Multi-step adaptation

“Irreducibly complex” adaptations (those requiring multiple independent mutations in order to function) are often used as the paradigmatic example of what evolution cannot accomplish. But we have found that standard evolutionary theory predicts that it should actually happen fairly often in large populations. Now we need to figure out what factors (such as spatial structure or adaptation elsewhere in the genome) determine how much complexity different populations should be able to evolve, and what signatures we can use to detect it in nature.

More generally, complex adaptations might involve a mutation and a migration rather than multiple mutations. For example, a zoonotic pathogen may need to both cross to humans and adapt to them in order to spread. What does our theory predict about this case?