Editing Natural selection (section)

==Classification==

[[File:Genetic Distribution.svg|thumb|1: [[directional selection]]: a single extreme [[phenotype]] favoured.<br />2, [[stabilizing selection]]: intermediate favoured over extremes.<br />3: disruptive selection: extremes favoured over intermediate.<br />X-axis: [[phenotypic trait]]<br />Y-axis: number of organisms<br />Group A: original population<br />Group B: after selection]]

Natural selection can act on any heritable [[phenotypic trait]],<ref>{{harvnb|Zimmer|Emlen|2013}}</ref> and selective pressure can be produced by any aspect of the environment, including sexual selection and [[Competition (biology)|competition]] with members of the same or other species.<ref>{{harvnb|Miller|2000|p=8}}</ref><ref name="ArnqvistRowe2005">{{cite book |last1=Arnqvist |first1=Göran |last2=Rowe |first2=Locke |title=Sexual Conflict |url=https://books.google.com/books?id=JLfvwPqsHnMC&pg=PA15 |year=2005 |publisher=Princeton University Press |isbn=978-0-691-12218-2 |oclc=937342534 |pages=14–43}}</ref> However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.<ref name=Michigan/>

Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.

===By effect on a trait===

Selection has different effects on traits. [[Stabilizing selection]] acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous. [[Directional selection]] favours extreme values of a trait. The uncommon [[disruptive selection]] also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and [[univariate]] then both higher and lower trait levels are favoured. Disruptive selection can be a precursor to [[speciation]].<ref name=Michigan/>

===By effect on genetic diversity===

Alternatively, selection can be divided according to its effect on [[genetic diversity]]. [[Negative selection (natural selection)|Purifying or negative selection]] acts to remove genetic variation from the population (and is opposed by [[Mutation#By inheritance|''de novo'' mutation]], which introduces new variation.<ref>{{harvnb|Lemey|Salemi|Vandamme|2009}}</ref><ref>{{cite web |url=http://www.nature.com/scitable/topicpage/Negative-Selection-1136 |title=Negative Selection |last=Loewe |first=Laurence |year=2008 |work=Nature Education |publisher=[[Nature Publishing Group]] |location=Cambridge, MA |oclc=310450541}}</ref> In contrast, [[balancing selection]] acts to maintain genetic variation in a population, even in the absence of ''de novo'' mutation, by negative [[frequency-dependent selection]]. One mechanism for this is [[heterozygote advantage]], where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human [[ABO blood group]] locus has been explained in this way.<ref>{{cite journal |last1=Villanea |first1=Fernando A. |last2=Safi |first2=Kristin N. |last3=Busch |first3=Jeremiah W. |title=A General Model of Negative Frequency Dependent Selection Explains Global Patterns of Human ABO Polymorphism |journal=PLOS ONE |date=May 2015 |volume=10 |issue=5 |pages=e0125003 |doi=10.1371/journal.pone.0125003 |pmid=25946124 |pmc=4422588|bibcode=2015PLoSO..1025003V |doi-access=free }}</ref>

[[File:Life cycle of a sexually reproducing organism.svg|thumb|upright=1.1|Different types of selection act at each [[Biological life cycle|life cycle stage]] of a sexually reproducing organism.<ref name=Christiansen1984/>]]

===By life cycle stage===

Another option is to classify selection by the [[Biological life cycle|life cycle]] stage at which it acts. Some biologists recognise just two types: [[Natural selection#Types of selection|viability (or survival) selection]], which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase the rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on [[gamete]] survival, and compatibility selection, acting on [[zygote]] formation.<ref name=Christiansen1984>{{harvnb|Christiansen|1984|pp=65–79}}</ref>

===By unit of selection===

Selection can also be classified by the level or [[unit of selection]]. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. [[Gene selection]] acts directly at the level of the gene. In [[kin selection]] and [[intragenomic conflict]], gene-level selection provides a more apt explanation of the underlying process. [[Group selection]], if it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.<ref>{{cite journal |author=Wade, Michael J. |display-authors=etal |title=Multilevel and kin selection in a connected world |journal=Nature |date=2010 |volume=463 |issue=7283 |pages=E8–E9 | doi=10.1038/nature08809 |pmid=20164866 |pmc=3151728|bibcode=2010Natur.463....8W }}</ref>

===By resource being competed for===

[[File:Pavo cristatus in Barbados Wildlife Reserve 12.jpg|thumb|upright=1.2|right|The [[Peafowl|peacock]]'s elaborate plumage is mentioned by Darwin as an example of [[sexual selection]],<ref name=DarwinSexualSelection>Darwin, Charles (1859). On the Origin of Species (1st edition). Chapter 4, page 88. "And this leads me to say a few words on what I call Sexual Selection. This depends ..." http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=12</ref> and is a classic example of [[Fisherian runaway]],<ref name=Greenfield/> driven to its conspicuous size and [[animal coloration|coloration]] through [[mate choice]] by females over many generations.]]

