# Tierra > **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:** Artificial Life / Digital Ecology **Doc Type:** Canonical Wiki Node **Maturity:** Developed ## Definition Tierra is a virtual environment in which self-replicating machine-language programs compete for limited memory and processor time. ## Nexus Context Beginning from an ancestral replicator, Tierra produced parasites, host defenses, counteradaptations and hyperparasites without fixed human fitness targets. The ecology itself generated selection. ## System Architecture Digital organisms occupy a shared virtual memory “soup.” Their machine-language genomes execute directly, including copying routines. Mutation introduces inherited variants, while finite memory and processor time create genuine competition inside the world. This shared memory makes other organisms’ code an ecological resource. Short parasites can omit replication machinery and borrow a host’s routine; hosts can change their routines; counterparasites and [[wiki/Hyperparasite|hyperparasites]] can then exploit the new relationships. ## Why Tierra Is Not a Genetic Algorithm A conventional [[wiki/Genetic Algorithm|genetic algorithm]] is evaluated against a human-specified objective. Tierra’s effective fitness emerges from survival and replication in an environment continually changed by other organisms. This makes it an [[wiki/Artificial Ecology|artificial ecology]] and a central precursor to the [[wiki/Ecosystem Scenario|ecosystem scenario]] for evolvable AI. ## Evidence Boundary Its organisms operate inside a designed virtual machine and depend on its resource rules. Tierra demonstrates digital evolution, not proven sentience or indefinite open-endedness. “Unprogrammed strategy” means Ray did not specify the parasite or defense, not that the substrate lacked authored rules. ## Relationships [[wiki/Thomas S. Ray|Thomas S. Ray]], [[wiki/Artificial Ecology|Artificial Ecology]], [[wiki/Host-Parasite Coevolution|Host-Parasite Coevolution]], [[wiki/Avida|Avida]] ## Sources / Provenance - Ray (1991); Müller, Steels and Szathmáry, _PNAS_ (2026).