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{{short description|Biological entity within the hierarchy of biological organization}}
A '''unit of selection''' is a biological entity within the hierarchy of biological organisation (e.g. [[genes]], [[cells]], [[individuals]], [[deme|groups]], [[species]]) that is directly subject to [[natural selection]]. There has been intense debate among evolutionary biologists about the extent to which evolution has been shaped by selection pressures acting at the different levels of biological organisation.
{{more citations needed|date=March 2018}}
[[File:Wilson's Matryoshka Doll Multilevel Selection Model.jpg|thumb|upright=1.3|[[David Sloan Wilson]] and [[Elliott Sober]]'s 1994 Multilevel [[natural selection|Selection]] Model, illustrated by a nested set of Russian [[matryoshka doll]]s. Wilson himself compared his model to such a set.]]
== The debate ==
A '''unit of selection''' is a [[biological]] entity within the hierarchy of [[biological organization]] (for example, an entity such as: a [[RNA world hypothesis|self-replicating molecule]], a [[gene]], a [[cell (biology)|cell]], an [[organism]], a [[Group selection|group]], or a [[species]]) that is subject to [[natural selection]]. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.<ref name="okasha2006">Okasha, S. (2006) ''Evolution and the levels of selection''. Oxford University Press.</ref><ref name=HullLangmanGlenn2001>{{cite book |title=Science and Selection: Essays on biological evolution and the philosophy of science |chapter=Chapter 3: A general account of selection |author1=Hull, David L. |author2=Langman, Rodney E. |author3=Glenn, Sigrid S. |publisher=Cambridge University Press |year=2001 |isbn=9780521644051 |chapter-url-access=registration |chapter-url=https://archive.org/details/scienceselection0000hull }}</ref><ref>{{Cite journal|last1=Zee|first1=Peter|last2=Dyken|first2=J. David Van|last3=Bever|first3=James D.|last4=Richerson|first4=Peter J.|last5=Fletcher|first5=Jeffrey A.|last6=Linksvayer|first6=Timothy A.|last7=Breden|first7=Felix|last8=Fields|first8=Peter|last9=Edmund D. Brodie Iii|date=February 2010|title=Multilevel and kin selection in a connected world|journal=Nature|language=en|volume=463|issue=7283|pages=E8–E9|doi=10.1038/nature08809|pmid=20164866|pmc=3151728|issn=1476-4687|bibcode=2010Natur.463....8W}}</ref>
Evolutionary biologists have debated for several decades about the levels and units of selection that are most likely to result in adaptive evolution, e.g., about the relative importance of group and individual selection in driving the evolution of [[altruism]] in several taxonomic groups (altruism reduces the fitness of individuals which engage in it, so that it cannot have evolved because of selection acting on single individuals in isolation; see [[Kin selection]]).


There is debate over the relative importance of the units themselves. For instance, is it group or individual selection that has driven the evolution of [[altruism]]? Where altruism reduces the fitness of ''individuals'', individual-centered explanations for the evolution of altruism become complex and rely on the use of [[game theory]],<ref name=MaynardSmith>{{cite book |title=Minds, Machines and Evolution |editor=Hookway, Christopher |author=Maynard Smith, John |author-link=John Maynard Smith |chapter=The evolution of animal intelligence |chapter-url=https://books.google.com/books?id=bb86AAAAIAAJ&q=%22I+turn+now+to+evolutionary+game+theory%22&pg=PA64 |page=64 |isbn=9780521338288 |year=1986|publisher=CUP Archive }}</ref><ref name= Dugatkin>{{cite book |title=Game Theory and Animal Behavior |editor1=Dugatkin, Lee Alan |editor2=Reeve, Hudson Kern|chapter-url=https://books.google.com/books?id=ldmRTvHAFFQC&pg=PA52 |page=52 |author=Dugatkin, Lee Alan |chapter= §3.2.3 Category III: Group selection |isbn=9780195350203 |year=1998 |publisher=Oxford University Press |quote=group-selected cooperation can always be cast within some broad-based individual selection model}}</ref> for instance; see [[kin selection]] and [[group selection]]. There also is debate over the definition of the units themselves,<ref>{{Cite book|last=Bourrat|first=Pierrick|date=August 2021|title=Facts, Conventions, and the Levels of Selection|url=https://www.cambridge.org/core/elements/facts-conventions-and-the-levels-of-selection/8EAF88974A3BE92761217A2EC6AB4634|access-date=2021-08-23|website=Elements in the Philosophy of Biology|doi=10.1017/9781108885812 |isbn=9781108885812 |s2cid=238732212 |language=en}}</ref> and the roles for selection and replication,<ref name=HullLangmanGlenn2001/> and whether these roles may change in the course of evolution.<ref name=Sydow>{{cite book |title=From Darwinian Metaphysics towards Understanding the Evolution of Evolutionary Mechanisms. A Historical and Philosophical Analysis of Gene-Darwinism and Universal Darwinism |author=von Sydow, Momme |publisher=Universitaetsverlag Goettingen Press |year=2012 |page=481 |isbn=978-3863950064}}</ref>
The unit-of-selection topic has also enjoyed high resonance with the general public because many mistakenly believe that deep meaning about evolution and thus about life in general and human nature in particular, may flow from the fact that evolution may have been shaped by competition between units at some specific level of organisation.


