Sexy son hypothesis
Encyclopedia
The sexy son hypothesis
of evolutionary biology was first proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University
in 1979. It proposes that a female
animal's optimal choice among potential mate
s is a male whose genes will produce male offspring
with the best chance of reproductive success
. In particular, the sexy son hypothesis implies that a potential mate
's capacity as a caregiver or any other direct benefits the male can offer the female (e.g., nuptial gifts, good territory
) are irrelevant to his value as the potential father of the female's offspring. What matters are her sexy sons' future breeding successes (like that of their promiscuous father) in creating large numbers of offspring
carrying copies of the female's genes.
had written in his groundbreaking book The Selfish Gene
:
The idea is that if females choose physically attractive males, they will tend to get physically attractive sons, and therefore more grandchildren, because choosy females will prefer their attractive, "sexy" sons. The theory will function regardless of the physical or behavioral trait a female chooses, as long as it is heritable, because it is possessing the trait that makes males attractive, and not the qualities of the trait in itself. Thus, traits culturally perceived as negative can still be seen as desirable; for example, females who stay with or are attracted to males they know to be disloyal in a monogamous relationship. If this trait is passed to any male children, they are more likely to themselves be non-monogamous, have several mates and spread the female's genes to multiple grandchildren. The sexy son hypothesis is one of several possible explanations for the highly diverse and often astonishing ornaments of animals.
Once a preference becomes established, females choosing males with elaborate secondary sexual traits will produce sons that carry allele
s for the trait and daughters that carry alleles for the preference,
generating genetic coupling that will drive self-reinforcing coevolution of both trait and preference, due to the mating advantage of males with the trait. Thus, the original viability benefits associated with the preference can be undermined by a Fisherian runaway
sexy sons process. Similar models have been proposed for postcopulatory female preferences, such as the time at which females removed the male's sperm ampulla
after mating. Sexual selection
by direct and/or indirect benefits as well as sexual conflict determine the evolution of animal mating systems.
In its original context, the “narrow-sense sexy son hypothesis” of Weatherhead and Robertson refers to mating systems with care from both parents. In these mating systems, females that mate with a polygynous male normally receive less assistance than females mated with a monogamous male, and thus suffer from direct fitness consequences that have to be (at least) compensated for by the breeding successes of their sexy sons. On the other hand, a “broad-sense sexy son hypothesis” encompasses both polygyny and promiscuous mating systems, with and without care from both parents. Alatalo (1998) argues that the costs of any additional choice may be so minor that female choice for honestly signaling males, that is good genes, may evolve even if the indirect benefits on offspring quality are small. A similar argument can be made for the sexy son hypothesis if mates of attractive males do not suffer any direct fitness consequences.
refers to the conflicting goals of breeding males and females. It describes the diverging interests of males and females in optimizing their fitness. From the viewpoint of any one partner, the best outcome would be for the partner's mate to care for the young, thus freeing up his or her own resources (e.g., time and energy) that s/he - but typically he - can invest in further sex that may create additional offspring. In polygynous mating systems, sexual conflict means the optimization of male reproductive success by having mated with multiple females, even though the reproductive success of a polygynously mated female is thereby reduced.
assumes that when females encounter better males than their previous mates, they re-mate in order to fertilize their eggs with the better male's sperm.
Significant sire effects on egg to adult viability and son’s pre-productive survival, as well as moderate to high coefficients of additive genetic variation for these traits have been found in Dung Beetles. Beetles who have selected mates with better genetics tend to have offspring that survive longer and are more able to reproduce than those that do not pick mates with genetic quality. This suggests that carefully choosing a mate is beneficial.
Another study notes that Pronghorn
females engage in an obvious and energetically expensive mate sampling process to identify vigorous males. Though each female selects independently, the outcome is that a small proportion of the herd's males sire most young. Offspring of attractive males were more likely to survive to weaning and to age classes as late as 5 years, apparently due to faster growth rates. Because pronghorn males do not have costly ornaments
, the authors conclude that female choice for good genes can exist in the absence of obvious sexual selection
cues
such as elaborate antlers.
The sexy son hypothesis is closely related to the good genes assumption and the Fisherian runaway selection process. Like good genes, the sexy son hypothesis assumes the existence of indirect genetic benefits that are able to compensate any inferior direct reproductive success (i.e., fewer offspring). The main difference between good genes and the sexy son hypothesis is that the latter assumes an indirect effect due to the attractiveness of the sons, whereas good genes focus on the viability of both sons and daughters. However, "attractiveness" is not narrowly defined, and can refer to every trait that increases a male's probability to become polygynous.
