Other models [of mutation]
Sequential substitution systems are probably more realistic than cellular automata as models of genetic programs, since elements can explicitly be added to their rules at will. As a rather different approach, one can consider a fixed underlying rule—say a class 4 cellular automaton—with modifications in initial conditions. The notion of universality in Chapter 11 implies that under suitable conditions this should be equivalent to modifications in rules. As an alternative to modelling individual organisms, one can also consider substitution systems which directly generate genealogical trees for populations of organisms, somewhat like Leonardo Fibonacci's original model of a rabbit population.