John Holland:
HIDDEN ORDER (Addison Wesley, 1995)

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Holland focuses on "complex adaptive systems". Such systems are governed by principles of anticipation and feedback. Based on a model of the world, an adaptive system anticipates what is going to happen. Models are improved based on feedback from the environment.
Complex adaptive system are ubiquitous in nature. They include brains, ecosystems and even economies. They share a number of features: each of these systems is a network of agents acting in parallel and interacting; behavior of the system arises from cooperation and competitiong among its agents; each of these systems has many levels of organization, with agents at each level serving as building blocks for agents at a higher level; such systems are capable of rearranging their structure based on their experience; they are capable of anticipating the future by means of innate models of the world; new opportunities for new types of agents are continously beeing created within the system.
All complex adaptive systems share four properties (aggregation, nonlinearity, flowing, diversity) and three mechanisms (categorization by tagging, anticipation through internal models, decomposition in building blocks).
Holland also reviews his own framework for representing adaptive agents, consisting of a performance system (to describe the system's skills), a credit-assignment algorithm (to reward the fittest rules) and a rule-discovery algorithm (to generate plausible hypotheses). His new visual model is called ECHO, and it "echoes" the creation of complex structures by natural selection. ECHO operates on a network of sites, each containing resources and agents. Each agent's structure is defined in terms of strings of resources, each string being a chromosome. Each chromosome contains three tags (offense, defense and adhesion), three conditions (exchange, mating and replication), and a list of resource transformations. Tags and conditions determine what happens when two agents interact.

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