The Extended Phenotype

Genes synthesize only proteins

In the main portion of the book, 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.

Genes do not affect the organism's body only

It is commonly suggested that there are three types of extended phenotypes. The first refers to the capacity of animals to modify their environment using architectural constructions. Dawkins cited as examples caddis houses and beaver dams.

He then goes further to point to first animal morphology and ultimately animal behaviour, which can seem advantageous not to the animal itself, but rather to a parasite which afflicts it. It is "parasite manipulation", which is the capacity of several groups of parasites to modify the host behaviour to increase their 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. However, a recent 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 by malaria parasites?

The third type of extended phenotype refers to an 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.

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

Gene-centred view of life

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, in the case where natural selection pressures can be modified by the organisms during the evolutionary process.