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The Extended Phenotype, in detail
The Extended Phenotype is Richard Dawkins's most technical and, by his own account, most important book. Where The Selfish Gene popularized the gene's-eye view of evolution for general audiences, The Extended Phenotype works out the logical implications of that view more carefully and argues for the concept that gives the book its title: the idea that a gene's phenotype — its effects on the world — need not be limited to the body it inhabits. Genes can have effects that reach beyond the organism into the environment, and natural selection acts on those extended effects just as it acts on anatomical features.
The central examples of extended phenotypes are striking. A caddisfly larva builds a case of stones and pebbles around itself; genetic variation among larvae produces measurable variation in the cases. Selection acts on the cases as surely as it acts on the bodies inside them. Beavers build dams; the dam is an expression of the beaver's genes in the world. Parasites manipulate host behavior — a fluke that causes snails to grow thicker shells, a wasp that parasitizes a caterpillar and causes it to stand guard over the wasp's cocoon. In all these cases, a gene influences the world beyond the boundary of its host organism's skin.
Dawkins also argues for the concept of the extended organism: when a parasite manipulates a host's behavior to serve the parasite's reproductive interests, the functional boundary between organism and parasite becomes blurred. The parasite's genes are in effect running the host's body for their own replication. This dissolves the intuitive boundary between where one organism ends and another begins, and suggests that the individual organism is not the natural unit of biological analysis.
The Extended Phenotype is not easy reading. It assumes knowledge of evolutionary biology and engages with technical arguments in population genetics and comparative ethology. But readers willing to follow the argument will find one of the most rigorous defenses of gene-centered evolutionary thinking available.
The big ideas
- 1.
A gene's phenotype — its effects that natural selection can act on — is not limited to the organism that contains it. Genes can influence the world through artifacts, other organisms, and behavioral manipulation.
- 2.
The extended phenotype concept includes cases like beaver dams, caddisfly cases, and parasite-manipulated host behavior, where genetic variation produces variation in structures or behaviors outside the organism's body.
- 3.
Parasites that manipulate host behavior illustrate how genes from one organism can effectively control the phenotype of another, blurring the functional boundary between organisms.
