# Host-Parasite Coevolution > **Machine-Evolution Nexus:** [[articles/Digital Darwinism and the Invisible World of Machine Evolution|Digital Darwinism and the Invisible World of Machine Evolution]] places this node within the history and governance of substrate-independent evolution. **Domain:** Evolution / Reciprocal Selection **Doc Type:** Canonical Wiki Node **Maturity:** Developed ## Definition Host–parasite coevolution is reciprocal evolutionary change as hosts evolve defenses and parasites evolve ways around them. ## Nexus Context Tierra produced attack and defense cycles; Avida experiments show parasites can increase host complexity and evolvability. The relation transfers Ehrlich and Raven’s coevolutionary logic into digital populations. ## Evidence Boundary Reciprocity does not imply equality of benefit or agency. The causal claim concerns selection feedback across lineages. ## Relationships [[wiki/Coevolution|Coevolution]], [[wiki/Parasite|Parasite]], [[wiki/Tierra|Tierra]], [[wiki/Avida|Avida]] ## Sources / Provenance - Ray (1991); Avida host–parasite studies.