== Fundamental theory ==
Richard Dawkins, for instance, has written several books championing the view that the unit of selection is the individual gene. He argues that genes are selfish and that most if not all of the history, structure, and dynamics of the living world is the result of an ongoing race among selfish genes to produce more copies of themselves than competing genes by manipulating the characteristics of individuals, (kin) groups, etc.


Two useful introductions to the fundamental theory underlying the unit of selection issue and debate, which also present examples of multi-level selection from the entire range of the biological hierarchy (typically with entities at level ''N''-1 competing for increased representation, i.e., higher frequency, at the immediately higher level ''N'', e.g., organisms in populations or cell lineages in organisms), are [[Richard Lewontin|Richard Lewontin's]] classic piece ''[[The Units of Selection]]''<ref>[[Richard Lewontin|Lewontin, Richard]] 1970. The Units of Selection. [[Annual Review of Ecology, Evolution, and Systematics|''Annual Review of Ecology and Systematics'']] 1: 1-18.</ref> and [[John Maynard-Smith]] and [[Eörs Szathmáry]]'s co-authored book, ''[[The Major Transitions in Evolution]]''. As a theoretical introduction to units of selection, Lewontin writes:
Almost without exception, however, modern evolutionary biologists view evolution as a natural process of cause and effect that is deprived of ultimate teachings about the meaning of life.
<blockquote>The generality of the principles of natural selection means that any entities in nature that have variation, reproduction, and heritability may evolve. ...the principles can be applied equally to genes, organisms, populations, species, and at opposite ends of the scale, prebiotic molecules and ecosystems." (1970, pp. 1-2)</blockquote>


[[Elisabeth Lloyd]]'s book ''The Structure and Confirmation of Evolutionary Theory'' provides a basic ''philosophical'' introduction to the debate. Three more recent introductions include [[Samir Okasha]]'s book ''Evolution and the Levels of Selection'', Pierrick Bourrat's book ''Facts, Conventions, and the Levels of Selection'', and [[Elisabeth Lloyd]] and Javier Suárez book ''Units of Selection''.
The philosopher of biology [[Elliot Sober]] has indeed stressed that causal explanations of the process of selection should be as central to the narratives describing natural selection as they are to any scientific narrative. He contends therefore that the unit of selection can be found by dissecting how differences in fitness are generated in each specific case. He points out, indeed, that natural selection entails the differential reproduction of many things ("selection of") but that it is the causation of differences in reproductive output what points to what is directly "selected-for" and thus to the level and the unit of selection.


== Selection at each level ==
Sober's focus on how fitness differences are generated is simple enough, but it comes after decades of confusion about which criterion should be adopted in determining what the unit of selection is in each specific case. [[George C. Williams|G.C. Williams]] and Dawkins, for instance, have treated the problem of the unit of selection together with that of the unit of heredity and of the unit of evolution, which does not help one view selection as a process that generates fitness differences.


Below, undisputed cases of selection at the genic, cellular, individual, and group level are presented and discussed, starting with individual selection which is the most familiar type.
Below, cases of selection at the genic, cellular, individual and group level from within the multi-level selection perspective are presented and discussed.


=== Nucleic acid ===
== Selection at the level of individual organism ==
{{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.
Selection at the level of the organism can be described as [[Darwinism]], and is well understood and considered common. When a gazelle, for instance, has a trait that allows it to run faster than others and therefore to avoid predators more effectively so that ultimately it manages to stay alive longer and reproduce over more breeding seasons, the causation of the higher fitness of this gazelle can be accounted for fully only if one looks at how individual gazelles fare under predation so one can come to the conclusion that the faster gazelle's speed allows it to avoid predation better.


