Mutations are the fundamental source of genetic variation but also the cause of deleterious effects on fitness. The mutation rate (the number of genetic changes within a given unit of time) is a central parameter in evolutionary biology. Thanks to the application of genomic sequencing technology, we can now precisely determine the exact number of de novo mutations arising in germ cells that are transmitted to offspring, as well as somatic mutations generated during somatic cell divisions. It has long been recognized that mutation rates vary substantially across taxa and even between populations within species. Although several hypotheses have been put forward to explain the proximal mechanisms that underlie the variations in mutation rate, including intrinsic and extrinsic environmental factors, the ultimate causes of evolutionary change on the mutation rate remain controversial even today.