A digital organism is a self-replicating computer program that mutates and evolves. Digital organisms are used as a tool to study the dynamics of Darwinian evolution, and to test or verify specific hypotheses or mathematical models of evolution.
Digital organisms can be traced back to the game Core War, in which computer programs had to compete with each other and try to stop the opponent from executing. It turned out that one of the winning strategies was to replicate as fast as possible, which had the result that the opponent was deprived of all computational resources. However, programs in the Core War game did not mutate.
Steen Rasmussen at Los Alamos National Laboratory took the idea from Core War one step further in his core world system. He introduced mutations, in the form of random changes in the instructions of the programs inhabiting the core world. However, Rasmussen did not observe the evolution of complex and stable programs. It turned out that the programming language in which core world programs were written was very brittle, and more often than not mutations would completely destroy the functionality of a program.
The first to solve the issue of program brittleness was Tom Ray with his Tierra system. Tierra was similar to core world. However, Ray made some key changes to the programming language such that mutations were much less likely to destroy a program. With these modifications, he observed for the first time computer programs that did indeed evolve in a meaningful and complex way.
Later, Chris Adami , Titus Brown , and Charles Ofria started developing their Avida system, which was inspired by Tierra but had again some crucial differences. In Tierra, all programs lived in the same address space, and could potentially overwrite or otherwise interfere with each other. In Avida, on the other hand, each program lives in its own address space. Through this modification, experiments with Avida became much cleaner and easier to interpret than those with Tierra. With Avida, digital organism research has begun to be accepted as a valid contribution to evolutionary biology by a growing number of evolutionary biologists. Evolutionary biologist Richard Lenski of Michigan State University has used Avida extensively in his work. Lenski, Adami, and their colleagues have published in journals such as Nature and the Proceedings of the National Academy of Sciences (USA).