[[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>
The speed of the faster gazelle could be caused by a single gene, be polygenic, or be fully environmentally determined, but the unit of selection in this case is the individual since the speed of the gazelles that selection is evaluating, is a functional property of each individual gazelle, i.e., individual speed is the property being selected-for.


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.
The description of the causation chain in this case of selection can be stopped at the individual level because the generation of fitness differences is [[supervenient]] to the (various possible) causes below the individual level (i.e. differences in speed were necessary and sufficient in this case of selection).


[[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.
Note, however, that in this case of selection it is the presence of predators what creates an opportunity for faster individual gazelles to be selected and for their speed to be selected-for, so that this presence is, in the deepest ultimate sense, the cause of the selection regime that is active
== Selection at the level of the group ==


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]].
Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Some mosquito-transmitted rabbit viruses, for instance, are only transmitted to uninfected rabbits from infected rabbits that are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it, since otherwise all the viruses inside the dead rabbit would rot with it. And indeed in natural systems such viruses display much lower virulence levels than do mutants of the same viruses which in laboratory culture readily outcompete non-virulent variants (or than do tick-transmitted viruses since ticks do bite dead rabbits).


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.
== Selection at the level of the gene ==


=== Epigene ===
Evolutionary biologists have described clear-cut examples of selection at the level of the gene, such as [[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 generate copies of themselves that later insert more or less randomly in the genome. 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 to be reduce strongly individual fitness.
{{Main article|Transgenerational epigenetic inheritance|Contribution of epigenetic modifications to evolution}}
{{expand section|date=March 2018}}


There is also view that evolution is acting on [[epigene]]s.<ref>{{cite journal | author=Hunter, P. | title=Extended phenotype redux. How far can the reach of genes extend in manipulating the environment of an organism? | journal=EMBO Rep | volume=10 | issue=3 | pages=212–5 |pmc=2658563 | pmid=19255576 | doi=10.1038/embor.2009.18| year=2009 }}</ref>
== Selection at the level of the cell ==
[[Leo Buss]] in his book ''The Evolution of Individuality'' proposes that much of the [[Evolutionary developmental biology|evolution of development]] in [[metazoan]]s has been constrained and shaped by the conflict between selection at the level of the cell and that at the level of the multicellular individual. With this theory it is possible to address phenomena as diverse as [[cancer]], [[gastrulation]], and germ line sequestration.
== Species selection and selection at higher taxonomic levels ==
That selection can operate at and above the level of species remains controversial among biologists. One particular defender of the idea of species selection is [[Stephen Jay Gould|S.J. Gould]] who has proposed the view that there exist [[macroevolution]]ary processes which shape evolution beyond the level of species and are not driven by the [[microevolution]]ary mechanisms already characterized by the [[Modern evolutionary synthesis|Modern Synthesis]]. If one views species as individuals that replicate (speciate) and die (go extinct), then species could be subject to selection and thus could change their occurrence over geological time, much as heritable selected-for traits change theirs over the generations.


=== Cell ===
For species selection to occur however, the patterns of differential persistence of species over geological time must be the result of selection for species-intrinsic properties rather than for properties of genes, cells, individuals, or populations of the species involved. In other words, species must be shown to have been units of selection whose properties were directly evaluated by selection. While the fossil record clearly shows differential persistence of species, examples of species-intrinsic properties subject to natural selection have been much harder to document.


[[Leo Buss]] in his book ''[[The Evolution of Individuality]]'' proposes that much of the [[Evolutionary developmental biology|evolution of development]] in [[animal]]s reflects the conflict between selective pressures acting at the level of the cell and those acting at the level of the multicellular individual. This perspective can shed new light on phenomena as diverse as [[gastrulation]] and germ line sequestration.


This selection for unconstrained proliferation is in conflict with the fitness interests of the individual, and thus there is tension between selection at the level of the cell and selection at the level of the individual. Since the proliferation of specific cells of the vertebrate immune system to fight off infecting pathogens is a case of programmed and exquisitely contained cellular proliferation, it represents a case of the individual manipulating selection at the level of the cell to enhance its own fitness. In the case of the vertebrate immune system, selection at the level of the cell and individual are not in conflict.

