The relative fitness of an individual is a simple function of
the number of mutations the individual carries. The
fitness procedure in the simulator is just a direct implementation
of the corresponding function in the mathematical model. In C, the
fitness ` w` of individual ` x` is

w = pow(1-s,x);

In nature, some unfit individuals will survive to reproduce, and some
fit individuals will die before reproducing, for example through accidents.
To model this uncertainty, we draw a uniform random number **x** between 0.0 and 1.0,
and say an individual will survive if .