Editing Unit of selection (section)

=== Nucleic acid ===
{{Main article|Gene-centered view of evolution}}
{{See also|Missing heritability problem}}

[[George C. Williams (biologist)|George C. Williams]] in his influential book ''[[Adaptation and Natural Selection]]'' was one of the first to present a [[gene-centered view of evolution]] with the gene as the unit of selection, arguing that a unit of selection should exhibit a high degree of permanence.

[[Richard Dawkins]] has written several books popularizing and expanding the idea. According to Dawkins, genes cause phenotypes and a gene is 'judged' by its phenotypic effects. Dawkins distinguishes entities which survive or fail to survive ("replicators") from entities with temporary existence that interact directly with the environment ("vehicles"). Genes are "replicators" whereas individuals and groups of individuals are "vehicles". Dawkins argues that, although they are both aspects of the same process, "replicators" rather than "vehicles" should be preferred as units of selection. This is because replicators, owing to their permanence, should be regarded as the ultimate beneficiaries of adaptations. Genes are replicators and therefore the gene is the unit of selection. Dawkins further expounded this view in an entire chapter called '[[God's utility function]]' in the book ''[[River Out of Eden]]'' where he explained that genes alone have [[utility function]]s.<ref name="River_Out_of_Eden">See the chapter ''[[God's utility function]]'' in {{cite book | first=Richard | last=Dawkins | author-link=Richard Dawkins | title=River Out of Eden | publisher=Basic Books | year=1995 | isbn=0-465-06990-8| title-link=River Out of Eden }}</ref>

Some clear-cut examples of selection at the level of the gene include [[meiotic drive]] and [[retrotransposon]]s.  In both of these cases, gene sequences increase their relative frequency in a population without necessarily providing benefits at other levels of organization.  Meiotic-drive mutations (see [[segregation distortion]]) manipulate the machinery of chromosomal segregation so that chromosomes carrying the mutation are later found in more than half of the gametes produced by individuals heterozygous for the mutation, and for this reason the frequency of the mutation increases in the population.

[[Retrotransposon]]s are DNA sequences that, once replicated by the cellular machinery, insert themselves in the genome more or less randomly.  Such insertions can be very mutagenic and thus reduce drastically individual fitness, so that there is strong selection against elements that are very active.  Meiotic-drive alleles have also been shown strongly to reduce individual fitness, clearly exemplifying the potential conflict between selection at different levels.

According to the [[RNA world]] hypothesis, RNA sequences performing both enzymatic and information storage roles in autocatalytic sets were an early unit of selection and evolution that would later transition into living cells.<ref>{{Cite journal |doi = 10.1186/1745-6150-7-23|pmid = 22793875|pmc = 3495036|title = The RNA world hypothesis: The worst theory of the early evolution of life (except for all the others)a|journal = Biology Direct|volume = 7|pages = 23|year = 2012|last1 = Bernhardt|first1 = Harold S. | doi-access=free }}</ref> It is possible that  [[RNA-based evolution]] is still taking place today. Other subcellular entities such as viruses, both  [[DNA virus|DNA-based]] and [[RNA virus|RNA-based]], [[Viral evolution|do evolve]].

The gene-centered view of evolution normally refers to selection among different [[allele]]s of the same gene. However, [[gene family|gene families]] also differ in their tendency to diversify and avoid loss during evolution.<ref name="james2023">{{cite journal |last1=James |first1=Jennifer E |last2=Nelson |first2=Paul G |last3=Masel |first3=Joanna |title=Differential Retention of Pfam Domains Contributes to Long-term Evolutionary Trends |journal=Molecular Biology and Evolution |date=4 April 2023 |volume=40 |issue=4 |pages=msad073 |doi=10.1093/molbev/msad073|pmid=36947137 |pmc=10089649 }}</ref> This latter form of selection more closely resembles clade selection of groups of species.