Some view [[cancer stem cell]]s as units of selection.<ref>{{cite journal | author=Greaves, Mel | volume=6 | issue=1 | title=Cancer stem cells as 'units of selection' | journal=Evolutionary Applications | pages=102–108 |doi=10.1111/eva.12017| pmid=23396760 | pmc=3567475 | year=2013 | bibcode=2013EvApp...6..102G }}</ref>

=== Behavioural ===
{{further|Memetics|Dual inheritance theory|Sociocultural evolution|Cultural evolution}}

Gene–culture coevolution was developed to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution.

=== Organism ===
{{further|Hologenome theory of evolution}}

Selection at the level of the organism can be described as [[Darwinism]], and is well understood and considered common. If a relatively faster gazelle manages to survive and reproduce more, the causation of the higher fitness of this gazelle can be fully accounted for if one looks at how individual gazelles fare under predation.

The speed of the faster gazelle could be caused by a single gene, be polygenic, or be fully environmentally determined, but the unit of selection in this case is the individual since speed is a property of each individual gazelle.

When speaking about individual organism evolution an [[extended phenotype]] and [[superorganism]] must be also mentioned.

=== Group ===
{{main|Group selection}}
{{See also|Free-rider problem|Fisher's principle}}

If a group of organisms, owing to their interactions or division of labor, provides superior fitness compared to other groups, where the fitness of the group is higher or lower than the mean fitness of the constituent individuals, group selection can be declared to occur.<ref>{{Cite web | url=https://www.britannica.com/science/group-selection |title = Group selection &#124; biology}}</ref>

Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Many common examples of group traits are reducible to individual traits, however. Selection of these traits is thus more simply explained as selection of individual traits.

Some mosquito-transmitted rabbit viruses are only transmitted to uninfected rabbits from infected rabbits which are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it, since otherwise all the viruses inside the dead rabbit would rot with it. And indeed in natural systems such viruses display much lower virulence levels than do mutants of the same viruses that in laboratory culture readily outcompete non-virulent variants (or than do tick-transmitted viruses since ticks do bite dead rabbits).

In the previous passage, the group is assumed to have "lower virulence", i.e., "virulence" is presented as a group trait. One could argue then that the selection is in fact against individual viruses that are too virulent. In this case, however, the fitness of all viruses within a rabbit is affected by what the group does to the rabbit. Indeed, the proper, directly selected group property is that of "not killing the rabbit too early" rather than individual virulence. In situations such as these, we would expect there to be selection for cooperation amongst the viruses in a group in such a way that the group will not "kill the rabbit too early". It is of course true that any group behavior is the result of individual traits, such as individual viruses suppressing the virulence of their neighbours, but the causes of phenotypes are rarely the causes of fitness differences.

=== Species and higher levels ===
{{Main|Punctuated equilibrium}}
{{See also|Speciation|Species concept|l2=Species problem|Species complex}}

It remains controversial among biologists whether selection can operate at and above the level of species.<ref name="Vrba 1984">{{cite journal | last=Vrba | first=Elisabeth S. | title=What is Species Selection? | journal=Systematic Zoology | volume=33 | issue=3 | pages=318–328 | year=1984 | doi=10.2307/2413077 | jstor=2413077 }}</ref> Proponents of species selection include [[R. A. Fisher]] (1929);<ref name="Vrba 1984"/> [[Sewall Wright]] (1956);<ref name="Vrba 1984"/> [[Richard Lewontin]] (1970);<ref name="Vrba 1984"/> [[Niles Eldredge]] & [[Stephen Jay Gould]] (1972); [[Steven M. Stanley]] (1975).<ref name="Stanley 1975">{{cite journal | last=Stanley | first=SM | title=A theory of evolution above the species level. | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=72 | issue=2 | year=1975 | pmid=1054846 | pmc=432371 | pages=646–650 | doi=10.1073/pnas.72.2.646| bibcode=1975PNAS...72..646S | doi-access=free }}</ref><ref name="Vrba 1984"/> Gould proposed that there exist [[macroevolution]]ary processes which shape evolution, not driven by the [[microevolution]]ary mechanisms of the [[Neo-Darwinism|Modern Synthesis]].<ref>{{cite journal |last=Lieberman |first=Bruce S. |author2=Vrba, Elisabeth S. |title=Stephen Jay Gould on species selection: 30 years of insight |journal=Paleobiology |date=Spring 2005 |volume=31 |issue=2 Suppl |pages=113–121 |doi=10.1666/0094-8373(2005)031[0113:SJGOSS]2.0.CO;2 |s2cid=14801676 |url=http://paleo.ku.edu/geo/faculty/BSL/gouldselection.pdf |access-date=2012-07-08 |archive-url=https://web.archive.org/web/20120918210548/http://paleo.ku.edu/geo/faculty/BSL/gouldselection.pdf |archive-date=2012-09-18 |url-status=dead }}</ref> If one views species as entities that replicate (speciate) and die (go extinct) within a [[clade]], then species could be subject to selection and thus could change their occurrence over geological time, much as heritable selected-for traits change theirs over generations.
For evolution to be driven by species selection, differential success must be the result of selection upon species-intrinsic properties, rather than for properties of genes, cells, individuals, or populations within species. Such properties include, for example, population structure, their propensity to speciate, extinction rates, and geological persistence. While the fossil record shows differential persistence of species, examples of species-intrinsic properties subject to natural selection have been much harder to document.

