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The "Vicar of Bray"
hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
(or Fisher-Muller Model) attempts to explain why sexual reproduction might have advantages over asexual reproduction.
Reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
is the process by which organisms give rise to offspring.
Asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
involves a single parent and results in offspring that are genetically identical to each other and to the parent.
In contrast to asexual reproduction,
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 ...
involves two parents. Both the parents produce
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s through
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
, a special type of cell division that reduces the
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
number by half. During an early stage of meiosis, before the chromosomes are separated in the two daughter cells, the chromosomes undergo
genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...
. This allows them to exchange some of their
genetic information
A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are usua ...
. Therefore, the gametes from a single organism are all genetically different from each other. The process in which the two gametes from the two parents unite is called
fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
. Half of the genetic information from both parents is combined. This results in offspring that are genetically different from each other and from the parents.
In short, sexual reproduction allows a continuous rearrangement of genes. Therefore, the offspring of a population of sexually reproducing individuals will show a more varied selection of
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
s. Due to faster attainment of favorable genetic combinations, sexually reproducing populations
evolve more rapidly in response to
environmental change Environmental change is a change or disturbance of the environment most often caused by human influences and natural ecological processes. Environmental changes include various factors, such as natural disasters, human interferences, or animal in ...
s. Under the Vicar of Bray hypothesis, sex benefits a population as a whole, but not individuals within it, making it a case of
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 behav ...
.
Disadvantage of sexual reproduction
Sexual reproduction often takes a lot of effort. Finding a mate can sometimes be an expensive, risky and time consuming process. Courtship, copulation and taking care of the new born offspring may also take up a lot of time and energy. From this point of view, asexual reproduction may seem a lot easier and more efficient. But another important thing to consider is that the individuals with the highest fitness are more likely to find a mate and reproduce. Therefore, the chances of offspring with a higher fitness increases. The Vicar of Bray hypothesis proposes that sexual reproduction is more beneficial than asexual reproduction, despite the cost of time and effort.
Origin of the name 'Vicar of Bray'
The hypothesis is called after
the Vicar of Bray
The Vicar of Bray is a satirical description of an individual fundamentally changing his principles to remain in ecclesiastical office as external requirements change around him. The religious upheavals in England from 1533 to 1559 (and then from ...
, a semi-fictionalized
cleric
Clergy are formal leaders within established religions. Their roles and functions vary in different religious traditions, but usually involve presiding over specific rituals and teaching their religion's doctrines and practices. Some of the ter ...
who retained his ecclesiastic office by quickly adapting to the prevailing religious winds in England, switching between various
Protestant
Protestantism is a Christian denomination, branch of Christianity that follows the theological tenets of the Reformation, Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century agai ...
and
Catholic
The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptized Catholics worldwide . It is among the world's oldest and largest international institutions, and has played a ...
rites as the ruling hierarchy changed.
[
Ridley, Matt. ''The Red Queen: Sex and the Evolution of Human Nature'' (1993), Penguin Books
] The figure described was
Simon Aleyn
Simon Aleyn (or Alleyn; died 17 October 1565) was a Canon of Windsor from 1559–63
He was educated in Oxford and graduated BA 1539, MA 1542.
He was appointed:
*Vicar of Cookham 1553
*Vicar of Strathfieldsaye 1559
He was also Vicar of St Micha ...
between 1540 and 1588. The main work of
Thomas Fuller
Thomas Fuller (baptised 19 June 1608 – 16 August 1661) was an English churchman and historian. He is now remembered for his writings, particularly his ''Worthies of England'', published in 1662, after his death. He was a prolific author, and ...
