A mutation is a permanent alteration of the DNA nucleotide sequence of a genome due to RNA copying errors, radiation or chemical exposure (how the Incredible Hulk and Spider-Man came to be, respectively), or viral infections. Mutations in the germ line can be inherited by offspring, but mutations in somatic body cells cannot be passed on.
Many different types of RNA copying error mutations are possible, including insertion, deletion, duplication, frameshift, and nonsense mutations. Sometimes these errors result in different proteins being created, sometimes they have no apparent effect but influence level and timing of gene expression, and sometimes they have no effect at all. The latter two are known as silent mutations, which occur either in non-coding regions of the genome known as introns or in coding regions known as exons.
Mutations occur relatively rarely, making determining the rate of mutations an important part of genetic analysis. It can be thought of as the number of mutations per a biological unit, such as mutations per gamete or mutations per cell division.
Albertin et al. (1) found evidence of extensive mRNA editing in the genome sequences of octopodes, suggesting perhaps a higher rate of mutation than previously predicted. This, they hypothesize, could account for the "remarkable morphological departures from the basic molluscan body plan," including the arms and neural system. This is not to mention the sophisticated social, problem-solving, and learning behaviors that octopodes display. Mutations can contribute to eventual divergence between species. Albertin et al. also estimated sequence divergence with respect to time in a genomic analysis of the octopus, looking at when cephalopods diverged into bivalves and gastropods around 540 million years ago (MYA). This allows an estimate of mean neural substitution rate, which provides one basis of comparison of closeness of related cephalopod species.
1. Albertin, C. B., et al. (2015). "The octopus genome and the evolution of cephalopod neural and morphological novelties." Nature 524(7564): 220-224.