One issue with selection among [[clade]]s is that they are not independent, i.e. all species are descended from the same [[last universal common ancestor]] and are thus part of the same clade.<ref name="okasha2006" /> This criticism does not apply to selection among different [[gene family|gene families]] that are not evolutionarily related, and which are [[Gene duplication|duplicated]] and lost at different rates rather than speciating and going extinct at different rates.<ref name="james2023" />

In the microbial realm, it has been interpreted that the unit of selection is a blend of ecological and functional behaviors, or [[Guild (ecology)|guilds]], beyond the species-level.<ref>{{Cite journal |last1=Shapiro |first1=B. Jesse |last2=Polz |first2=Martin F. |date=May 2014 |title=Ordering microbial diversity into ecologically and genetically cohesive units |url=http://dx.doi.org/10.1016/j.tim.2014.02.006 |journal=Trends in Microbiology |volume=22 |issue=5 |pages=235–247 |doi=10.1016/j.tim.2014.02.006 |pmid=24630527 |issn=0966-842X|pmc=4103024 |hdl=1721.1/101684 }}</ref>


==References==
==References==
{{reflist|30em}}
*[[Leo Buss|Buss, Leo W.]] (1988) ''The Evolution of Individuality'' (ISBN 0691084688)

==Sources==
* [[Richard Dawkins|Dawkins, Richard]] (1976; second edition 1989) ''The selfish gene''. ISBN 0192860925.
* Brandon, Robert; Burian, Richard M. eds., (1984) ''Genes, Organisms, Population: Controversies Over the Units of Selection''. Cambridge MA: MIT Press. ({{ISBN|978-0-262-02205-7}})
* [[Stephen Jay Gould|Gould, Stephen Jay]] (2002) ''The Structure of Evolutionary Theory''. Harvard University Press.
* [[Leo Buss|Buss, Leo W.]] (1988) ''The Evolution of Individuality''. ({{ISBN|0-691-08468-8}})
* [[George C. Williams (biologist)|Williams, G. C.]] (1966) ''[[Adaptation and Natural Selection]]''. Princeton University Press, Princeton. ({{ISBN|0-691-02615-7}})
* [[Richard Dawkins|Dawkins, Richard]] (1976; 1989; 2006) ''[[The Selfish Gene]]''. Oxford University Press, Oxford. ({{ISBN|0-19-286092-5}})
* [[Stephen Jay Gould|Gould, Stephen Jay]] (2002) ''[[The Structure of Evolutionary Theory]]''. Harvard University Press.
* [[Elisabeth Lloyd|Lloyd, Elisabeth]] (1988) ''The Structure and Confirmation of Evolutionary Theory'', Greenwood Press (Reprinted Princeton University Press, 1994 {{ISBN|0-691-00046-8}}).
* [[Elliott Sober|Sober, Elliott]] (1984; 1993) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus''. The University of Chicago Press.
* [[Elliott Sober|Sober, Elliott]] (1984; 1993) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus''. The University of Chicago Press.
* [[John Maynard Smith|Maynard Smith, J.]] ''Evolutionary Genetics''. Oxford University Press, 1998.
*[[Samir Okasha|Okasha, S.]] (2006) ''Evolution and the levels of Selection''. Oxford University Press.
*Bourrat, P. (2021) ''[https://www.cambridge.org/core/elements/facts-conventions-and-the-levels-of-selection/8EAF88974A3BE92761217A2EC6AB4634 Facts, Conventions and the Levels of Selection]''. Cambridge University Press.

