At the end of the first day Tom & Kurt presented an open lecture to the Santa Fe Institute community which was well attended. Tom presented as him main theme, the argument that the Tierra system offers the possibility of working with the more creative potentials of evolution: the ability to generate species and to generate complexity. Whereas all previous work with evolution, from the domestication of plants and animals to the fields of genetic algorithms have essentially worked at the level of managing evolution within the species, through breeding of captive populations.
Tom argued that a radically different approach is needed to work with the more creative potentials of evolution, such as its ability to generate the Cambrian explosion of diversity. The approach Tom suggested is to create an environment in which an ecological community of digital organisms can exist and evolve free from human interference. The strategy then is to get out of the way and let evolution work its magic, while at the same time thinking about what are the conditions that can spark a digital analog to the Cambrian explosion, and attempting to engineer these conditions. The network initiative is based on this approach.
Tom got a lot of flack from the audience when he presented a sketch of complexity over time in the evolution of life on Earth, which showed the bulk of complexity increase at the moment of the Cambrian explosion. Melanie Mitchell wanted to know precisely what was meant by the vertical axis, labeled ``complexity''. Tom didn't want to get into a discussion of how to define and measure complexity, but shared his conception of life on Earth as based on hierarchical organization of structures upon structures, through a dozen orders of magnitude of scale from the molecular level up through the ecosystem, the span of scales where evolution exerts its influence.
Several people criticized the graph for placing the bulk of complexity increase in the Cambrian explosion, pointing out that there were other major transitions as well. Tom freely admits that the graph is lousy and needs to be redone to reflect the other major transitions in evolution. Szathmary & Maynard Smith did a nice summary of them in the March 16, 1995 Nature (the article is a teaser for their new book: The Major Transitions in Evolution, by John Maynard Smith and Eors Szathmary. W. H. Freeman: 1995. Pp. 346. $29.95 pbk, we should probably all read the book). Tom will base reworking of the graph on their characterizations. Rephrased in the context of multiple major transitions, Tom's thesis is that most of the complexity generated by evolution has occurred in a series of bursts corresponding to these transitions. The largest of these is the Cambrian explosion. Between these bursts, complexity no doubt continues to increase, but relatively much more slowly.
Melanie stunned Tom by stating that Dan McShea (currently working at SFI) had concluded that evolution does not generate complexity. In subsequent email discussions it was clarified that what Melanie meant to say, and what Dan is stating, is that any claims that ``complexity increases over evolution'' are problematical, and that no empirical evidence has yet been given to establish this. Of course, to give empirical evidence, one has to define ``complexity'', which few evolutionary biologists have done. This is not to say that evolution does not generate complexity, but only that the lack of a complexity measure leaves us without actual relevant empirical evidence.
Kurt described the work with multi-cellular digital organisms based on an instruction set which allows individual digital organisms to parallelize by spawning additional processors. In this work, split and join instructions are added to the set, and the soup is seeded by a creature that splits once, and divides the work of replication between two processors. When these creatures are allowed to evolve, they split further and use as many as 32 processors in their replication. The most highly parallelized solutions show some interesting complexities.
Even so, the parallel solutions are all effectively SIMD style solutions, in which all of the processors execute the same code, but differentiate only to the degree of operating on different data. We are interested in seeing the evolution of differentiation at the level of what code is executed, effectively MIMD style parallelism, which would correspond to the differentiation of cells into different cell types.
It is expected however that this differentiation will not arise in the single node installation of Tierra. The network version of Tierra is being implemented in the hopes of creating an environment which would reward cell differentiation.