Coevolution is often invoked to explain open problems in evolution, including the origin of sexual reproduction, speciation, and the evolution of complex adaptations. But what makes it so special? One hallmark of coevolution is the dynamic “push-and-pull” of reciprocal adaptation, for example, when parasites push hosts to evolve resistance and resistant hosts pull their parasites to cope. I will present work with a digital host-parasite system that highlights the necessity of this reciprocal process for the evolution of complex traits. I will also discuss how we’ve begun testing if reciprocity is responsible for host diversification using custom continuous microbial culture devices. Together, these experiments are directly testing the consequences of feedback on the dynamics of adaptation.