As mentioned in the previous section, many things can cause a DNA mutation, including:
Therefore, mutations are fairly common. Mutations happen at a steady rate in any population, but the location and type of every mutation is completely random. According to Carl Sagan in "The Dragons of Eden":
Large organisms such as human beings average about one mutation per ten gametes [a gamete is a sex cell, either sperm or egg] -- that is, there is a 10 percent chance that any given sperm or egg cell produced will have a new and inheritable change in the genetic instructions that make up the next generation. These mutations occur at random and are almost uniformly harmful -- it is rare that a precision machine is improved by a random change in the instructions for making it.
According to "Molecular Biology of the Cell":
Only about one nucleotide pair in a thousand is randomly changed every 200,000 years. Even so, in a population of 10,000 individuals, every possible nucleotide substitution will have been "tried out" on about 50 occasions in the course of a million years, which is a short span of time in relation to the evolution of species. Much of the variation created in this way will be disadvantageous to the organism and will be selected against in the population. When a rare variant sequence is advantageous, however, it will be rapidly propagated by natural selection. Consequently, it can be expected that in any given species the functions of most genes will have been optimized by random point mutation and selection.
According to the book "Evolution," by Ruth Moore, it is possible to speed up mutations with radiation:
So Muller put hundreds of fruit flies in gelatin capsules and bombarded them with X-rays. The irradiated flies were then bred to untreated ones. In 10 days thousands of their offspring were buzzing around their banana-mash feed, and Muller was looking upon an unprecedented outburst of man-made mutations. There were flies with bulging eyes, flat eyes, purple, yellow and brown eyes. Some had curly bristles, some no bristles...
Mutations fuel the process of evolution by providing new genes in the gene pool of a species.
Then, natural selection takes over.