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A unit of selection is a
biological
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
entity within the hierarchy of
biological organization (for example, an entity such as: a
self-replicating molecule, a
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
, a
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
, an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
, a
group, or a
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
) that is subject to
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.
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
Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a cor ...
? 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
Game theory is the study of mathematical models of strategic interactions among rational agents. Myerson, Roger B. (1991). ''Game Theory: Analysis of Conflict,'' Harvard University Press, p.&nbs1 Chapter-preview links, ppvii–xi It has appli ...
,
for instance; see
kin selection
Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
and
group selection
Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene.
Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behavi ...
. There also is debate over the definition of the units themselves, and the roles for selection and replication,
[ and whether these roles may change in the course of evolution.]
Fundamental theory
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'' and John Maynard-Smith
John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics und ...
and Eörs Szathmáry
Eörs Szathmáry (born 1959) is a Hungarian theoretical evolutionary biologist at the now-defunct Collegium Budapest Institute for Advanced Study and at the Department of Plant Taxonomy and Ecology of Eötvös Loránd University, Budapest. He is ...
's co-authored book, ''The Major Transitions in Evolution
''The Major Transitions in Evolution'' is a book written by John Maynard Smith and Eörs Szathmáry (Oxford University Press, 1995). Also
Maynard Smith and Szathmary authored a review article in ''Nature''.
Maynard Smith and Szathmáry ident ...
''. 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. Two more recent introductions include Samir Okasha
Samir Okasha , is a Professor of Philosophy of Science at University of Bristol. He is a winner of Lakatos Award for his book '' Evolution and the Levels of Selection''. He was appointed a Fellow of the British Academy
The British Academy is ...
's book ''Evolution and the Levels of Selection'' an
Pierrick Bourrat
s book ''Facts, Conventions, and the Levels of Selection''.
Selection at each level
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
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
Richard Dawkins (born 26 March 1941) is a British evolutionary biologist and author. He is an emeritus fellow of New College, Oxford and was Professor for Public Understanding of Science in the University of Oxford from 1995 to 2008. An ...
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
''River Out of Eden: A Darwinian View of Life'' is a 1995 popular science book by Richard Dawkins. The book is about Darwinian evolution and summarizes the topics covered in his earlier books, '' The Selfish Gene'', '' The Extended Phenotype'' ...
' in the book '' River Out of Eden'' where he explained that genes alone have utility function
As a topic of economics, utility is used to model worth or value. Its usage has evolved significantly over time. The term was introduced initially as a measure of pleasure or happiness as part of the theory of utilitarianism by moral philosoph ...
s.[See the chapter '']God's utility function
''River Out of Eden: A Darwinian View of Life'' is a 1995 popular science book by Richard Dawkins. The book is about Darwinian evolution and summarizes the topics covered in his earlier books, '' The Selfish Gene'', '' The Extended Phenotype'' ...
'' in
Some clear-cut examples of selection at the level of the gene include meiotic drive
Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will effect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenoty ...
and retrotransposon
Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations ( transposon) by converting RNA back into DNA throu ...
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
Intragenomic conflict refers to the evolutionary phenomenon where genes have phenotypic effects that promote their own transmission in detriment of the transmission of other genes that reside in the same genome. The selfish gene theory postulates ...
) 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
Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations ( transposon) by converting RNA back into DNA throu ...
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
The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existen ...
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. It is possible that RNA-based evolution
RNA-based evolution is a theory that posits that RNA is not merely an intermediate between Watson and Crick model of the DNA molecule and proteins, but rather a far more dynamic and independent role-player in determining phenotype. By regulating ...
is still taking place today. Other subcellular entities such as viruses, both DNA-based and RNA-based, do evolve.
Epigene
There is also view that evolution is acting on epigene
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are ...
s.
Cell
Leo Buss in his book '' The Evolution of Individuality'' proposes that much of the evolution of development in animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
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
Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...
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
Cancer stem cells (CSCs) are cancer cells (found within tumors or hematological cancers) that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found in a particular cancer sample ...
s as units of selection.
Behavioural
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
Selection at the level of the organism can be described as Darwinism
Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations tha ...
, 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
''The Extended Phenotype'' is a 1982 book by the evolutionary biologist Richard Dawkins, in which the author introduced a biological concept of the same name. The main idea is that phenotype should not be ''limited'' to biological processes suc ...
and superorganism must be also mentioned.
Group
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.
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
It remains controversial among biologists whether selection can operate at and above the level of species. Proponents of species selection include R. A. Fisher
Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who a ...
(1929); Sewall Wright
Sewall Green Wright FRS(For) Honorary FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. He was a founder of population genetics alongsi ...
(1956); Richard Lewontin (1970); Niles Eldredge & Stephen Jay Gould
Stephen Jay Gould (; September 10, 1941 – May 20, 2002) was an American paleontologist, evolutionary biologist, and historian of science. He was one of the most influential and widely read authors of popular science of his generation. Goul ...
(1972); Steven M. Stanley (1975). Gould proposed that there exist macroevolutionary processes which shape evolution, not driven by the microevolution
Microevolution is the change in allele frequencies that occurs over time within a population. This change is due to four different processes: mutation, selection ( natural and artificial), gene flow and genetic drift. This change happens over ...
ary mechanisms of the Modern Synthesis
Modern synthesis or modern evolutionary synthesis refers to several perspectives on evolutionary biology, namely:
* Modern synthesis (20th century), the term coined by Julian Huxley in 1942 to denote the synthesis between Mendelian genetics and ...
. If one views species as entities 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 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.
References
Sources
* Brandon, Robert; Burian, Richard M. eds., (1984) ''Genes, Organisms, Population: Controversies Over the Units of Selection''. Cambridge MA: MIT Press. ()
* Buss, Leo W. (1988) ''The Evolution of Individuality''. ()
* Williams, G. C. (1966) '' Adaptation and Natural Selection''. Princeton University Press, Princeton. ()
* Dawkins, Richard (1976; 1989; 2006) ''The Selfish Gene
''The Selfish Gene'' is a 1976 book on evolution by the ethologist Richard Dawkins, in which the author builds upon the principal theory of George C. Williams's '' Adaptation and Natural Selection'' (1966). Dawkins uses the term "selfish gen ...
''. Oxford University Press, Oxford. ()
* Gould, Stephen Jay (2002) ''The Structure of Evolutionary Theory
''The Structure of Evolutionary Theory'' (2002) is Harvard paleontologist Stephen Jay Gould's technical book on macroevolution and the historical development of evolutionary theory. The book was twenty years in the making, published just two mo ...
''. Harvard University Press.
* Lloyd, Elisabeth (1988) ''The Structure and Confirmation of Evolutionary Theory'', Greenwood Press (Reprinted Princeton University Press, 1994 ).
* Sober, Elliott (1984; 1993) ''The Nature of Selection: Evolutionary Theory in Philosophical Focus''. The University of Chicago Press.
* Maynard Smith, J. ''Evolutionary Genetics''. Oxford University Press, 1998.
* Okasha, S. (2006) ''Evolution and the levels of Selection''. Oxford University Press.
*Bourrat, P. (2021)
Facts, Conventions and the Levels of Selection
'. Cambridge University Press.
External links
*
*
* Lloyd, Elisabeth
"Units and Levels of Selection."
''The Stanford Encyclopedia of Philosophy'', (Fall 2005 Edition), Edward N. Zalta (ed.)
* Mayr, Ernst (1997)
"The objects of selection
''Proc. Natl. Acad. Sci. USA'' 94 (March): 2091-94.
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