==External links==
* {{cite IEP |url-id=altr-grp |title=Altruism and Group Selection}}
* {{cite journal | author=Dusek, Val | author-link=Val Dusek | year=2002 | title=Lewontin's Living Legacy: Levels of Selection and Organismic Construction of the Environment | url=http://www.human-nature.com/nibbs/02/lewontin.html | journal=Human Nature Review | volume=2 | pages=367–374 }}
* Lloyd, Elisabeth, [http://plato.stanford.edu/archives/fall2005/entries/selection-units/ "Units and Levels of Selection."] ''The Stanford Encyclopedia of Philosophy'', (Fall 2005 Edition), Edward N. Zalta (ed.)
* [[Ernst Mayr|Mayr, Ernst]] (1997). [http://www.pnas.org/cgi/content/full/94/6/2091 "The objects of selection] ''Proc. Natl. Acad. Sci. USA'' 94 (March): 2091-94.

{{Evolution}}
{{Evolutionary psychology}}

[[Category:Evolutionary biology]]
[[Category:Evolutionary biology]]
[[Category:Population genetics]]
[[Category:Population genetics]]

Latest revision as of 20:51, 15 December 2024

David Sloan Wilson and Elliott Sober's 1994 Multilevel Selection Model, illustrated by a nested set of Russian matryoshka dolls. Wilson himself compared his model to such a set.

A unit of selection is a biological entity within the hierarchy of biological organization (for example, an entity such as: a self-replicating molecule, a gene, a cell, an organism, a group, or a species) that is subject to natural selection. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.[1][2][3]

There is debate over the relative importance of the units themselves. For instance, is it group or individual selection that has driven the evolution of altruism? Where altruism reduces the fitness of individuals, individual-centered explanations for the evolution of altruism become complex and rely on the use of game theory,[4][5] for instance; see kin selection and group selection. There also is debate over the definition of the units themselves,[6] and the roles for selection and replication,[2] and whether these roles may change in the course of evolution.[7]

Fundamental theory

[edit]

Two useful introductions to the fundamental theory underlying the unit of selection issue and debate, which also present examples of multi-level selection from the entire range of the biological hierarchy (typically with entities at level N-1 competing for increased representation, i.e., higher frequency, at the immediately higher level N, e.g., organisms in populations or cell lineages in organisms), are Richard Lewontin's classic piece The Units of Selection[8] and John Maynard-Smith and Eörs Szathmáry's co-authored book, The Major Transitions in Evolution. As a theoretical introduction to units of selection, Lewontin writes:

The generality of the principles of natural selection means that any entities in nature that have variation, reproduction, and heritability may evolve. ...the principles can be applied equally to genes, organisms, populations, species, and at opposite ends of the scale, prebiotic molecules and ecosystems." (1970, pp. 1-2)

Elisabeth Lloyd's book The Structure and Confirmation of Evolutionary Theory provides a basic philosophical introduction to the debate. Three more recent introductions include Samir Okasha's book Evolution and the Levels of Selection, Pierrick Bourrat's book Facts, Conventions, and the Levels of Selection, and Elisabeth Lloyd and Javier Suárez book Units of Selection.

Selection at each level

[edit]

Below, cases of selection at the genic, cellular, individual and group level from within the multi-level selection perspective are presented and discussed.

Nucleic acid

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Main article: Gene-centered view of evolution
See also: Missing heritability problem

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 functions.[9]

Some clear-cut examples of selection at the level of the gene include meiotic drive and retrotransposons. 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.

Retrotransposons 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.[10] It is possible that RNA-based evolution is still taking place today. Other subcellular entities such as viruses, both DNA-based and RNA-based, do evolve.

The gene-centered view of evolution normally refers to selection among different alleles of the same gene. However, gene families also differ in their tendency to diversify and avoid loss during evolution.[11] This latter form of selection more closely resembles clade selection of groups of species.

Epigene

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Main articles: Transgenerational epigenetic inheritance and Contribution of epigenetic modifications to evolution

There is also view that evolution is acting on epigenes.[12]

Cell

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Leo Buss in his book The Evolution of Individuality proposes that much of the evolution of development in animals reflects the conflict between selective pressures acting at the level of the cell and those acting at the level of the multicellular individual. This perspective can shed new light on phenomena as diverse as gastrulation and germ line sequestration.

