Richard Dawkins, Edinburgh International Book Festival, august 2005.

Muriel Gray: I’ll dig out the cuttings for you. They’ll be like holiday snaps. Your next book, The Extended Phenotype, was of enormous importance to you. You said that out of all your early work that is the book that you most wanted us to read. It deals with the way in which genetics makes connections with other individuals, not simply as an individual. Why was that so important to you?

 Richard Dawkins: That really was something like an original contribution in a way that The Selfish Gene possibly wasn’t. To explain the idea of The Extended Phenotype, I suppose I have to say what a phenotype is. It is that which is shown—that which is manifest. The genes are inside every cell and they show themselves in their effects on the body, which are the phenotypes. Your eye colour, your hair colour, your nose shape, your behaviour and so on are all phenotypes. To the extent that they’re caused by genes, they are relevant to natural selection. In Selfish Gene terms, natural selection will favour certain genes by virtue of their phenotypic effect. That’s the ordinary phenotype.

The Extended Phenotype recognises that the effects that a gene has—which are important to whether it is successful in being passed on to future generations—don’t have to be limited to the body in which the gene sits. I used the example of animal artefacts—things like spider webs or caddis larvae houses. Caddis larvae are little insect larvae that live in streams and build beautiful little houses for themselves out of stones, sticks or tiny snail shells. It’s like a snail shell; the larva goes around in this house that it has built. The house is built by the animals’ building behaviour, which is under genetic control, as it must be or it couldn’t have been favoured by natural selection. That is the orthodox way of putting it.

The extended phenotype way of putting it is to say that the house itself is the phenotype of the genes. You could do a genetic study of caddis houses. You could do an artificial selection; just as you might select dogs for their long shaggy coats, long noses, snub noses or whatever, you could select caddis houses as the phenotype and it would be exactly as though you were selecting caddises for the length of their antennae or the colour of their legs. The attributes of the house—the features of the house that we can see—are a perfectly good genetic phenotype. That is genuinely unorthodox. It’s a purely logical point. It’s not a point that you could go out and test; it’s obviously got to be true, but it is a different way of looking at biology.

I used caddis houses and animal artefacts as a way of softening up my readership so that they would accept the idea of the extended phenotype. Having done that, I moved on to say that the extended phenotype doesn’t have to be inanimate like sticks and stones. It could actually be another animal. When a parasitic worm sits inside an ant it modifies the behaviour of the ant, in just the same way as the caddis changes the shape of its house. The changed behaviour of the ant, which benefits the worm, is actually a phenotypic effect of genes in the worm.

The reason why it is worth putting it like that is that the changed behaviour of the ant benefits the genes of the worm. The normal behaviour of an ant in the middle of the day would be to go down to the ground to get away from the sun. The worm that is sitting inside the ant needs the ant to be eaten by a sheep so that the worm can pass on to the next stage in its life cycle, which is to live inside a sheep. The worm burrows into the brain of the ant and makes a tiny lesion there, which changes the behaviour of the ant so that it goes up grass stems to the top. That is a suicidal thing for the ant to do in the middle of the day but it’s very good for the worm.

That is the conventional way of putting it. Because that is a Darwinian adaptation for the benefit of the worm, it follows from the logic of Darwinism that it is the genes in the worm that are being benefited. What is the phenotype that the genes are using to get themselves into the next generation—in this case, into the sheep? The phenotype is the behaviour of the ant. The changed behaviour of the ant is part of the phenotype of the worm’s genes. It would be perfectly conventional to say that the change in the ant’s behaviour was part of the phenotype of the ant’s genes, but the extended phenotype logic is that it is part of the phenotype of the worm’s genes.

If you can buy that, the parasite doesn’t actually have to be sitting inside the host. A cuckoo sits in the nest of the host and manipulates the host’s behaviour. It makes the foster parent feed it by having a supernormal bright gape that the foster parent drops food into. That is a manipulation of the host. It is a Darwinian adaptation; the phenotype of the cuckoo genes is the changed behaviour of the host. There is a reaching out from one body to another. A gene in one organism reaches out and changes the behaviour of another organism. The Extended Phenotype is a radically different way of looking at things. You can see how a gene in one organism can have a phenotypic effect in a different organism.

The final step is to say that when a male bird sings, say, the hormones, gonads and ovaries of a female bird on another side of the wood increase in size and secrete hormones. That is an adaptation by the male bird; a phenotypic effect of genes in the male bird influences the phenotype in the female bird.