Editing Tierra (computer simulation) (section)

== Simulations ==
The basic Tierra model has been used to experimentally explore ''[[in silico]]'' the basic processes of [[evolution]]ary and [[ecology|ecological]] dynamics. Processes such as the dynamics of [[punctuated equilibrium]], [[Host–parasite_coevolution|host-parasite co-evolution]] and density-dependent [[natural selection]] are amenable to investigation within the Tierra framework. A notable difference between Tierra and more conventional models of [[evolutionary computation]], such as [[genetic algorithm]]s, is that there is no explicit, or [[exogenous]] [[fitness (biology)|fitness function]] built into the model. Often in such models there is the notion of a function being "optimized"; in the case of Tierra, the fitness function is endogenous: there is simply survival and death.

According to Thomas S. Ray and others, this may allow for more "open-ended" evolution, in which the dynamics of the feedback between evolutionary and ecological processes can itself change over time (see [[evolvability]]), although this claim has not been realized – like other digital evolution systems, it eventually reaches a point where novelty ceases to be created, and the system at large begins either looping or ceases to 'evolve'. The issue of how true [[open-ended evolution]] can be implemented in an artificial system is still an open question in the field of [[artificial life]].<ref>Bedau M.A., McCaskill J.S. et al., "Open problems in artificial life", ''Artificial Life'', 2000 Fall '''6'''(4):363-76</ref>

[[Mark Bedau]] and [[Norman Packard]] developed a statistical method of classifying evolutionary systems and in 1997, Bedau ''et al.'' applied these statistics to Evita, an [[Artificial life]] model similar to Tierra and [[Avida (software)|Avida]], but with limited organism interaction and no parasitism, and concluded that Tierra-like systems do not exhibit the open-ended evolutionary signatures of naturally evolving systems.<ref>Bedau, M.A., Snyder, E., Brown, C.T. and Packard, N.H. 1997, "A Comparison of Evolutionary Activity in Artificial Evolving Systems and in the Biosphere", in ''Fourth European Conference on Artificial Life'', Husbands and Harvey (eds), MIT press, p125</ref>

Russell K. Standish has measured the informational complexity of Tierran 'organisms', and has similarly not observed complexity growth in Tierran evolution.<ref>Standish, R.K. 2003 "Open-ended artificial evolution", ''International Journal of Computational Intelligence and Applications 3(2)'', 167-175</ref>

Tierra is an abstract model, but any quantitative model is still subject to the same validation and verification techniques applied to more traditional [[mathematical model]]s, and as such, has no special status. The creation of more detailed models in which more realistic dynamics of biological systems and organisms are incorporated is now an active research field (see [[systems biology]]).