Considering that the average gene remains unchanged for millions of years, this is as good as permanent when compared with the time span of human history. This gives a good indication of their immutability. Recently, the entire genome of a small worm has been sequenced and it was found that almost half of its genes are still alive and useful in the human genome after some 300 million years of separate evolution. It appears, therefore, that at least some genes are amazingly durable, practically immutable. Well, nothing is absolute in this world.
Mutations are like typos that can occur when somebody copies a text.
Most of these are fatal for the new organism, and therefore, they disappear again as fast as they occurred. These alleles are very valuable since they generate necessary diversity within a population. Within a stable population, no favorable mutations normally occur, since such a population has long ago reached the optimal balance with its environment. Sharks, for instance have lived almost unchanged for well over 100 million years. When the environment changes, then things start to happen.