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A cathedral termite mount – a small animal with a very noticeable extended phenotype

The Extended Phenotype (subtitled "The Gene as the Unit of Selection" later "The Long Reach of the Gene") is a 1982 book by Richard Dawkins. A revised edition was published in 1999 with an afterword by the philosopher Daniel Dennett. Dawkins considers the concept of the Extended Phenotype to be his principal contribution to evolutionary theory.

Dawkins starts from the ideas of his earlier book The Selfish Gene, which portrayed the organism as a survival machine constructed by its genes to maximise their chances of replicating. In a much more technical presentation than the earlier book, Dawkins devotes a significant portion of this work to an attempt to remedy incomprehension and rebut criticism of The Selfish Gene.

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, citing as examples caddis houses and beaver dams. He then goes further to point to first animal morphology and ultimately animal behaviour, which appears advantageous not to the animal itself, but rather to a parasite which afflicts it. 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.

In conducting this argument, Dawkins aims to strengthen the case for a gene-centred view of life, to the point where it is recognised that the organism itself needs to be explained. This is the challenge which he takes up in the final chapter entitled "Rediscovering the Organism."

• From The World of Richard Dawkins
• Video of cricket exhibiting behavioral extended phenotype of parasite