{{Further|Sexual selection}}

Finally, selection can be classified according to the [[Resource (biology)|resource]] being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. [[Ecological selection]] is natural selection via any means other than sexual selection, such as kin selection, competition, and [[Infanticide (zoology)|infanticide]]. Following Darwin, natural selection is sometimes defined as ecological selection,<ref name="Blute 2019">{{cite journal |last=Blute |first=Marion |title=A New, New Definition of Evolution by Natural Selection |journal=Biological Theory |volume=14 |issue=4 |date=2019 |issn=1555-5542 |doi=10.1007/s13752-019-00328-4 |pages=280–281 |url=https://www.researchgate.net/profile/Marion-Blute/publication/335802423_A_New_New_Definition_of_Evolution_by_Natural_Selection/links/65b562c779007454973ead62/A-New-New-Definition-of-Evolution-by-Natural-Selection.pdf}}</ref> in which case sexual selection is considered a separate mechanism.<ref>{{harvnb|Mayr|2006}}</ref>

Sexual selection as first articulated by Darwin (using the example of the [[Peafowl|peacock]]'s tail)<ref name=DarwinSexualSelection/> refers specifically to competition for mates,<ref>{{harvnb|Andersson|1994}}</ref> which can be ''intrasexual'', between individuals of the same sex, that is male–male competition, or ''intersexual'', where one gender [[mate choice|chooses mates]], most often with males displaying and females choosing.<ref name="Hosken2011">{{cite journal |last1=Hosken |first1=David J. |last2=House |first2=Clarissa M. |title=Sexual Selection |journal=Current Biology |date=January 2011 |doi=10.1016/j.cub.2010.11.053 |pmid=21256434 |volume=21 |issue=2 |pages=R62–R65|s2cid=18470445 |doi-access=free |bibcode=2011CBio...21..R62H }}</ref> However, in some species, mate choice is primarily by males, as in some fishes of the family [[Syngnathidae]].<ref name="Eens">{{cite journal |last1=Eens |first1=Marcel |last2=Pinxten |first2=Rianne |date=5 October 2000 |title=Sex-role reversal in vertebrates: behavioural and endocrinological accounts |journal=Behavioural Processes |volume=51 |issue=1–3 |pages=135–147 |doi=10.1016/S0376-6357(00)00124-8 |pmid=11074317|s2cid=20732874 }}</ref><ref name="Barlow">{{cite journal |last=Barlow |first=George W. |date=March 2005 |title=How Do We Decide that a Species is Sex-Role Reversed? |journal=[[The Quarterly Review of Biology]] |volume=80 |issue=1 |pages=28–35 |doi=10.1086/431022 |pmid=15884733|s2cid=44774132 }}</ref>

<!--[[File:Alligator Pipefish 2.jpg|upright=1.2|thumb|[[Alligator pipefish]] males select females.]]-->
Phenotypic traits can be [[signalling theory|displayed]] in one sex and desired in the other sex, causing a [[positive feedback]] loop called a [[Fisherian runaway]], for example, the extravagant plumage of some male birds such as the peacock.<ref name=Greenfield>{{cite journal |author1=Greenfield, M.D. |author2=Alem, S. |author3=Limousin, D. |author4=Bailey, N.W. |title=The dilemma of Fisherian sexual selection: Mate choice for indirect benefits despite rarity and overall weakness of trait-preference genetic correlation |journal=Evolution |date=2014 |volume=68 |issue=12 |pages=3524–3536 |doi=10.1111/evo.12542 |pmid=25308282 |s2cid=2619084 |doi-access=free }}</ref> An alternate theory proposed by the same [[Ronald Fisher]] in 1930 is the [[sexy son hypothesis]], that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of [[Deer|stags]], which are used in combat with other stags. More generally, intrasexual selection is often associated with [[sexual dimorphism]], including differences in body size between males and females of a species.<ref name="Hosken2011"/>