Continuing the discussion from Perry Marshall: What is Random?:
In discussing wither mutations were “random with respect to fitness,” some interesting back and forth took place.
This brings us to an important classic experiment, the Luria-Delbruck Experiment. There were two possible outcomes A and B. They observed B, which showed that in this experiment that beneficial mutations arose before selection, not in direct response to it. To a first approximation, this is usually the case. However…
- Even spontaneous mutations are biased towards beneficial (or at least away from detrimental)
- There are well known cases where this general rule is violated.
As is the case for many things in biology, there is a rule, but there are exceptions, and the exceptions are important. Also, I’d add, that biologists do not always speak with mathematical precision.