Three fairly indicative papers on the subject (via http://www.reed.edu/~mab/papers.html)

• M. A. Bedau, E. Snyder, N. H. Packard, “A Classification of Long-Term Evolutionary Dynamics”. In Artificial Life VI; C. Adami, R. Belew, H. Kitano, and C. Taylor, eds., MIT Press (1998), pp. 228-237.
• M. A. Bedau, E. Snyder, C. T. Brown, and N. H. Packard, “A Comparison of Evolutionary Activity in Artificial Evolving Systems and the Biosphere”. In P. Husbands and I. Harvey, eds., 1997, Fourth European Conference on Artificial Life pp. 125-134. MIT Press/Bradford Books.
• M. A. Bedau, N. H. Packard, “Measurement of Evolutionary Activity, Teleology, and Life”. In C. Langton, C. Taylor , D. Farmer, and S. Rasmussen, eds., Artificial Life II, Santa Fe Institute Studies in the Sciences of Complexity, Vol. X, Redwood City, CA: Addison-Wesley, pp. 431-461. 1992.)