Tierra simulated an evolutionary system by introducing computer programs that competed for computer resources, specifically processor (CPU) time and access to main memory. In this respect it is similar to core wars, but differs in that the programs being run in the simulation are able to modify themselves, and thereby evolve. Tierra's programs are artificial life organisms.
In Avida, every digital organism lives in its own protected region of memory, and is executed by its own virtual CPU. By default, other digital organisms cannot access this memory space, neither for reading nor for writing, and cannot execute code that is not in their own memory space. Whereas in Tierra the organisms effectively share and compete for one "brain", in Avida each one has its own brain.
A second major difference is that the virtual CPUs of different organisms can run at different speeds, such that one organism executes, for example, twice as many instructions in the same time interval as another organism. The speed at which a virtual CPU runs is determined by a number of factors, but most importantly, by the tasks that the organism performs: Tasks are logical computations that the organisms can carry out to reap extra CPU speed as bonus.
Adami and Ofria, in collaboration with others, have used Avida to conduct research in digital evolution, and the scientific journals Nature and Science have published several of their papers. Nature published "The Evolutionary Origin of Complex Features" in 2003, in which the evolution of a mathematical equals operation is constructed of at least 19 simpler, precisely ordered instructions.