What is the extended phenotype?

The extended phenotype is a concept introduced by the evolutionary biologist Richard Dawkins in his 1982 book of the same name. It says that a phenotype should not be limited to processes inside an organism, such as protein synthesis or tissue growth. It should include every effect a gene has on its environment, whether inside or outside the body of the individual carrying it. [1]

The book builds on The Selfish Gene. Its central thesis is that individual organisms are not the true units of natural selection. The unit is the gene, which Dawkins calls the active, germ-line replicator. Genes succeed or fail by replicating across generations, and they are selected indirectly, through their phenotypic effects. [1]

Dawkins argues that the only thing genes control directly is the synthesis of proteins. Confining the phenotype to an organism's own body is therefore an arbitrary limit. In his words, as quoted in a 2009 article, a replicator should be thought of as having extended phenotypic effects, consisting of all its effects on the world at large, not just on the body in which it sits. [1] [2]

Dawkins describes three forms. The first is the construction of structures that modify the environment, such as caddis houses and beaver dams. The second is manipulation of other organisms. A well-known case is the suicidal drowning of crickets infected by hairworms, a behaviour that the parasite's life cycle requires. Another is a nematode that infects ants and makes them resemble ripe fruit, so that birds eat them and spread the nematode eggs. The ant's fruit-like appearance is the nematode's extended phenotype, because the nematode's genes produce it without changing the ant's own genes. [1] [2]

The third form is action at a distance, where the two organisms are not in direct contact. Cuckoo chicks, which elicit intensive feeding from host birds, are one example. Another, given by David Hughes, is an orchid that tricks male bees into attempting to mate with it, and so pollinates it, by mimicking a female bee. [1] [2]

The concept has been debated for more than 20 years. One line of debate is between supporters of the extended phenotype and supporters of niche construction. Niche construction proposes that factors besides genes matter, so that descendants inherit an environmental legacy as well as a genetic one. The 2009 article describes the distinction this way: the benefit of an extended phenotype passes on solely through an organism's genes, rather than as an altered niche. A 2008 European Science Foundation meeting concluded that the concept had become more relevant in light of recent research. It also concluded that its role will be explanatory, not a tool for designing meaningful experiments. [2]

Dawkins himself set a condition: extended phenotypes count only if they are candidate adaptations for the benefit of the alleles responsible for variation in them. The idea is still applied. One evolutionary medicine paper uses it to describe how genes in one organism can influence the phenotype of another, and a recent Trends in Genetics collection presents new work on it. [2] [3] [4]