Definition
The extended phenotype is the gene's full causal reach — every phenotypic effect a gene has on the world, including effects beyond the boundary of the body it directly builds. The concept was introduced in topic 13 of The Selfish Gene and developed at book length in Dawkins's 1982 The Extended Phenotype, which he considers his most original contribution.
A beaver's dam is part of the beaver's extended phenotype. A spider's web is part of the spider's. A caddis-fly's case is part of the caddis fly's. More radically, a parasite's manipulation of its host — Toxoplasma altering a rat's behavior, a trematode swelling a snail's eye-stalks, a cuckoo chick's begging signal exploiting host parental circuitry — is the parasite's extended phenotype reaching into the host's body.
Why it matters
How it works
Selection acts on whatever increases or decreases a gene's representation in the next gene pool. If a gene's effects extend beyond the body's skin — into a beaver's dam, a host's brain, an entire ecosystem — those effects still count. The genes that build better dams produce more beaver descendants; the genes are selected through their dam-building, even though the dam is not part of the beaver's body.
The host-manipulation cases are the most counterintuitive. The trematode worm needs to be eaten by a bird. The worm's genes build a phenotype — the swollen, pulsating eye-stalks — in the snail's body. That phenotype attracts the bird. The selection acts on the trematode's genes through their effect on the snail. The snail is, in this aspect of its biology, partly a survival machine for the trematode, not for itself.