On the evolution of complexity
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It is often taken for granted that as systems evolve over time they tend to become more complex. But little is understood about the mechanisms that might cause evolution to favor increases in complexity over time. This paper proposes three means by which complexity tends to grow as systems evolve. In co-evolutionary 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 subsystems or sub-functions or sub-parts to break through performance limitations, or enhance its range of operation, or 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 complexity in all three mechanisms is intermittent and epochal. And in the first two is reversible, so that collapses in complexity may occur randomly from time to time. Ilustrative examples are drawn from biology, but from economics, adaptive computation, artificial life, and evolutionary game theory.