Evolution - the eyes have it

Another book by Richard Dawkins so soon after his best-selling River Out of Eden. What more is there to say about evolution and the transfer of genes through the replication of DNA after The Selfish Gene and The Blind Watchmaker? In Climbing Mount Improbable we have more of the same familiar themes with his reductionist approach to life, but also many new examples and expansions of earlier ones recounted with journalistic flair. Dawkins's fourth major book promoting Darwinian evolution is as readable and as fascinating as the previous ones. Here is a text with the fervour and enthusiasm for which he is well-known and which led to his appointment to the first chair of public understanding of science at Oxford University.

The central parable for this new book is a snow-capped mountain with forbidding cliffs on one side and forgiving slopes and well-worn footpaths on the other. This is used to show that evolution is a gradual process up the easy side of the mountain and not of sudden leaps up the cliff face. The process of natural selection is continuous and slow as random mutations which are advantageous to an organism are selected and gradually change the way a spider weaves a web or the number of wings on an insect.

Dawkins, with his usual evangelical atheistic approach, feels that only a mad God would attempt the task of leaping up the precipice in a single bound. He has shown through many examples that evolution has followed the gentle slope up the mountain; all but a few scientists who are extreme exponents of punctuated equilibrium would agree. In spite of taking the tedious route up the mountain, evolution has reached many peaks, and it is some of these that are so lucidly described here.

The first example chosen is that of mimicry where one organism has evolved to resemble another unrelated one such as beetles that look like ants and live in ants' nests. The leafy sea dragon is a sea horse that closely resembles the seaweed in which it lives. These are given as examples of organisms that look as if they have been designed but have actually come about through the gradual process of evolution. Dawkins terms these products of nature "designoid". Other examples of such well-designed objects include the pitchers of carnivorous plants that trap and digest insects. Yet the pitcher is also a home to a rich community of maggots which are able to survive without being digested and thrive in the digestive liquid of the pitcher because it is so well aerated by the plant.

In convergent evolution, however, different organisms evolve to a similar shape because that shape is most appropriate for their ecosystem and way of life. Examples given include a series of hedgehog-like animals and the independently evolved streamlining of oceanic animals such as the bottlenose dolphin, the blue marlin, the Galapagos penguin and even the long-extinct ichthyosaur.

To demonstrate these processes we are reintroduced to computer bimorphs that featured in The Blind Watchmaker. Through the modelling process of inheritance of genetic characters and randomly introduced genetic mutations the computer program can demonstrate the gradual process of natural selection as an organism advances up the slope of Mount Improbable to produce a most unlikely but highly functional design such as a pitcher or an ant-beetle.

The second chapter demonstrates through the evolution of spider webs what was demonstrated by the evolution of echo location in bats, in The Blind Watchmaker. Spiders webs are the most elaborate and sophisticated structures and so it is hard to contemplate how such a skill evolved. A typical orb-weaving spider produces six different kinds of silk from the nozzles on its rear end, each made by separate glands. The spider is able to switch on and off the appropriate silk as it goes about the construction of its web. Such a process could not have come about in a single step and we are taken through both the process of evolution and the construction of a web. Spiders are truly fascinating animals. In some species the male plucks the threads much as one might pluck a harp. The vibrations lure the female along a mating thread to the source of the twanging. After mating, many female spiders eat their mate, but to Dawkins this is a happy ending because the male has performed his task and successfully passed on his genes to the next generation and even helped to nourish his offspring. For anyone who is dubious about spiders this whole chapter is compulsory reading.

We are also introduced to a computer program of artificially generated webs bombarded with computer flies in order to analyse the efficiency of different web designs and demonstrate how natural selection has produced webs that are more efficient in catching flies. We are taken through the evolution of the elephant's trunk and the modern day textbook demonstration of industrial melanism, where moths in the industrial heart of England changed from light coloured to dark through selection favouring those that matched the darker colour of smoke-polluted trees.

In chapter five, entitled "The forty-fold path to enlightenment", Dawkins returns to one of his favourite themes, the evolution of eyes. Sight and the eye are concepts that creationists have found impossible to conceive as the result of evolution. However, eyes have evolved independently many times as the chapter heading indicates. The human eye and the insect eye are entirely different in origin but both provide the ability to see and recognise objects. Perhaps Dawkins dwells on the eye because it was the one structure about which even Darwin was not always confident. Darwin wrote in a letter: "The eye, to this day, gives me a cold shudder, but when I think of the fine known gradation, my reason tells me I ought to conquer the cold shudder." Dawkins has no such doubts and has produced another graphic account of the evolution of sight which also climbed the gradual slopes of Mount Improbable.

As a botanist I am glad to see that a plant, the fig, takes prime position in the book, featuring in the first and last chapters. The narrative begins with an account of a lecture on the fig in literature which an unfortunate speaker delivered in Dawkins's presence. The lecturer included the story of Adam and Eve in the Garden of Eden and suggested that the tree of knowledge was a fig rather than an apple. Dawkins is quick to rebuke the speaker for his literal-mindedness and this seems to have challenged him to use the fig as the ultimate example of the pathway up Mount Improbable. Dawkins says: "There is a genuine paradox and real poetry lurking in the fig, with subtleties to exercise an inquiring mind and wonders to uplift the aesthetic one."

One cannot help feeling sorry for the poor lecturer who was only pursuing his chosen field of literature, but the fig is indeed a wonder of nature's design. The fig may appear like a fruit, but it is a cluster of flowers turned inside out and surrounded by a fleshy case. Dawkins compares it to a garden, which to me underestimates its marvels. There are about 900 species of fig, each with its own species of wasplet pollinator. The male wasp passes its entire life within a fig and only the female emerges loaded with pollen to fly on to another fig. The pollen load is no casual dusting as it is on most insect pollinators - it is gathered by the fig wasps to pollinate the flowers inside the new fig entered by the wasps. There are many variations in the different species of figs and wasps, and even parasitic wasps that prey on the real pollinators. Figs and their wasps are the epitome of co-evolution where two organisms have evolved "in step with each other and out of step with other fig and wasp species". They represent a spectacular pinnacle of Mount Improbable and they have reached their present status through a long and tortuous evolutionary pathway.

There are no more powerful explanations of the process of evolution than Richard Dawkins's books and any rational biologist accepts evolution and natural selection as the processes through which the organisms he discusses have come about. I am convinced about evolution but my faith also remains undiminished. How can Dawkins know that God is not involved in the process of evolution? Why can the origins of the unseen not be explained by a Creator? To me there is still a divine design in the complexity and diversity by which we are surrounded.

The reductionist approach of Dawkins primarily focuses back to the molecule of DNA, and although he uses many examples from the work of naturalists and ecologists, he ignores much of their findings on ecosystem dynamics and social systems among the component organisms.

We may expect a further book soon since Dawkins concludes: "Evolution is an enchanted loom of shuttling DNA codes whose evanescent patterns, as they chase their partners through geological deep time, weave a massive database of ancestral wisdom, a digitally coded description of ancestral worlds and what it takes to survive in them. But that is the train of thought that must wait for another book."

I for one will be one of the first to obtain such a volume because, even though I would like to see more of a naturalist's approach, each new book adds to the understanding of biology which is so vital as the world addresses the environmental problems that we now face.

Sir Ghillean Prance is director, Royal Botanic Gardens, Kew.