Tierra C source code creates a virtual computer
and its Darwinian operating system, whose architecture
has been designed in such a way that the executable
machine codes are evolvable. This means that the
machine code can be mutated (by flipping bits
at random) or recombined (by swapping segments
of code between algorithms), and the resulting
code remains functional enough of the time for
natural (or presumably artificial) selection to
be able to improve the code over time.
Along with the C source code which generates the
virtual computer, we provide several programs
written in the assembler code of the virtual computer.
Some of these were written by a human and do nothing
more than make copies of themselves in the RAM
of the virtual computer. The others evolved from
the first, and are included to illustrate the
power of natural selection.
operating system of the virtual computer provides
memory management and timesharing services. It
also provides control for a variety of factors
that affect the course of evolution: three kinds
of mutation rates, disturbances, the allocation
of CPU time to each creature, the size of the
soup, etc. In addition, the operating system provides
a very elaborate observational system that keeps
a record of births and deaths, sequences the code
of every creature, and maintains a genebank of
successful genomes. The operating system also
provides facilities for automating the ecological
analysis, that is, for recording the kinds of
interactions taking place between creatures."
Tom Ray http://www.isd.atr.co.jp/%7Eray/tierra/whatis.html