Important to point out that existing variation can be reshuffled by a variety of mechanisms that we don't always consider as mutations leading to increases or decreases in variation and thus altering the potential for evolution. The mutations which are of primary concern are those in the germ line as these are the one that will be passed on. August Weismann was the first to point out the distinction between germ and soma . Weismann's doctrine was a serious blow to Lamarkian inheritance of acquired characteristics. Trends in Ecology and Evolution vol.
Mutation is an ongoing process.
Mutations in the replication or repair machinery of DNA can alter mutation rates. Deletions and insertion will cause frameshift mutations . Transposable elements are mobile genetic elements that can move from one part of the genome to another. In moving from one location to another they can cause mutations. If the element landed in the middle of the coding sequence of a gene, it most likely would lead to a frameshift mutation or introduce a stop codon , and knock out the function of that gene.