Fisher's principle is an
evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
ary model that explains why the
sex ratio
A sex ratio is 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. However, many species deviate from an even sex ratio, ei ...
of most
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
that produce
offspring
In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...
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 tha ...
is approximately 1:1 between
male
Male (Planet symbols, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or Egg cell, ovum, in the process of fertilisation. A male organism cannot sexual repro ...
s and
female
An organism's sex is female ( symbol: ♀) if it produces the ovum (egg cell), the type of gamete (sex cell) that fuses with the male gamete (sperm cell) during sexual reproduction.
A female has larger gametes than a male. Females and ...
s.
A. W. F. Edwards
Anthony William Fairbank Edwards, Fellow of the Royal Society, FRS One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 1935) is a British statistician, geneticist and evolutionary biologist. Ed ...
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 such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biolo ...
".
[
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 a ...
in his 1930 book ''The Genetical Theory of Natural Selection
''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher which combines Mendelian inheritance, Mendelian genetics with Charles Darwin's theory of natural selection, with Fisher being the first to argue that "Mendelism therefore va ...
''[ (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.][
]
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 (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 mechanism of evolution in which members of one sex mate choice, choose mates of the other sex to mating, 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
Frequency-dependent selection is an evolutionary process by which the fitness (biology), fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population.
* In positive frequency-dependent selection, the fit ...
, 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. R.H. MacArthur (1965) first suggested applying to sex ratios the language of game theory
Game theory is the study of mathematical models of strategic interactions. It has applications in many fields of social science, and is used extensively in economics, logic, systems science and computer science. Initially, game theory addressed ...
, 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 and George R. Price (1973) into their concept of the evolutionarily stable strategy, 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 (197 ...
, is now an important concept in ecology
Ecology () is the natural science of the relationships among living organisms and their Natural environment, environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosystem, and biosphere lev ...
.
George C. Williams, Richard Dawkins
Richard Dawkins (born 26 March 1941) is a British evolutionary biology, evolutionary biologist, zoologist, science communicator and author. He is an Oxford fellow, emeritus fellow of New College, Oxford, and was Simonyi Professor for the Publ ...
, and Randolph M. Nesse cite Fisher's principle as an example that illustrates the dominance of individual-level selection over 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 beha ...
because, regardless of the hypothetical sex imbalance of a group, the genes of a parental organism produce offspring of the less frequent sex in order to maximize their transmission to subsequent generations but at substantial cost to the group growth rate.
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 Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
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 natural history, naturalist Charles Darwin, first published in 1871, which applies evolutionary theory to human evolution, and details his theory of sexual selection, ...
'' 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
''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher which combines Mendelian inheritance, Mendelian genetics with Charles Darwin's theory of natural selection, with Fisher being the first to argue that "Mendelism therefore va ...
''.[
Carl Düsing of the ]University of Jena
The University of Jena, officially the Friedrich Schiller University Jena (, abbreviated FSU, shortened form ''Uni Jena''), is a public research university located in Jena, Thuringia, Germany.
The university was established in 1558 and is cou ...
published a similar argument in three works between 1883 and 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 such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biolo ...
'', (Edwards, 1998, pp. 564–569) Fisher's principle is a staple of popular science
Popular science (also called pop-science or popsci) is an interpretation of science intended for a general audience. While science journalism focuses on recent scientific developments, popular science is more broad ranging. It may be written ...
books on evolution. For example, see:
*
*
For those wishing more advanced treatment, see
*
{{Evolutionary psychology
Evolutionary biology
Sex ratio
Evolutionary game theory
1930 introductions
Ronald Fisher