I recently read: Parasites may affect the evolution of complexity in their hosts. The presence of a parasitic form within a host population tends to select for a more complex instruction set in the hosts, full of old and emerging resistance strategies. The two forms together drive an increase in complexity relative to a single population on its own. ........... Your thoughts?
This certainly could be the case. However, the line between parasite and symbiote is a fluid one. Certain relationships could drive a reduction in complexity. See, for example, the various forms of Wolbachia. https://en.wikipedia.org/wiki/Wolbachia Wolbachia infection could conceivably lead a population to shed some of its immune response mechanisms.
