The Extended Phenotype

A cathedral termite mound – a small animal with a large extended phenotype

Dawkins argues that the only thing that genes control directly is the synthesis of proteins. He points to the arbitrariness of restricting the idea of the phenotype to apply only to the phenotypic expression of an organism's genes in its own body. Dawkins develops this idea by pointing to the effect that a gene may have on an organism's environment through that organism's behaviour.

Dawkins suggests that there are three forms of extended phenotype. The first is the capacity of animals to modify their environment using architectural constructions. Dawkins cited as examples caddis houses and beaver dams. The second is manipulating other organisms. Dawkins points out that animal morphology, and ultimately animal behaviour, may be advantageous not to the animal itself but, for instance, to a parasite which afflicts it – "parasite manipulation". This refers to the capacity, found in several groups of parasites, to modify the behaviour of the host to increase the parasite's own fitness. One famous example of this second type of extended phenotype is the suicidal drowning of crickets infected by hairworm, a behaviour that is essential to the parasite's reproductive cycle. Another example of such behaviour is seen in female mosquitoes carrying malaria parasites. The mosquitoes are significantly more attracted to human breath and odours than uninfected mosquitoes.[3] A 2013 study shows that an immune challenge with heat-killed Escherichia coli can generate the same changes in the behaviour as is seen in infection by Plasmodium yoelii. It raises an unanswered question: to what extent is the alteration of host behaviour due to active manipulation selected for in malaria parasites?[4]

The third form of extended phenotype is action at a distance of the parasite on its host. A common example is the manipulation of host behaviour by cuckoo chicks, which elicit intensive feeding by the parasitized host birds. These behavioural modifications are not physically associated with the host but influence the expression of its behavioural phenotype.[5]

Dawkins summarizes these ideas in what he terms the Central Theorem of the Extended Phenotype:

An animal's behaviour tends to maximize the survival of the genes "for" that behaviour, whether or not those genes happen to be in the body of the particular animal performing it.[2]

Nests are typical examples of extended phenotypes.

In conducting this argument, Dawkins aims to strengthen the case for a gene-centric view of life, to the point where it is recognized that the organism itself needs to be explained. This is the challenge which he takes up in the final chapter entitled "Rediscovering the Organism." The concept of extended phenotype has been generalized in an organism-centered view of evolution with the concept of niche construction,[6] in the case where natural selection pressures can be modified by the organisms during the evolutionary process.[7]