  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. Complex processing logic can carve long, deep
fitness valleys that 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. Furthermore, we cite evidence
that the immune control logic is phylogenetically conserved in mammalian
lineages. Thus our model of defensive complexity suggests a new hypothesis for
the lower rates of evolution for immune control logic compared to other immune
structures.
