Effect on fitness of mutations
To a good first approximation $\overline{\Delta f} = 0$. Where $\overline{\Delta f}$ is the mean change in fitness down to any point or indel mutation. The reasons for this are as follows:


In the genome of higher organisms, most of the genome is non-functional ("junk") so most mutations will not have any effect regardless of the change made.
A substantial proportion of in-frame point mutations will be synonymous mutations that result in the same amino acid being coded for (actually this can have an effect on protein expression but I don't believe anyone has - yet - shown a fitness difference?)
Even where a mutation does alter an amino acid many amino acid changes have no measurable effect on the protein produced. Especially where the new amino acid has similar properties to the one it has replaced.
Even when a mutation does alter the protein function, or render the product non-functional, in many cases this will not impact fitness since fitness is conditional on the environment in which it is measured and not all genes impact all environments.


So the distribution, whatever it is, will have a large spike at 0. Probably this spike is several orders of magnitude higher than the next highest value. Further, we can be reasonably certain that $\overline{\Delta f} &lt; 0$ since there are more possible ways to break a Gene through a point or indel mutation than there are to improve it. If this is the case, so that there is a large excess of mutations with a negative effect on fitness, we can conclude that the distribution of mutational changes in fitness will be non-normal (negative skew and spike at zero), and that the normal distribution will be a poor approximation.