Genes don’t elbow each other directly. They do it by proxy. They boot up bodies and minds—phenotypes—and let them duke it out. Phenotypes that fare better at the brawl are, like their respective genotypes, said to be fitter. The fitness of a phenotype depends, of course, not just on genes, but also on the vagaries of disease, development, nutrition, and the common depredations of time. Identical twins, for instance, can differ in their phenotypic fitness. But make no mistake: even though genes battle by proxy, they have skin in the game. Like pilots in a plane, genes sit strapped into their
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