(d. 1661), ''Worthies of England'', describes this man:
The vivacious vicar Bray.html" ;"title="Bray,_Berkshire.html" ;"title="f Bray, Berkshire">Bray">Bray,_Berkshire.html" ;"title="f Bray, Berkshire">Brayliving under King Henry VIII, King Edward VI, Queen Mary, and Queen Elizabeth, was first a Papist, then a Protestant, then a Papist, then a Protestant again. He had seen some martyrs burnt (two miles off) at Windsor, Berkshire, Windsor and found this fire too hot for his tender temper. This vicar, being taxed ttackedby one for being a turncoat
A turncoat is a person who shifts allegiance from one loyalty or ideal to another, betraying or deserting an original cause by switching to the opposing side or party. In political and social history, this is distinct from being a traitor, as the ...
and an inconstant changeling, said, "Not so, for I always kept my principle, which is this – to live and die the Vicar of Bray."
– ''Worthies of England'', published 1662
Origin of the hypothesis
The hypothesis was first expressed in 1889 by
August Weismann
August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Cha ...
and later by Guenther (1906). Afterwards, the hypothesis was formulated in terms of population genetics by Fisher (1930) and Muller (1932) and with greater mathematical formalism, by Muller (1958, 1964) and Crow and Kimura (1965). The doubts about the validity of the Vicar of Bray hypothesis caused the upcoming of alternative hypotheses such as:
* Best-Man (Williams, 1966; Emlen, 1973; Treisman, 1976): The Best-Man hypothesis proposes that, on average, sexually produced offspring may be of somewhat lower
fitness than the asexually produced offspring, but the much greater diversity of the sexually produced offspring implies that they will include a few individuals of extraordinary high fitness. If these individuals have a high chance on survival and reproducing, their offspring might also be of high fitness. If you consider this going on for multiple generations, the proportion of individuals with a high fitness might increase so rapidly that it will be more than sufficient to offset the cost of sex.
* Tangled Bank (Ghiselin, 1974; Burt and Bell, 1987; Ridley, 1993): The Tangled Bank hypothesis proposes that sexual reproduction is beneficial when there exists intense
competition
Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
for space, food and resources. It argues that genetically diverse offspring from sexually reproducing individuals are able to extract more food from their environment than genetically identical offspring from asexually reproducing individuals.
* Red Queen (Van Valen, 1973; Hamilton, 1975; Levin, 1975; Charlesworth, 1976; Glesener and Tilman, 1978; Glesener, 1979; Bell, 1982; Bell and Maynard Smith, 1987; Ridley, 1993; Peters and Lively, 1999, 2007; Otto and Nuismer, 2004; Kouyos et al., 2007; Salathé et al., 2008): The
Red Queen hypothesis
The Red Queen hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended t ...
proposes that organisms must constantly adapt and evolve in order to survive. If a species does not adapt to its evolving natural enemies and the changing environment, it will go
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
. This hypothesis also proposes that sexual reproduction is beneficial. But in contrast to the Vicar of Bray hypothesis, the Red Queen hypothesis states that sexual reproduction does not only benefit the population as a whole, but it benefits individual genes directly.
Mathematical models have been used in order to try to prove or disprove these hypotheses. However, for a mathematical model, assumptions must be made. Assumptions on the size of the population, the breeding process, the environment, natural enemies and so on. That is why there will always be populations for which the model does not apply. Some models are better in explaining the ‘average’ population, while others better explain the smaller populations or populations that live in a more extreme environment. A good way to decide which model is the best might be to compare the expected result from the model with data from natural observations.
[Libertini G. (2011): Concordance of the Predictions of a Simulation Model for the Evolutionary Advantage of Sex with Observational Evidence. WebmedCentral; ZOOLOGY; 2(11):WMC002464.]
People who criticize the Vicar of Bray hypothesis (and all other hypotheses that propose sexual reproduction has an advantage over asexual reproduction) say that sexual reproduction might be beneficial in some situations, but not always, which is why both ways of reproduction still exist. If either sexual reproduction or asexual reproduction would be much more beneficial, evolution should result in one of the two ways of reproduction to disappear and the other one to persist.
Notes
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Evolutionary biology