. In particular, it has been shown that human females are more attracted to men of higher physical attractiveness, and to men whose physical features are indicative of a higher exposure to testosterone during key developmental periods, during the most fertile
times of their menstrual cycle
s, and more attracted to relatively feminine
men ("dads") during the remainder of the cycle. However, the methodology of these claims is disputed. These observations have led to the conclusion that infidelity
is a natural occurrence in women, and evolution
arily advantageous, on the grounds that it will enable them to secure both the best genes
and the best caregiver for their offspring.
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
of evolutionary biology was first proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University
Queen's University
Queen's University, , is a public research university located in Kingston, Ontario, Canada. Founded on 16 October 1841, the university pre-dates the founding of Canada by 26 years. Queen's holds more more than of land throughout Ontario as well as Herstmonceux Castle in East Sussex, England...
in 1979. It proposes that a female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
animal's optimal choice among potential mate
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
s is a male whose genes will produce male offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
with the best chance of reproductive success
Reproductive success
Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. In practice, this is often a tally of the number of offspring produced by an individual. A more correct definition, which incorporates inclusive fitness, is the...
. In particular, the sexy son hypothesis implies that a potential mate
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
's capacity as a caregiver or any other direct benefits the male can offer the female (e.g., nuptial gifts, good territory
Territory (animal)
In ethology the term territory refers to any sociographical area that an animal of a particular species consistently defends against conspecifics...
) are irrelevant to his value as the potential father of the female's offspring. What matters are her sexy sons' future breeding successes (like that of their promiscuous father) in creating large numbers of offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
carrying copies of the female's genes.
Context
Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits. In 1976, prior to Weatherhead and Robertson's paper, Richard DawkinsRichard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
had written in his groundbreaking book The Selfish Gene
The Selfish Gene
The Selfish Gene is a book on evolution by Richard Dawkins, published in 1976. It builds upon the principal theory of George C. Williams's first book Adaptation and Natural Selection. Dawkins coined the term "selfish gene" as a way of expressing the gene-centred view of evolution as opposed to the...
:
The idea is that if females choose physically attractive males, they will tend to get physically attractive sons, and therefore more grandchildren, because choosy females will prefer their attractive, "sexy" sons. The theory will function regardless of the physical or behavioral trait a female chooses, as long as it is heritable, because it is possessing the trait that makes males attractive, and not the qualities of the trait in itself. Thus, traits culturally perceived as negative can still be seen as desirable; for example, females who stay with or are attracted to males they know to be disloyal in a monogamous relationship. If this trait is passed to any male children, they are more likely to themselves be non-monogamous, have several mates and spread the female's genes to multiple grandchildren. The sexy son hypothesis is one of several possible explanations for the highly diverse and often astonishing ornaments of animals.
Once a preference becomes established, females choosing males with elaborate secondary sexual traits will produce sons that carry allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s for the trait and daughters that carry alleles for the preference,
generating genetic coupling that will drive self-reinforcing coevolution of both trait and preference, due to the mating advantage of males with the trait. Thus, the original viability benefits associated with the preference can be undermined by a Fisherian runaway
Fisherian runaway
Fisherian runaway is a model of sexual selection, first proposed by R.A. Fisher in 1915, and expanded upon in his 1930 book The Genetical Theory of Natural Selection, that suggests an explanation for sexual selection of traits that do not obviously increase fitness of survival, based upon a...
sexy sons process. Similar models have been proposed for postcopulatory female preferences, such as the time at which females removed the male's sperm ampulla
Spermatophore
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
after mating. Sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
by direct and/or indirect benefits as well as sexual conflict determine the evolution of animal mating systems.
In its original context, the “narrow-sense sexy son hypothesis” of Weatherhead and Robertson refers to mating systems with care from both parents. In these mating systems, females that mate with a polygynous male normally receive less assistance than females mated with a monogamous male, and thus suffer from direct fitness consequences that have to be (at least) compensated for by the breeding successes of their sexy sons. On the other hand, a “broad-sense sexy son hypothesis” encompasses both polygyny and promiscuous mating systems, with and without care from both parents. Alatalo (1998) argues that the costs of any additional choice may be so minor that female choice for honestly signaling males, that is good genes, may evolve even if the indirect benefits on offspring quality are small. A similar argument can be made for the sexy son hypothesis if mates of attractive males do not suffer any direct fitness consequences.
Sexual conflict
Sexual conflictSexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
refers to the conflicting goals of breeding males and females. It describes the diverging interests of males and females in optimizing their fitness. From the viewpoint of any one partner, the best outcome would be for the partner's mate to care for the young, thus freeing up his or her own resources (e.g., time and energy) that s/he - but typically he - can invest in further sex that may create additional offspring. In polygynous mating systems, sexual conflict means the optimization of male reproductive success by having mated with multiple females, even though the reproductive success of a polygynously mated female is thereby reduced.
