Conclusion
The Von Neumann Self-reproducing
On Genetic Relativism
Let me say then that I actually accept all this: that for the solution of von Neumann's problem, as I have stated it, adopting the framework of Genetic Absolutism seems to be quite the simplest and most efficacious approach, and I endorse it as such. Nonetheless, I think it worthwhile to point out the possibility of working in the alternative framework of Genetic Relativism for a number of distinct reasons.
Firstly, it would be easy, otherwise, to mistake what is merely a pragmatic preference for using Genetic Absolutism in solving von Neumann's problem with the minimum of effort, for a claim that Genetic Absolutism is, in some sense, necessary for the solution of this problem. It is not. More generally, our chosen problem is only concerned with what may be possible, or sufficient--not what is necessary.
A second closely related point is this: prima facie, our
solution based on Genetic Absolutism may seem to imply that a
general constructive automaton (i.e., capable of
constructing a very wide range of target machines) is a
pre-requisite to any evolutionary growth of complexity.
It is not. Indeed, we may say that, if such an implication
were present, we should probably have to regard our
solution as defective, for it would entirely beg the question of
how such a relatively complex entity as 
(or something
fairly close to it) could arise in the first place. Conversely,
once we recognise the possibility of evolution within the
framework of Genetic Relativism, we can at least see how such
prior elaboration of the powers of the constructive automata
could occur ``in principle''; this insight remains valid, at
least as a coherent conjecture, even if we have not demonstrated
it in operation. This has a possible advantage in relation to
the solution of von Neumann's problem in that it may permit us to
work, initially at least, with significantly more primitive
constructive automata as the bases of our self-reproducers.
Thirdly, Genetic Absolutism views all the self-reproducers under investigation as connected by a single ``genetic network'' of mutational changes. This is sufficient to solve von Neumann's problem, as stated, which called only for exhibiting the possibility of mutational growth of complexity. In practice, however, we are interested in this as a basis for a Darwinian growth of complexity. Roughly speaking, this can only occur, if at all, along paths in the genetic network which lead ``uphill'' in terms of ``fitness''. If the genetic network is fixed then this may impose severe limits on the practical paths of Darwinian evolution (and thus on the practical growth of complexity). Again, once we recognise the possibility of evolution within a framework of Genetic Relativism--which offers the possibility, in effect, of changing, or jumping between, different genetic networks--the practical possibilities for the (Darwinian) growth of complexity are evidently greatly increased.
This last point represents a quite different reason for favouring the framework (or perhaps we may now say ``research programme'') of Genetic Relativism, and it is independent of the ``power'' of particular core constructive automata. In particular, even if we can exhibit a single full blown general constructive automaton, which yields a mutationally connected set of self-reproducers spanning (virtually) every possible behaviour supported in the system, there could still be advantages, from the point of view of supporting Darwinian evolution, in identifying alternative constructive automata, defining alternative genetic networks (viewed now as evolutionarily accessible pathways through the space of possible automaton behaviours).
Indeed, this need not be all that difficult to do: it provides a particular reason to consider combining a basic constructive automaton with a turing machine (or something of similar computational powers): the latter is arranged so that it ``pre-processes'' the description tape in some (turing computable) fashion. The program of the turing machine could then effectively encode a space of alternative genetic languages (subject to the primitive constructional abilities of the original constructive automaton); with moderately careful design, it should be possible to open up an essentially infinite set of constructive automata, which are themselves connected under mutation (of the program for the embedded turing machine--another tape of some sort), thus permitting a multitude of different genetic networks for potential exploitation by a Darwinian evolutionary process. This should greatly enhance the possibilities for Darwinian evolution of any sort, and thus, in turn, for evolution involving the growth of complexity.
This particular idea seems to have been anticipated by Codd:
A further special case of interest is that in which both a universal computer and a universal constructor [sic] exist and the set of all tapes required by the universal constructor is included in the Turing domain. For in this case it is possible to present in coded form the specifications of configurations to be constructed and have the universal computer decode these specifications ...Then the universal constructor can implement the decoded specifications. Codd (1968, pp. 13-14)
While Codd did not elaborate on why such flexibility in ``coding'' should be of any special interest, it seems plausible that he had in mind precisely the possibility of opening up alternative genetic networks.
Conclusion
The Von Neumann Self-reproducing
On Genetic Relativism
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Timestamp: 2000-08-16