Basics of Avida
The Avida “world” is inhabited by replicating critters whose genomes are strings of opcode computer instructions. Populations of critters evolve when mutations change instructions and, by chance, those changes allow the critters to replicate more efficiently and/or to earn additional replication resources by performing logic operations.
The major evolutionary variables are under experimental control, so, for example, one can change the kind and rate of mutations and/or alter the fitness landscape that defines the ‘reward’ for performing logic operations by editing appropriate control files. A great deal of data on runs can be saved to disk for later analysis.