## Definition The **extended phenotype** is the thesis, advanced by Richard Dawkins in his 1982 book of the same name and summarised in chapter 13 of *The Selfish Gene*, that the effects of a gene on the world — its *phenotypic expression* — need not be limited to the body of the organism carrying that gene. A gene's influence can extend to artefacts, to the behaviour of other organisms, and to the wider environment, all of which can be subject to natural selection as surely as bones or enzymes. ## Standard vs. Extended Phenotype | Scope | Example | |---|---| | Standard phenotype | A gene producing longer beaver incisors | | Extended phenotype | The same gene lineage (via incisors → felling behaviour → dam building) producing a larger pond | The standard phenotype stops at the organism's skin. The extended phenotype asks: what are all the downstream physical consequences of a gene's activity, wherever they fall? ## Three Classes of Extension **1. Artefacts.** Structures built by organisms — beaver dams, bird nests, caddisfly cases — are the external expression of the genes that shaped the building behaviour. Just as a longer incisor gene can be selected for, so can a better-nest-building gene, even though the nest is not part of the organism's body. **2. Parasite manipulation of host behaviour.** The most striking illustrations involve parasites whose genes alter the phenotype of a host organism: - The *Ophiocordyceps* fungi (zombie-ant fungi) control ant locomotion and cause the ant to die in a location optimal for spore dispersal. - The *Thisbe irenea* caterpillar exudes a nectar that drives ants into a guard-like trance, giving the caterpillar a powerful bodyguard. - The rabies virus alters dog behaviour toward aggression and wandering, increasing virus transmission. In each case, what appears to be the host's behaviour is partly an expression of the parasite's genome. **3. Action at a distance through signals and communication.** Genes influence behaviour through communication — a bird's warning call, a cuckoo chick's begging cry — which then alters the behaviour of other organisms. The receiving organism's nervous system becomes, in effect, part of the extended phenotype of the signaller's genes. ## Conceptual Implication The extended phenotype dissolves the boundary between organism and environment as the natural limit of genetic influence. It reinforces the [[Replicator and Vehicle]] distinction: the vehicle (organism) is not the boundary of selection's reach; the replicator's effects can, in principle, ramify arbitrarily far through an ecosystem. Dawkins also notes that sneezing may be partly a viral extended phenotype — the cold virus has "designed" the sneeze reflex to maximise its own dispersal. ## Related - [[Replicator and Vehicle]] - [[Gene-Centred View of Evolution]] - [[The Selfish Gene]] ## Sources - [[The Extended Selfish Gene (Dawkins 2016)]]