I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

The Extended Phenotype, Richard Dawkins introduces the idea that an animal’s genes (its genotype) do more than sculpt its body (its phenotype). They also indirectly shape the animal’s environment. Beaver genes build beaver bodies, but since those bodies go on to make dams, the genes are also redirecting the flow of rivers. A bird’s genes create a bird, but they also make a nest. My genes made my eyes, hands, and brain, and in doing so they also made this book. All of these things – dams, nests, and books – are what Dawkins calls extended phenotypes. They are products of a creature’s genes that extend beyond its body. In a way, that’s what our microbiomes are. They too are shaped by animal genes, which create environments that encourage specific microbes to grow. Although they lie inside their owners, they are just as much an extended phenotype as a beaver dam.