The process through which two or more systems or populations adapt to each other.
The systems must be compatible and more or less complementary in order to allow for a process of reciprocal adaptation.
For example, some plants, when attacked by a parasite, produce terpenes, which in turn attract parasites of the parasite. As a result, a global fluctuating equilibrium (even if frequently chaotic) tends to become established.
This process has been called "rivalrous adaptability" by H.G. BURGER, who sees it as a requirement for complexity (1967, p.211)
The stability of reciprocal adaptation can be altered among interacting populations if one of the partners starts to evolve. In such a case, reciprocal adaptation can still be maintained if the other partners succeed in turn in producing workable evolutive ways to respond to the change. We then have co-evolution.