An important discovery, but equally important to keep it in proper context: "If you want to get a rough grasp of how the leopard might get its spots, then buil

An important discovery, but equally important to keep it in proper context: "If you want to get a rough grasp of how the leopard might get its spots, then building a CA model (or something similar) can be very illuminating. It will not tell you whether that's actually how it works. This is an important example, because there is a classic theory of biological pattern formation, or morphogenesis, first formulated by Turing in the 1950s, which lends itself very easily to modeling in CAs, and with a little fine-tuning produces things which look like animal coats, butterfly wings, etc., etc. The problem is that there is absolutely no reason to think that's how those patterns actually form; no one has identified even a single pair of Turing morphogens, despite decades of searching. For a good account of the actual mechanisms of biological pattern formation, as they are being revealed by molecular developmental biology, see John Gerhart and Marc Kirschner, Cells, Embryos and Evolution: Toward a Cellular and Developmental Understanding of Phenotypic Variation and Evolutionary Adaptability (Oxford: Blackwell Scientific, 1997). This book provides an excellent discussion of the interactions between genetic control, self-organization, evolutionary forces and functional adpatations in development. (There is a nice review by Danny Yee.) — The failure of experiment to turn up any biological system working according to Turing's principles was remarked by the late, great John Maynard Smith in his review of Depew and Weber's Darwinism Evolving (in the New York Review of Books, vol. 42, no. 4, 2 March 1995); my search of the subsequent literature doesn't indicate that the situation has changed. Update, 4 March 2012: There is now a fairly convincing example of a pair of Turing morphogens in actual biology: Andrew D. Economou, Atsushi Ohazama, Thantrira Porntaveetus, Paul T Sharpe, Shigeru Kondo, M. Albert Basson, Amel Gritli-Linde, Martyn T. Cobourne and Jeremy B. A. Green, "Periodic stripe formation by a Turing mechanism operating at growth zones in the mammalian palate", Nature Genetics 44 (2012): 348--351 Thanks to a reader for letting me know about this." http://vserver1.cscs.lsa.umich.edu/~crshalizi/reviews/wolfram/ http://vserver1.cscs.lsa.umich.edu/~crshalizi/reviews/wolfra...