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regular, or irregular, array (see Figures 17 and 18). Each site has a well-defined set of neighboring sites, and each site can contain a subpopulation of agents. In addition, each site is assigned a production function that determines how rapidly the site produces and accumulates the various resources. For example, one site may produce 10 units of resource a per time step, and nothing of b, c, or d,whereas another may produce 4 units each of a, b, c,and d. If the site is unoccupied by any agents, these resouces accumulate, up to some maximum value. In the example of the site that produces 10 units of resource a per time step, the site could continue to accumulate the resource until it had accumulated, say, a total of 100 units. Agents present at a site can "consume" these resources. Thus an agent located at a site that produces the resources it needs can manage reproduction without combat or trade, if it survives combat interactions with other agents. Different agents may have intrinsic limits on the resources they can take up from the site. For example, an agent may only be able to consume resource b from the environment, being dependent upon agent-agent interactions to obtain other needed resources. Resouces available at a site are shared among the agents that can consume them. |
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When neither agent-agent nor agent-environment interactions are providing at least one needed resource at a given site, an agent may migrate from that site to a neighboring site. For example, consider an agent that has already acquired enough of resources a and b to make copies of its chromosomes but that is not acquiring needed resource c. Then that agent will migrate to some neighboring site; in the simplest models the new site is simply selected at random from the neighboring sites. |
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The actual Echo simulation is designed so that, in effect, the populations at each of the sites in the model undergo their interactions simultaneously. In other words, Echo is well suited to execution on a massively parallel computer. The interactions at each site are carried out by repetition of the following basic cycle. |
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(1) Pairs of agents from within the site are selected for interaction. (In the simplest model, these pairs are simply selected at random from the local population). Each pair is tested for the kind(s) of action that will ensue following the procedures outlined above. |
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(1.1) First the pair is tested for combat, which may be invoked unilaterally. |
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(1.2) If combat is not invoked, then the pair is tested for trade, which can only be invoked bilaterally. The same pair is then tested for mating compatibility. If the agents are compatible, then, with low probability, recombination of their chromosomes will follow. |
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