The details and mechanisms of the evolution of creatures in the Tierran computer have been described in detail [2,3,4,5,6,7], and will only be summarized here. Running of the self-replicating program on the error prone computer creates a situation that is in fact identical to the one outlined by Darwin. Those genotypes that are most efficient at replicating leave more descendants in the future generations, and increase in frequency in the population.
An interesting characteristic of this process is the surprising variety and inventiveness of evolved means of increasing efficiency of replication. Some of the increase in efficiency is achieved through straightforward optimization of the replication algorithm. However, efficiency is also achieved through more surprising avenues involving interactions between creatures.
Evolution increases the adaptation of organisms to their environment. In the Tierran universe, initially the environment consists largely of the memory which is fairly uniform and always available, and the CPU which allocates time to each creature in a consistent and uniform fashion. In such a simple environment the most obvious route to efficiency is optimization of the algorithm. However, once the memory is filled with creatures, the creatures themselves become a prominent feature of the environment. Now evolution also discovers ways for creatures to exploit one another, and to defend against such exploitation.