We often take for granted that systems evolving over time tend to become more complicated. When this paper was written, little was understood about what mechanisms might cause evolution to favor increases in complication. This paper proposes three means by which complication tends to grow as systems evolve. In coevolutionary systems it may grow by increases in “species” diversity: under certain circumstances new species may provide further niches that call forth further new species in a steady upward spiral. In single systems it may grow by increases in structural sophistication: the system steadily cumulates increasing numbers of internal subsystems or sub-functions or subparts to break through performance limitations, or to enhance its range of operation, or to handle exceptional circumstances. Or, it may suddenly increase by “capturing software”: the system captures simpler elements and learns to “program” these as “software” to be used to its own ends. Growth in complication in all three mechanisms is intermittent and epochal. And in the first two it is reversible; collapses in complexity may occur randomly from time to time. Illustrative examples are drawn not just from biology, but from economics, adaptive computation, artificial life, and evolutionary game theory.
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