Good genes theory
"Good genes" theory proposes that females select males seen to have genetic advantages that increase offspring quality. Increased viability of offspring provides compensation for any lower reproductive success that results from their being "picky". The good-gene hypothesis for polyandryPolyandry
Polyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered...
assumes that when females encounter better males than their previous mates, they re-mate in order to fertilize their eggs with the better male's sperm.
Significant sire effects on egg to adult viability and son’s pre-productive survival, as well as moderate to high coefficients of additive genetic variation for these traits have been found in Dung Beetles. Beetles who have selected mates with better genetics tend to have offspring that survive longer and are more able to reproduce than those that do not pick mates with genetic quality. This suggests that carefully choosing a mate is beneficial.
Another study notes that Pronghorn
Pronghorn
The pronghorn is a species of artiodactyl mammal endemic to interior western and central North America. Though not an antelope, it is often known colloquially in North America as the prong buck, pronghorn antelope, or simply antelope, as it closely resembles the true antelopes of the Old World and...
females engage in an obvious and energetically expensive mate sampling process to identify vigorous males. Though each female selects independently, the outcome is that a small proportion of the herd's males sire most young. Offspring of attractive males were more likely to survive to weaning and to age classes as late as 5 years, apparently due to faster growth rates. Because pronghorn males do not have costly ornaments
Biological ornament
A biological ornament is a structure of an animal that appears to serve a decorative function rather than an ostensible, utilitarian function. Ornaments are used in displays to attract mates. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite...
, the authors conclude that female choice for good genes can exist in the absence of obvious sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
cues
Sensory cue
A sensory cue is a statistic or signal that can be extracted from the sensory input by a perceiver, that indicates the state of some property of the world that the perceiver is interested in perceiving....
such as elaborate antlers.
The sexy son hypothesis is closely related to the good genes assumption and the Fisherian runaway selection process. Like good genes, the sexy son hypothesis assumes the existence of indirect genetic benefits that are able to compensate any inferior direct reproductive success (i.e., fewer offspring). The main difference between good genes and the sexy son hypothesis is that the latter assumes an indirect effect due to the attractiveness of the sons, whereas good genes focus on the viability of both sons and daughters. However, "attractiveness" is not narrowly defined, and can refer to every trait that increases a male's probability to become polygynous.
Sperm models
‘‘Good-sperm’’ models predict positive genetic associations between a male’s sperm competitiveness and the general viability of his offspring, whereas ‘‘sexy-sperm’’ models predict that multiple mating females produce sons successful in sperm competition, and daughters that incite sperm competition through multiple mating. As with precopulatory processes, postcopulatory models predict that the trait in males that determines fertilization success will become genetically coupled with the mechanism by which females bias sperm use toward preferred males.Human sexual behavior
The sexy son hypothesis has also been put forward as the origin of some aspects of human sexual behaviorHuman sexual behavior
Human sexual activities or human sexual practices or human sexual behavior refers to the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts from time to time, and for a wide variety of reasons...
. In particular, it has been shown that human females are more attracted to men of higher physical attractiveness, and to men whose physical features are indicative of a higher exposure to testosterone during key developmental periods, during the most fertile
Fertility
Fertility is the natural capability of producing offsprings. As a measure, "fertility rate" is the number of children born per couple, person or population. Fertility differs from fecundity, which is defined as the potential for reproduction...
times of their menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....
s, and more attracted to relatively feminine
Femininity
Femininity is a set of attributes, behaviors, and roles generally associated with girls and women. Though socially constructed, femininity is made up of both socially defined and biologically created factors...
men ("dads") during the remainder of the cycle. However, the methodology of these claims is disputed. These observations have led to the conclusion that infidelity
Infidelity
In many intimate relationships in many cultures there is usually an express or implied expectation of exclusivity, especially in sexual matters. Infidelity most commonly refers to a breach of the expectation of sexual exclusivity.Infidelity can occur in relation to physical intimacy and/or...
is a natural occurrence in women, and evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
arily advantageous, on the grounds that it will enable them to secure both the best genes
Human genome
The human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...
and the best caregiver for their offspring.
See also
- Sexual selectionSexual selectionSexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
- Mate choiceMate choiceMate choice, or intersexual selection, is an evolutionary process in which selection of a mate depends on attractiveness of its traits. It is one of two components of sexual selection...
- Fisherian runawayFisherian runawayFisherian runaway is a model of sexual selection, first proposed by R.A. Fisher in 1915, and expanded upon in his 1930 book The Genetical Theory of Natural Selection, that suggests an explanation for sexual selection of traits that do not obviously increase fitness of survival, based upon a...
- Sexual selection in human evolutionSexual selection in human evolutionCharles Darwin described sexual selection as depending "on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction". In animals, he saw the competition for advantage as occurring between males - the most successful of which were chosen...