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Fisher's principle is an
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary model that explains why the
sex ratio The sex ratio (or gender ratio) is usually defined as the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species d ...
of most
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 appropriate s ...
that produce
offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
through
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
is approximately 1:1 between
male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...
s and
female Female ( symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females ...
s.
A. W. F. Edwards Anthony William Fairbank Edwards, FRS (born 1935) is a British statistician, geneticist and evolutionary biologist. He is the son of the surgeon Harold C. Edwards, and brother of medical geneticist John H. Edwards. He has sometimes been called ...
has remarked that it is "probably the most celebrated argument in
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
". Fisher's principle was outlined by
Ronald 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 ...
in his 1930 book '' The Genetical Theory of Natural Selection'' (but has been incorrectly attributed as original to Fisher). Fisher couched his argument in terms of parental expenditure, and predicted that parental expenditure on both sexes should be equal. Sex ratios that are 1:1 are hence known as "", and those that are ''not'' 1:1 are "" or "" and occur because they break the assumptions made in Fisher's model.Charnov, E., and Bull, J. (1989).
Non-fisherian sex ratios with sex change and environmental sex determination
, ''Nature 338,'' pp. 148–150. https://doi.org/10.1038/338148a0 Retrieved 29 March 2022.


Basic explanation

W.D. Hamilton gave the following simple explanation in his 1967 paper on "Extraordinary sex ratios", given the condition that males and females cost equal amounts to produce: # Suppose male births are less common than female. # A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. # Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. # Therefore the genes for male-producing tendencies spread, and male births become more common. # As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. # The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio. In modern language, the 1:1 ratio is the
evolutionarily stable strategy An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set o ...
(ESS).


Parental investment

Fisher wrote the explanation described by Eric Charnov and James J. Bull as being "characteristically terse" and "cryptic": in Chapter 6: "Sexual Reproduction and
Sexual Selection Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ( ...
":


Development of the argument

Fisher's principle is an early example of a model in which genes for greater production of either sex become equalized in the population, because each sex supplies exactly half the genes of all future generations. Fisher's principle is rooted in the concept of frequency-dependent selection, though Fisher's principle is not frequency-dependent selection ''per se''. Frequency-dependent selection, in this scenario, is the logic that the probability of an individual being able to breed is dependent on the frequency of the opposite sex in relation to its own sex. It was first described by Darwin in 1871. Fisher's principle extends frequency dependence to explain how natural selection can act on genes that affect the frequency of an individual's grandchildren without affecting the frequency of their children. Fisher predicted that parents will invest their resources equally between each sex of offspring, because each sex supplies exactly half the genes of all future generations. As a result, those genes that cause parents to invest unequally in the sexes will tend to be selected against. Fisher was aware that in humans, more boys are born, but boys are also more likely to die in infancy. As a consequence, he reasoned that because parents tend to invest less in boys because more boys die before the end of the period of parental care there is a higher rate of male births to equalise parental investment in each sex. Fisher's principle is also a precursor to
evolutionary game theory Evolutionary game theory (EGT) is the application of game theory to evolving populations in biology. It defines a framework of contests, strategies, and analytics into which Darwinian competition can be modelled. It originated in 1973 with John M ...
. R.H. MacArthur (1965) first suggested applying to sex ratios the language of game theory, and this was subsequently picked up by W.D. Hamilton (1967) who termed the equilibrium point the " unbeatable strategy". Hamilton's unbeatable strategy was refined by
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 un ...
and George R. Price (1973) into their concept of the
evolutionarily stable strategy An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set o ...
, i.e. one which cannot be invaded by a mutant strategy. Fisher's concept of parental expenditure (now termed parental investment), developed particularly by
Robert Trivers Robert Ludlow "Bob" Trivers (; born February 19, 1943) is an American evolutionary biologist and sociobiologist. Trivers proposed the theories of reciprocal altruism (1971), parental investment (1972), facultative sex ratio determination (1973), ...
, is now an important concept in
ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
.


Fisher's sources

Historical research by A.W.F. Edwards has shown that, although the idea has been attributed to Fisher,
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
had originally formulated a similar argument in the first edition of ''
The Descent of Man ''The Descent of Man, and Selection in Relation to Sex'' is a book by English naturalist Charles Darwin, first published in 1871, which applies evolutionary theory to human evolution, and details his theory of sexual selection, a form of biol ...
'' but removed it for the second edition – Fisher only had a copy of the second edition – and quotes Darwin in '' The Genetical Theory of Natural Selection''.
Carl Düsing Carl may refer to: *Carl, Georgia, city in USA *Carl, West Virginia, an unincorporated community *Carl (name), includes info about the name, variations of the name, and a list of people with the name *Carl², a TV series * "Carl", an episode of tel ...
of the University of Jena published a similar argument in three works between 1883–1884, which is essentially identical to Shaw and Mohler's later model. It has been argued that Darwin's argument and Fisher's are different, in that Darwin assumes monogamy whereas Fisher's does not; the same author contends that Fisher's argument is very much like Dusing's, except that Fisher introduced the idea of parental investment and calculated the sex ratio that should exist at the age of independence, which may precede the age of reproduction.


References


Further reading

As ''the most celebrated argument in
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
'', (Edwards, 1998, pp. 564–569) Fisher's principle is a staple of popular science books on evolution. For example, see: * * For those wishing more advanced treatment, see * {{Evolutionary psychology Evolutionary biology Sex ratio Evolutionary game theory 1930 introductions