Editing Molecular evolution (section)

===Selection===

{{Main|Natural selection}}

Selection can occur when an allele confers greater [[fitness (biology)|fitness]], i.e. greater ability to survive or reproduce, on the average individual than carries it. A '''selectionist''' approach emphasizes e.g. that biases in [[codon usage bias|codon usage]] are due at least in part to the ability of even [[weak selection]] to shape molecular evolution.<ref>{{cite journal | vauthors = Hershberg R, Petrov DA | title = Selection on codon bias | journal = Annual Review of Genetics | volume = 42 | issue = 1 | pages = 287–299 | date = December 2008 | pmid = 18983258 | doi = 10.1146/annurev.genet.42.110807.091442 | s2cid = 7085012 }}</ref>

Selection can also operate at the gene level at the expense of organismal fitness, resulting in [[intragenomic conflict]]. This is because there can be a selective advantage for [[Selfish DNA|selfish genetic elements]] in spite of a host cost. Examples of such selfish elements include [[transposable elements]], [[meiotic drive]]rs, and [[Selfish genetic element#Selfish mitochondria|selfish mitochondria]].

Selection can be [[Population genetics#Detecting selection|detected]] using the [[Ka/Ks ratio]], the [[McDonald–Kreitman test]]. Rapid [[Adaptation|adaptive evolution]] is often found for genes involved in [[intragenomic conflict]], [[sexual antagonistic coevolution]], and the [[immune system]].