morphological structures get their forms from arrangements of very limited numbers of types of cells or other elements.

But just how are the programs for these and other features of organisms actually determined? Over the past century or so it has become almost universally believed that at some level these programs must end up being the ones that maximize the fitness of the organism, and the number of viable offspring it produces.

The notion is that if a line of organisms with a particular program typically produce more offspring, then after a few generations there will inevitably be vastly more organisms with this program than with other programs. And if one assumes that the program for each new offspring involves small random mutations then this means that over the course of many generations biological evolution will in effect carry out a random search for programs that maximize the fitness of an organism.

But how successful can one expect such a search to be?

The problem of maximizing fitness is essentially the same as the problem of satisfying constraints that we discussed at the end of Chapter 7. And what we found there is that for sufficiently simple constraints—particularly continuous ones—iterative random searches can converge fairly quickly to an optimal solution. But as soon as the constraints are more complicated this is no longer the case. And indeed even when the optimal solution is comparatively simple it can require an astronomically large number of steps to get even anywhere close to it.

Biological systems do appear to have some tricks for speeding up the search process. Sexual reproduction, for example, allows large-scale mixing of similar programs, rather than just small-scale mutation. And differentiation into organs in effect allows different parts of a program to be updated separately. But even with a whole array of such tricks, it is still completely implausible that the trillion or so generations of organisms since the beginning of life on Earth would be sufficient to allow optimal solutions to be found to constraints of any significant complexity.

And indeed one suspects that in fact the vast majority of features of biological organisms do not correspond to anything close to optimal solutions: rather, they represent solutions that were fairly easy to find, but are good enough not to cause fatal problems for the organism.