Use of a computer-generated system to represent the dynamic responses and behaviour of a real or proposed system. A mathematical description of a system is developed as a computer program that uses equations to represent the functional relationships within the system. When the program is run, the resulting mathematical dynamics form an analog, usually represented graphically, of the behaviour of the modeled system. Variables in the program can be adjusted to simulate varying conditions in the system. Computer simulations are used to study the behaviour of objects or systems that cannot be easily or safely tested in real life, such as weather patterns or a nuclear blast. Simpler simulations performed by personal computers are business models and geometric models. Seealso scientific visualization.

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Tierra is a computer simulation developed by ecologist Thomas S. Ray in the early 1990s in which computer programs compete for central processing unit (CPU) time and access to main memory. At present, the commonly accepted definition of life does not consider any computer program to be alive, however, Tierra is a frequently cited example of an artificial life model; in the metaphor of the Tierra, the evolvable computer programs can be considered as digital 'organisms' which compete for energy (CPU time) and resources (main memory). In this context, the computer programs in Tierra could be considered evolvable and can mutate, self-replicate and recombine.

Tierra is a derivative of the computer programmer's game Core War.

Simulations

The basic Tierra model has been used to experimentally explore in silico the basic processes of evolutionary and ecological dynamics. Processes such as the dynamics of punctuated equilibrium, host-parasite co-evolution and density-dependent natural selection are amenable to investigation within the Tierra framework. A notable difference to more conventional models of evolutionary computation, such as genetic algorithms is that there is no explicit, or exogenous 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 promise has not been realized – like other digital evolution systems, it eventually comes to 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.

Mark Bedau and Norman Packard developed 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, 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.

Russell K. Standish has measured the informational complexity of Tierran 'organisms', and has similarly found limited complexity growth in Tierran evolution.

While the dynamics of Tierra are highly suggestive, the significance of the dynamics for real ecological and evolutionary behavior are still a subject of debate within the scientific community. Tierra is an abstract model, but any quantitative model is still subject to the same validation and verification techniques applied to more traditional mathematical models, and as such, has no special status. More detailed models in which more realistic dynamics of biological systems and organisms are incorporated is now an active research field (see systems biology).

See also

• Avida
• Artificial life
• Digital organisms
• Evolutionary computation
• Fitness landscape
• Other Simulators for ALife
• Artificial intelligence

References

Aditional reading

• Ray, T. S. 1991, "Evolution and optimization of digital organisms", in Billingsley K.R. et al (eds), Scientific Excellence in Supercomputing: The IBM 1990 Contest Prize Papers, Athens, GA, 30602: The Baldwin Press, The University of Georgia. Publication date: December 1991, pp. 489-531.
• Casti, John L. (1997). Would-Be-Worlds. John Wiley & Sons, Inc. New York ISBN 0-471-12308-0

External links

• Tierra home page