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Unit of selection

Aunit of selectionis abiologicalentity within the hierarchy ofbiological organization(for example, an entity such as: aself-replicating molecule,agene,acell,anorganism,agroup,or aspecies) that is subject tonatural 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]

David Sloan WilsonandElliott Sober's 1994 MultilevelSelectionModel, illustrated by a nested set of Russianmatryoshka dolls.Wilson himself compared his model to such a set.

There is debate over the relative importance of the units themselves. For instance, is it group or individual selection that has driven the evolution ofaltruism?Where altruism reduces the fitness ofindividuals,individual-centered explanations for the evolution of altruism become complex and rely on the use ofgame theory,[4][5]for instance; seekin selectionandgroup 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

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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 levelN-1 competing for increased representation, i.e., higher frequency, at the immediately higher levelN,e.g., organisms in populations or cell lineages in organisms), areRichard Lewontin'sclassic pieceThe Units of Selection[8]andJohn Maynard-SmithandEörs 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 bookThe Structure and Confirmation of Evolutionary Theoryprovides a basicphilosophicalintroduction to the debate. Three more recent introductions includeSamir Okasha's bookEvolution and the Levels of Selection,Pierrick Bourrat's bookFacts, Conventions, and the Levels of Selection,andElisabeth Lloydand Javier Suárez bookUnits of Selection.

Selection at each level

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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. Williamsin his influential bookAdaptation and Natural Selectionwas one of the first to present agene-centered view of evolutionwith the gene as the unit of selection, arguing that a unit of selection should exhibit a high degree of permanence.

Richard Dawkinshas 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 bookRiver Out of Edenwhere he explained that genes alone haveutility functions.[9]

Some clear-cut examples of selection at the level of the gene includemeiotic driveandretrotransposons.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 (seesegregation 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.

Retrotransposonsare 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 theRNA worldhypothesis, 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 thatRNA-based evolutionis still taking place today. Other subcellular entities such as viruses, bothDNA-basedandRNA-based,do evolve.

The gene-centered view of evolution normally refers to selection among differentallelesof the same gene. However,gene familiesalso 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 inheritanceandContribution of epigenetic modifications to evolution

There is also view that evolution is acting onepigenes.[12]

Cell

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Leo Bussin his bookThe Evolution of Individualityproposes that much of theevolution of developmentinanimalsreflects 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 asgastrulationand 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 viewcancer stem cellsas units of selection.[13]

Behavioural

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Further information:Memetics,Dual inheritance theory,Sociocultural evolution,andCultural 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 asDarwinism,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 anextended phenotypeandsuperorganismmust be also mentioned.

Group

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Main article:Group selection
See also:Free-rider problemandFisher'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,andSpecies complex

It remains controversial among biologists whether selection can operate at and above the level of species.[15]Proponents of species selection includeR. 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 existmacroevolutionaryprocesses which shape evolution, not driven by themicroevolutionarymechanisms of theModern Synthesis.[17]If one views species as entities that replicate (speciate) and die (go extinct) within aclade,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 amongcladesis that they are not independent, i.e. all species are descended from the samelast universal common ancestorand are thus part of the same clade.[1]This criticism does not apply to selection among differentgene familiesthat are not evolutionarily related, and which areduplicatedand 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, orguilds,beyond the species-level.[18]

References

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  1. ^abOkasha, S. (2006)Evolution and the levels of selection.Oxford University Press.
  2. ^abHull, 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.ISBN9780521644051.
  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.ISSN1476-4687.PMC3151728.PMID20164866.
  4. ^Maynard Smith, John(1986)."The evolution of animal intelligence".In Hookway, Christopher (ed.).Minds, Machines and Evolution.CUP Archive. p. 64.ISBN9780521338288.
  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.ISBN9780195350203.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.ISBN9781108885812.S2CID238732212.Retrieved2021-08-23.{{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.ISBN978-3863950064.
  8. ^Lewontin, Richard1970. The Units of Selection.Annual Review of Ecology and Systematics1: 1-18.
  9. ^See the chapterGod's utility functioninDawkins, Richard(1995).River Out of Eden.Basic Books.ISBN0-465-06990-8.
  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.PMC3495036.PMID22793875.
  11. ^abJames, 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.PMC10089649.PMID36947137.
  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.PMC2658563.PMID19255576.
  13. ^Greaves, Mel (2013)."Cancer stem cells as 'units of selection'".Evolutionary Applications.6(1): 102–108.doi:10.1111/eva.12017.PMC3567475.PMID23396760.
  14. ^"Group selection | biology".
  15. ^abcdeVrba, Elisabeth S. (1984). "What is Species Selection?".Systematic Zoology.33(3): 318–328.doi:10.2307/2413077.JSTOR2413077.
  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.PMC432371.PMID1054846.
  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.S2CID14801676.Archived fromthe original(PDF)on 2012-09-18.Retrieved2012-07-08.
  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.ISSN0966-842X.PMC4103024.PMID24630527.

Sources

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