The Extended Phenotype
|The Extended Phenotype|
|Publisher||Oxford University Press|
|Dewey Decimal||575 20|
|LC Classification||QH375 .D38 1983|
|Preceded by||The Selfish Gene|
|Followed by||The Blind Watchmaker|
The Extended Phenotype (EP) is a biological concept introduced by Richard Dawkins in a 1982 book with the same title. The main idea is that phenotype should not be limited to biological processes such as protein biosynthesis or tissue growth, but extended to include all effects that a gene has on its environment, inside or outside of the body of the individual organism.
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 EP. 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 EP 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 EP 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:
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.
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."
- Dawkins, Richard (1989). The Extended Phenotype. Oxford: Oxford University Press. p. xiii. ISBN 0-19-288051-9.
- D. P. HUGHES, J. BRODEUR and F. THOMAS (2012) Host manipulation by parasites. OXFORD.
- From The World of Richard Dawkins
- Video of cricket exhibiting behavioral extended phenotype of parasite