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The distribution of rates for diverse kinds of mutations is called the "mutation spectrum" (see App. B of <ref name="Stoltzfus2021" />). Mutations of different types occur at widely varying rates. Point mutation rates for most organisms are very low, roughly 10<sup>−9</sup> to 10<sup>−8</sup> per site per generation,<ref>{{cite journal |last1=Wang |first1=Yiguan |last2=Obbard |first2=Darren J |title=Experimental estimates of germline mutation rate in eukaryotes: a phylogenetic meta-analysis |journal=Evolution Letters |date=19 July 2023 |volume=7 |issue=4 |pages=216–226 |doi=10.1093/evlett/qrad027|pmid=37475753 |pmc=10355183 |hdl=20.500.11820/8ffd5b76-77ae-4764-ae31-de2fb8aa35cf |hdl-access=free }}</ref> though some viruses have higher mutation rates on the order of 10<sup>−6</sup> per site per generation.<ref>{{cite journal |last1=Peck |first1=Kayla M. |last2=Lauring |first2=Adam S. |title=Complexities of Viral Mutation Rates |journal=Journal of Virology |date=15 July 2018 |volume=92 |issue=14 |pages=e01031-17 |doi=10.1128/JVI.01031-17|pmid=29720522 |pmc=6026756 }}</ref> [[Transition (genetics)|Transitions]] (A ↔ G or C ↔ T) are more common than [[transversion]]s ([[purine]] (adenine or guanine)) ↔ [[pyrimidine]] (cytosine or thymine, or in RNA, uracil)).<ref>{{Cite web | url=https://www.mun.ca/biology/scarr/Transitions_vs_Transversions.html | title=Transitions vs transversions}}</ref> Perhaps the most common type of mutation in humans is a change in the length of a [[short tandem repeat]] (e.g., the CAG repeats underlying various disease-associated mutations). Such STR mutations may occur at rates on the order of 10<sup>−3</sup> per generation.<ref name=WeberWong1993>{{cite journal | author=J. L. Weber and C. Wong | year=1993 | title=Mutation of human short tandem repeats | journal=Hum Mol Genet | volume=2 | issue=8 | pages=1123–8 | doi=10.1093/hmg/2.8.1123 | pmid=8401493 }}</ref>
Different frequencies of different types of mutations can play an important role in evolution via [[bias in the introduction of variation]] (arrival bias), contributing to parallelism, trends, and differences in the navigability of adaptive landscapes.<ref name=CanoPayne2020>{{cite journal | author=A. V. Cano and J. L. Payne | year=2020 | title=Mutation bias interacts with composition bias to influence adaptive evolution | journal=PLOS Computational Biology | volume=16 | issue=9 | pages=e1008296 | doi=10.1371/journal.pcbi.1008296 | pmid=32986712 | pmc=7571706 | bibcode=2020PLSCB..16E8296C | doi-access=free }}</ref><ref name=Nei2013>{{cite book | author=M. Nei | year=2013 | title=Mutation-Driven Evolution | publisher=Oxford University Press }}</ref> Mutation bias makes systematic or predictable contributions to [[parallel evolution]].<ref name=Stoltzfus2021>{{cite book | author=A. Stoltzfus | year=2021 | title=Mutation, Randomness and Evolution | publisher=Oxford, Oxford }}</ref> Since the 1960s, genomic [[GC content]] has been thought to reflect mutational tendencies.<ref name=Freese1962>{{cite journal | author=E. Freese | year=1962 | title=On the Evolution of the Base Composition of DNA | journal=J.
| pmid=13918161 | pmc=220819 | bibcode=1962PNAS...48..582S | doi-access=free }}</ref> Mutational biases also contribute to [[codon usage bias]].<ref name=StoltzfusYampolsky2009>{{cite journal | author=A. Stoltzfus and L. Y. Yampolsky | year=2009 | title=Climbing mount probable: mutation as a cause of nonrandomness in evolution | journal=J Hered | volume=100 | issue=5 | pages=637–47 | doi=10.1093/jhered/esp048 | pmid=19625453 | doi-access=free }}</ref> Although such hypotheses are often associated with neutrality, recent theoretical and empirical results have established that mutational tendencies can influence both neutral and adaptive evolution via [[bias in the introduction of variation]] (arrival bias).
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