Is the frequency at which a species mutate affected by natural selection?
Drake's rule

Let $L$ be the size of the genome. Let $\mu$ be the average per nucleotide mutation rate, then the genome-wide mutation rate $U$ is $U=L \mu$. There is a general relationship between the genome size of an organism and the nucleotide mutation rate that makes that the genome-wide mutation rate is often approximatively 1. There is of course several orders of magnitude of variation around this value of 1 among organisms.

This rule is called Drake's rule after the name of the author who first discovered it (Drake, 1991).

To my knowledge, this relationship is not fully well understood. Because you are looking for an adapted mutation rate in your question, I would like to highlight that I would not go about assuming this is adaptive. I could imagine that some molecular genetic constraint of the replication process could be causing this type of relationship between $L$ and $\mu$.

You will probably want to read this post about Drake's rule.

Mutation rate in humans

In humans, the average mutation rate is estimated to be around $\mu=2.5 \cdot 10^{-8}$ (Nachman and Crowell, 2000; see also this post), with a genome size of about $L=3 \cdot 10^9$, leading to an estimate of the genome-wide mutation rate of 80. 

Interestingly, the mutation rate is in fathers is age-dependent. See the post Gender and age-specific mutation rate in plants.