This selection for unconstrained proliferation is in conflict with the fitness interests of the individual, and thus there is tension between selection at the level of the cell and selection at the level of the individual. Since the proliferation of specific cells of the vertebrate immune system to fight off infecting pathogens is a case of programmed and exquisitely contained cellular proliferation, it represents a case of the individual manipulating selection at the level of the cell to enhance its own fitness. In the case of the vertebrate immune system, selection at the level of the cell and individual are not in conflict.

Some view cancer stem cells as units of selection.[13]

Behavioural

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Further information: Memetics, Dual inheritance theory, Sociocultural evolution, and Cultural evolution

Gene–culture coevolution was developed to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution.

Organism

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Further information: Hologenome theory of evolution

Selection at the level of the organism can be described as Darwinism, and is well understood and considered common. If a relatively faster gazelle manages to survive and reproduce more, the causation of the higher fitness of this gazelle can be fully accounted for if one looks at how individual gazelles fare under predation.

The speed of the faster gazelle could be caused by a single gene, be polygenic, or be fully environmentally determined, but the unit of selection in this case is the individual since speed is a property of each individual gazelle.

When speaking about individual organism evolution an extended phenotype and superorganism must be also mentioned.

Group

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Main article: Group selection
See also: Free-rider problem and Fisher's principle

If a group of organisms, owing to their interactions or division of labor, provides superior fitness compared to other groups, where the fitness of the group is higher or lower than the mean fitness of the constituent individuals, group selection can be declared to occur.[14]

Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Many common examples of group traits are reducible to individual traits, however. Selection of these traits is thus more simply explained as selection of individual traits.

Some mosquito-transmitted rabbit viruses are only transmitted to uninfected rabbits from infected rabbits which are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it, since otherwise all the viruses inside the dead rabbit would rot with it. And indeed in natural systems such viruses display much lower virulence levels than do mutants of the same viruses that in laboratory culture readily outcompete non-virulent variants (or than do tick-transmitted viruses since ticks do bite dead rabbits).

In the previous passage, the group is assumed to have "lower virulence", i.e., "virulence" is presented as a group trait. One could argue then that the selection is in fact against individual viruses that are too virulent. In this case, however, the fitness of all viruses within a rabbit is affected by what the group does to the rabbit. Indeed, the proper, directly selected group property is that of "not killing the rabbit too early" rather than individual virulence. In situations such as these, we would expect there to be selection for cooperation amongst the viruses in a group in such a way that the group will not "kill the rabbit too early". It is of course true that any group behavior is the result of individual traits, such as individual viruses suppressing the virulence of their neighbours, but the causes of phenotypes are rarely the causes of fitness differences.

Species and higher levels

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Main article: Punctuated equilibrium
See also: Speciation, Species problem, and Species complex

It remains controversial among biologists whether selection can operate at and above the level of species.[15] Proponents of species selection include R. A. Fisher (1929);[15] Sewall Wright (1956);[15] Richard Lewontin (1970);[15] Niles Eldredge & Stephen Jay Gould (1972); Steven M. Stanley (1975).[16][15] Gould proposed that there exist macroevolutionary processes which shape evolution, not driven by the microevolutionary mechanisms of the Modern Synthesis.[17] If one views species as entities that replicate (speciate) and die (go extinct) within a clade, then species could be subject to selection and thus could change their occurrence over geological time, much as heritable selected-for traits change theirs over generations. For evolution to be driven by species selection, differential success must be the result of selection upon species-intrinsic properties, rather than for properties of genes, cells, individuals, or populations within species. Such properties include, for example, population structure, their propensity to speciate, extinction rates, and geological persistence. While the fossil record shows differential persistence of species, examples of species-intrinsic properties subject to natural selection have been much harder to document.

One issue with selection among clades is that they are not independent, i.e. all species are descended from the same last universal common ancestor and are thus part of the same clade.[1] This criticism does not apply to selection among different gene families that are not evolutionarily related, and which are duplicated and lost at different rates rather than speciating and going extinct at different rates.[11]

In the microbial realm, it has been interpreted that the unit of selection is a blend of ecological and functional behaviors, or guilds, beyond the species-level.[18]

