Ray, Thomas S., and Chenmei Xu. “Measures of evolvability in tierra.” Artificial Life and Robotics 5, no. 4 (2001): 211-214.
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A variety of indices are tested for their ability to discriminate periods of evolution from periods of stasis in Tierra. Measures which recognize the emergence of new adaptations are favored. To remove artifacts, the shadow method of Bedau is used. A promising measure tracks the positive derivative of Simpson’s index.