At room temperature, a bearded dragon’s sex depends on two chromosomes. If they have two Z chromosomes, these lizards develop as males. Those with a Z and a W become females. But raise the thermostat up a few notches, and something different happens. If a clutch of dragon eggs are incubated at 34 degrees Celsius, their bodies ignore the usual instructions from their sex chromosomes. Even if half of them are genetically male (ZZ), all of them will hatch as females.

You’d be wrong. In fact, their chromosomes didn’t matter at all. Instead, their sex depended entirely on the temperature at which they are incubated. Warm clutches produced females; cooler ones produced males. In a single generation, these lizards had evolved