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Sex ratio

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SSG Rivera is always wrong. there are way more men then women on this planet and every other planet in the solar system.

Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms.[1] Typically, as the population increases today the ratio from men to women is uneven; for every 96 men there are 100 women due to the high death rate in males.

The human sex ratio is of particular interest to anthropologists and demographers. In human societies, however, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth[2], and by sex-selective abortion and infanticide. The CIA estimates that the current world wide sex ratio at birth is 107 boys to 100 girls.[3] In 2010, the global sex ratio was 986 males per 1,000 females and trended to reduce to 984 in 2011.[4]

Sex ratio theory

The theory of sex ratio is a field of study concerned with the accurate prediction of sex ratios in all sexual species, based on a consideration of their natural history. The field continues to be heavily influenced by Eric Charnov's 1982 book, Sex Allocation.[5] He defines five major questions, both for his book and the field in general (slightly abbreviated here):

  1. For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
  2. For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
  3. For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
  4. Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
  5. When does selection favour the ability of an individual to alter its allocation to male versus female function, in response to particular environmental or life history situations?

Biological research mostly concerns itself with sex allocation rather than sex ratio, sex allocation denoting the allocation of energy to either sex. Common research themes are the effects of local mate and resource competition (often abbreviated LMC and LRC, respectively).

Fisher's principle

Main article: Fisher's principle

Fisher's principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher's argument in his 1967 paper on "Extraordinary sex ratios"[6] as follows, given the assumption of equal parental expenditure on offspring of both sexes.

  1. Suppose male births are less common than female.
  2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
  3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
  4. Therefore the genes for male-producing tendencies spread, and male births become more common.
  5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
  6. 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).[7]

Types of sex ratio

The sex ratio varies according to the age profile of the population. It is generally divided into four:

  • primary sex ratio — ratio at fertilization
  • secondary sex ratio — ratio at birth
  • tertiary sex ratio — ratio in sexually active organisms
  • quaternary sex ratio — ratio in post-reproductive organisms

Measuring these is a problem since there are no clear boundaries between them.

Examples in non-human species

Environmental

Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategy: if the general population deviates from this equilibrium by favoring one sex, one can obtain higher reproductive success with less effort by producing more of the other. For species where the cost of successfully raising one offspring is roughly the same regardless of its sex, this translates to an approximately equal sex ratio.

Bacteria of the genus Wolbachia cause skewed sex ratios in some arthropod species as they kill males. Sex-ratio of adult populations of pelagic copepods is usually skewed towards dominance of females. However, there are differences in adult sex ratios between families: in families in which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); in families in which females can produce eggs continuously after only one mating, sex ratios are strongly skewed towards females.[8]

Several species of reptiles have temperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. In the American Alligator, for example, females are hatched from eggs incubated between 27.7° to 30 °C, whereas males are hatched from eggs 32.2–33.8 °C. In this method, however, all eggs in a clutch (20–50) will be of the same gender. In fact, the natural sex ratio of this species is five females to one male.[9]

Dioecious plants secondary sex ratio and amount of pollen

It was found that the amount of fertilizing pollen can influence secondary sex ratio in dioecious plants. Increase in pollen amount leads to decrease in number of male plants in the progeny. This relationship was confirmed on four plant species from three families – Rumex acetosa (Polygonaceae),[10][11] Melandrium album (Cariophyllaceae),[12][13] Cannabis sativa[14] and Humulus japonicus (Cannabinaceae).[15]

Individual

In birds, mothers can influence the gender of their chicks. In peafowl, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.[16]

In several different groups of fish, such as the Wrasses, Parrotfish, Clownfish, dichogamy — or sequential hermaphoditism — is normal. This can cause a discrepancy in the sex ratios as well. In the Bluestreak cleaner wrasse, there is only one male for every group of 6-8 females. If the male fish dies, the strongest female changes its sex to become the male for the group. All of these wrasse are born female, and only become male in this situation. Other species, like clownfish, do this in reverse, where all start out as non-reproductive males, and the largest male becomes a female, with the second-largest male maturing to become reproductive.

Economical

Traditionally, farmers have discovered that the most economically efficient community of animals will have a large number of females and a very small number of males. A herd of cows with a few prize bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.

See also

Notes

  1. ^ Coney, N.S. Mackey, W.C. (1998). "The woman as final arbiter: a case for the facultative character of the human sex ratio". Journal of Sex Research. 35 (2): 169–175. doi:10.1080/00224499809551930. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  2. ^ "Trend Analysis of the Sex Ratio at Birth in the United States" (PDF). U.S. Department of Health and Human Services, National Center for Health Statistics.
  3. ^ "CIA Fact Book". The Central Intelligence Agency of the United States. {{cite web}}: Missing or empty |url= (help); Text "https://www.cia.gov/library/publications/the-world-factbook/fields/2018.html" ignored (help)
  4. ^ "Decline in sex ratio is a global trend". The Times Of India. August 17, 2011.
  5. ^ Eric L. Charnov. (1982) Sex allocation. Princeton University Press, Princeton, New Jersey. ISBN 0-691-08312-6
  6. ^ Hamilton W.D. (1967). "Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology". Science. 156 (3774): 477–88. Bibcode:1967Sci...156..477H. doi:10.1126/science.156.3774.477. PMID 6021675. references {{cite journal}}: External link in |quote= (help); Unknown parameter |month= ignored (help)
  7. ^ Maynard Smith, J., Price, G.R. (1973). "The logic of animal conflict". Nature. 246 (5427): 15–8. Bibcode:1973Natur.246...15S. doi:10.1038/246015a0.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Kiørboe, T. (2006). "Sex, sex-ratios, and the dynamics of pelagic copepod populations". Oecologia. 148 (1): 40–50. doi:10.1007/s00442-005-0346-3. PMID 16425044.
  9. ^ Ferguson, M.W.J., Joanen, T. (1982). "Temperature of egg incubation determines sex in Alligator mississippiensis". Nature. 296 (5860): 850–3. Bibcode:1982Natur.296..850F. doi:10.1038/296850a0. PMID 7070524. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  10. ^ Соrrеns С. (1922). "Geschlechtsbestimmung und Zahlenverhaltnis der Geschlechter beim Sauerampfer (Rumex acetosa)". Biol. Zbl. 42: 465–80.
  11. ^ Rychlewski J., Kazlmierez Z. (1975). "Sex ratio in seeds of Rumex acetosa L. as a result of sparse or abundant pollination". Acta Biol Crac Ser Bot. 18: 101–14.
  12. ^ Correns C. (1928). "Bestimmung, Vererbung und Verteilung des Geschlechter bei den hoheren Pflanzen". Handb. Vererbungswiss. 2: 1–138.
  13. ^ Mulcahy D.L. (1967). "Optimal sex ratio in Silene alba". Heredity. 22 (3): 41.
  14. ^ Riede W. (1925) Beitrage zum Geschlechts- und Anpassungs-problem. “Flora” 18/19
  15. ^ Kihara H., Hirayoshi J. (1932) Die Geschlechtschromosomen von Humulus japonicus. Sieb. et. Zuce. In: 8th Congr. Jap. Ass. Adv. Sci., p. 363—367 (cit.: Plant Breeding Abstr., 1934, 5, № 3, p. 248, ref. № 768).
  16. ^ Pike, T.W., Petrie, M. (2005). "Maternal body condition and plasma hormones affect offspring sex ration in peafowl". Animal Behaviour. 70 (4): 745–51. doi:10.1016/j.anbehav.2004.12.020. {{cite journal}}: Unknown parameter |laysummary= ignored (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

References

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