extension of that pattern accommodates all the other creatures that have ever been looked at. The conclusion is especially persuasive if you look at molecular evidence, mostly because the sheer number of separate pieces of mutually- agreeing evidence is so huge.
The hierarchical tree pattern that builds up has exactly the shape and properties we should expect to see if it is, in fact, a tree of cousinship: a family tree, with all animals and plants ultimately descended from a common ancestor. Where fossils of past animals exist, they always support this conclusion. An alternative explanation is conceivable. It is exactly analogous to Bertrand Russell's humorous conjecture that we all sprang into existence five minutes ago complete with ready-made memories and holes in our socks.
The writer is author of 'The Selfish Gene' (OUP) and 'The Blind Watchmaker' (Penguin), and a fellow of New College, Oxford.
Bryan Appleyard is on holiday.