  One strategy for winning a coevolutionary struggle is to evolve rapidly. Most
of the literature on host-pathogen coevolution focuses on this phenomenon, and
looks for consequent evidence of coevolutionary arms races. An alternative
strategy, less often considered in the literature, is to deter rapid
evolutionary change by the opponent. To study how this can be done, we
construct an evolutionary game between a controller that must process
information, and an adversary that can tamper with this information processing.
In this game, a species can foil its antagonist by processing information in a
way that is hard for the antagonist to manipulate. We show that the structure
of the information processing system induces a fitness landscape on which the
adversary population evolves, and that complex processing logic is required to
make that landscape rugged. Drawing on the rich literature concerning rates of
evolution on rugged landscapes, we show how a species can slow adaptive
evolution in the adversary population. We suggest that this type of defensive
complexity on the part of the vertebrate adaptive immune system may be an
important element of coevolutionary dynamics between pathogens and their
vertebrate hosts.
