Hidden Order: How Adaptation Builds Complexity.

Computer simulations of artificial ecologies typically model the interactions of a population of independently acting, spatially situated, resource-restricted, differently structured, self-reproducing agents. The interactions in such systems are typically highly nonlinear, subject to random perturbation, executed in parallel, and coevolutionary in the sense that the agents interact with each other as well as their environment. Examples of artificial ecologies that possess the above characteristics include Skipper's "computer zoo" of migrating flocks of hierarchically invokable fragments of assembly code [10], Shanahan's populations of evolutionary automata [9], Lindgren's coevolving populations of strategies for playing the iterated prisoner's dilemma game [5], the Turing gas of self-organizing groups of autocatalytic algorithmic fragments of Rasmussen, KnudLsen, and Feldberg [6], Ray's populations of parasitic and symbiotic self-reproducing assembly code programs [8], the populations of redcode coreworld creatures of Rasmussen, Knudsen, Feldberg, and Hindsholm [7], Werner and Dyer's populations of males and females that coevolve communication [11], and Ackley and Littman's artificial world for testing the Baldwin effect concerning learning and evolution [1]. Each of these complex adaptive systems (and many others like them) succeed in illustrating one or more key features of living systems. However, each of these systems live up to their name in the worst possible way—they are all exceedingly complex. The emergent behavior and other interesting phenomena are obscured by so much modelspecific detail that it is rarely clear whether the observed phenomena represent any important general principles or are merely artifacts of the details. Moreover, all of these existing systems are computationally expensive and deliver little in the way of important emergent phenomena in relation to the amount of computational effort expended. This excessive overhead is not merely a matter of inefficiency or inconvenience; it may actually preclude emergence of important phenomena that can only materialize in the presence of certain minimum amounts of time or matter. John Holland's new book, Hidden Order: How Adaptation Builds Complexity [3],