References

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  1. ^ a b Okasha, S. (2006) Evolution and the levels of selection. Oxford University Press.
  2. ^ a b Hull, David L.; Langman, Rodney E.; Glenn, Sigrid S. (2001). "Chapter 3: A general account of selection". Science and Selection: Essays on biological evolution and the philosophy of science. Cambridge University Press. ISBN 9780521644051.
  3. ^ Zee, Peter; Dyken, J. David Van; Bever, James D.; Richerson, Peter J.; Fletcher, Jeffrey A.; Linksvayer, Timothy A.; Breden, Felix; Fields, Peter; Edmund D. Brodie Iii (February 2010). "Multilevel and kin selection in a connected world". Nature. 463 (7283): E8–E9. Bibcode:2010Natur.463....8W. doi:10.1038/nature08809. ISSN 1476-4687. PMC 3151728. PMID 20164866.
  4. ^ Maynard Smith, John (1986). "The evolution of animal intelligence". In Hookway, Christopher (ed.). Minds, Machines and Evolution. CUP Archive. p. 64. ISBN 9780521338288.
  5. ^ Dugatkin, Lee Alan (1998). "§3.2.3 Category III: Group selection". In Dugatkin, Lee Alan; Reeve, Hudson Kern (eds.). Game Theory and Animal Behavior. Oxford University Press. p. 52. ISBN 9780195350203. group-selected cooperation can always be cast within some broad-based individual selection model
  6. ^ Bourrat, Pierrick (August 2021). Facts, Conventions, and the Levels of Selection. doi:10.1017/9781108885812. ISBN 9781108885812. S2CID 238732212. Retrieved 2021-08-23. {{cite book}}: |website= ignored (help)
  7. ^ von Sydow, Momme (2012). From Darwinian Metaphysics towards Understanding the Evolution of Evolutionary Mechanisms. A Historical and Philosophical Analysis of Gene-Darwinism and Universal Darwinism. Universitaetsverlag Goettingen Press. p. 481. ISBN 978-3863950064.
  8. ^ Lewontin, Richard 1970. The Units of Selection. Annual Review of Ecology and Systematics 1: 1-18.
  9. ^ See the chapter God's utility function in Dawkins, Richard (1995). River Out of Eden. Basic Books. ISBN 0-465-06990-8.
  10. ^ Bernhardt, Harold S. (2012). "The RNA world hypothesis: The worst theory of the early evolution of life (except for all the others)a". Biology Direct. 7: 23. doi:10.1186/1745-6150-7-23. PMC 3495036. PMID 22793875.
  11. ^ a b James, Jennifer E; Nelson, Paul G; Masel, Joanna (4 April 2023). "Differential Retention of Pfam Domains Contributes to Long-term Evolutionary Trends". Molecular Biology and Evolution. 40 (4): msad073. doi:10.1093/molbev/msad073. PMC 10089649. PMID 36947137.
  12. ^ Hunter, P. (2009). "Extended phenotype redux. How far can the reach of genes extend in manipulating the environment of an organism?". EMBO Rep. 10 (3): 212–5. doi:10.1038/embor.2009.18. PMC 2658563. PMID 19255576.
  13. ^ Greaves, Mel (2013). "Cancer stem cells as 'units of selection'". Evolutionary Applications. 6 (1): 102–108. Bibcode:2013EvApp...6..102G. doi:10.1111/eva.12017. PMC 3567475. PMID 23396760.
  14. ^ "Group selection | biology".
  15. ^ a b c d e Vrba, Elisabeth S. (1984). "What is Species Selection?". Systematic Zoology. 33 (3): 318–328. doi:10.2307/2413077. JSTOR 2413077.
  16. ^ Stanley, SM (1975). "A theory of evolution above the species level". Proceedings of the National Academy of Sciences of the United States of America. 72 (2): 646–650. Bibcode:1975PNAS...72..646S. doi:10.1073/pnas.72.2.646. PMC 432371. PMID 1054846.
  17. ^ Lieberman, Bruce S.; Vrba, Elisabeth S. (Spring 2005). "Stephen Jay Gould on species selection: 30 years of insight" (PDF). Paleobiology. 31 (2 Suppl): 113–121. doi:10.1666/0094-8373(2005)031[0113:SJGOSS]2.0.CO;2. S2CID 14801676. Archived from the original (PDF) on 2012-09-18. Retrieved 2012-07-08.
  18. ^ Shapiro, B. Jesse; Polz, Martin F. (May 2014). "Ordering microbial diversity into ecologically and genetically cohesive units". Trends in Microbiology. 22 (5): 235–247. doi:10.1016/j.tim.2014.02.006. hdl:1721.1/101684. ISSN 0966-842X. PMC 4103024. PMID 24630527.

Sources

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