The Extended Phenotype is a 1982 book by the evolutionary biologist Richard Dawkins, in which the author introduced a biological concept of the same name. The book's 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 the body of the individual organism.
Dawkins considers The Extended Phenotype to be a sequel to The Selfish Gene (1976) aimed at professional biologists, and as his principal contribution to evolutionary theory. In the 1999 reissue and subsequent reprintings an afterword by Daniel Dennett is included.
Summary
Genes as the unit of selection in evolution
The central thesis of The Extended Phenotype, and of its predecessor by the same author, The Selfish Gene, is that individual organisms are not the true units of natural selection. Instead, the gene — or the 'active, germ-line replicator' — is the unit upon which the forces of evolutionary selection and adaptation act. It is genes that succeed or fail in evolution, meaning that they either succeed or fail in replicating themselves across multiple generations. Another example is seen in female mosquitoes carrying malaria parasites. The mosquitoes infected with the parasites whose preferred hosts are humans have been shown in a field experiment to be significantly more attracted to human breath and odours than uninfected mosquitoes when the parasites are at a point in their life cycle where they can infect a human target.
left|thumb|A [[Acrocephalus (bird)|reed warbler raising the young of a common cuckoo]]
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 host birds. Here the cuckoo does not interact directly with the host (which could be meadow pipits, dunnocks or reed warblers). The relevant adaptation lies in the cuckoo producing eggs and chicks that resemble sufficiently those of the host species so that they are not immediately ejected from the nest. These behavioural modifications are not physically associated with individuals of the host species but influence the expression of its behavioural phenotype.
Dawkins summarizes these ideas in what he terms the Central Theorem of the Extended Phenotype: