Citizendia
Your Ad Here

Sex ratio is the ratio of males to females in a population. A ratio is an expression which compares quantities relative to each other Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells In Biology a population is the collection of inter-breeding organisms of a particular Species; in Sociology 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].

The human sex ratio is of particular interest to anthropologists and demographers. In Anthropology and Demography, the human sex ratio is the Sex ratio for Homo sapiens (i In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls (which sometimes is shortened to "a ratio of 105"). BOY is a Canadian Indie pop band The band consists of vocalist and general instrumentalist Stephen Kozmeniuk, drummer Maurie Kaufmann, A girl is any Female Human from birth through Childhood and Adolescence to attainment of Adulthood The term may also be used to mean In human societies, however, sex ratios at birth or among infants may be considerably skewed by sex-selective abortion and infanticide. Sex-selective abortion is the targeted Abortion of a Fetus based upon its Sex.

In biology, sex ratio is defined as the proportion of males in the population. Foundations of modern biology There are five unifying principles

Contents

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. Natural history is the Scientific research of Plants or Animals leaning more towards the Observational than Experimental methods The field continues to be heavily influenced by Eric Charnov's 1982 book, Sex Allocation. Eric L Charnov (born Oct 29 1947) is an American theoretical ecologist at the University of New Mexico. [2] 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. Plant sexuality covers the wide variety of Sexual reproduction systems found across the Plant kingdom A hermaphrodite is an organism having both male and female reproductive organs Sex change in animals Some species are known to change sex, including reproductive functions in special circumstances such as the clownfish 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. Fisher's principle explains why the Sex ratio of most species is approximately 11 Assuming equal parental investment:

W.D. Hamilton gave the following basic explanation in his 1967 paper on "Extraordinary sex ratios"[3], given the condition that males and females cost equal amounts to produce:

  1. Suppose male births are less common than female. In Evolutionary biology, parental investment (PI is any Parental expenditure (time energy etc William Donald Hamilton, FRS ( 1 August 1936 &mdash 7 March 2000) was a British Evolutionary biologist
  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 commoner.
  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 through-out. Therefore 1:1 is the equilibrium ratio.

In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS)[4]

Types of sex ratio

The sex ratio varies according to the age profile of the population. In Game theory and Behavioural ecology, an evolutionarily stable strategy (ESS is a strategy which if adopted by a population of players It is generally divided into four:

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

Human sex ratio

Main article: human sex ratio

In anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens. In Anthropology and Demography, the human sex ratio is the Sex ratio for Homo sapiens (i Anthropology (/ˌænθɹəˈpɒlədʒi/ from Greek grc ἄνθρωπος anthrōpos, "human" -λογία -logia) is the study of Demography is the statistical study of all Populations. It can be a very general science that can be applied to any kind of dynamic population that is one that changes over In Anthropology and Demography, the human sex ratio is the Sex ratio for Homo sapiens (i Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Humans have a Fisherian sex ratio. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls (which sometimes is shortened to "a ratio of 105"). BOY is a Canadian Indie pop band The band consists of vocalist and general instrumentalist Stephen Kozmeniuk, drummer Maurie Kaufmann, A girl is any Female Human from birth through Childhood and Adolescence to attainment of Adulthood The term may also be used to mean In human societies, however, sex ratios at birth or among infants may be considerably skewed by sex-selective abortion and infanticide. Sex-selective abortion is the targeted Abortion of a Fetus based upon its Sex.

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. In Game theory and Behavioural ecology, an evolutionarily stable strategy (ESS is a strategy which if adopted by a population of players 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.

The bacterium wolbachia causes skewed sex ratios in some arthropod species as it kills males. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Arthropods are Animals belonging to the Phylum Arthropoda (from Greek ἄρθρον arthron, " Joint " Sex-ratio of adult populations of pelagic copepods is usually skewed towards dominance of females. Copepods are a group of small Crustaceans found in the sea and nearly every freshwater habitat and they constitute the biggest source of protein in the oceans However, there are differences in adult sex ratios between families: in families which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); and in families which females can produce eggs continuously after only one mating, sex ratios are strongly skewed towards females. [5].

Several species of reptiles have temperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. Temperature-dependent sex determination (TSD also called environmental sex determination, is where the surrounding temperature determines the sex of an organism In the American Alligator, for example, females are hatched from eggs incubated between 27. The American Alligator, Alligator mississippiensis, (known colloquially as simply gator) is one of the two living Species of Alligator, a genus 7° to 30 °C, where 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. [6]

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),[7][8] Melandrium album (Cariophyllaceae),[9][10] Cannabis sativa[11] and Humulus japonicus (Cannabinaceae). Common sorrel ( Rumex acetosa) also known as spinach dock, is a perennial herb that is cultivated as a garden Herb or Polygonaceae is a family of flowering plants also known as the "knotweed family" or "smartweed family" Cannabis sativa is an annual plant in the Cannabaceae family It is a Herb that has been used throughout recorded history by humans for various purposes Humulus japonicus ( Japanese hop, synonym H scandens) is an Ornamental plant in the Cannabaceae family Cannabaceae is a small family of Flowering plants According to the Royal Botanical Gardens database there are 170 species grouped in nine to fifteen [12]

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%. The term peafowl refers to Gallinaceous Birds classified within the genera Rheinardia Argusianus Afropavoand Pavo [13]

In several different groups of fish, such as the Wrasses, Parrotfish, Clownfish, dichogamy — or sequential hermaphoditism — is normal. The wrasses are a family, Labridae, of marine Fish, many of which are brightly colored Parrotfish are mostly tropical Perciform marine Fish of the family Scaridae. Clownfish and anemonefish are Fishes from the subfamily Amphiprioninae in the family Pomacentridae. Dichogamy is the separation in time of gender expression in a hermaphroditic organism a characteristic of some Fishes, Gastropods, and most Flowering plants 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. The bluestreak cleaner wrasse ( Labroides dimidiatus) is a species of Wrasse found on Coral reefs in the Indian Ocean and much of the Pacific 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 and a few prize bulls or a flock of chickens and one rooster are the most economical sex ratios for domesticated livestock.

See also

Notes

  1. ^ Nancy S. In the Evolutionary biology of Sexual reproduction, the operational sex ratio (OSR is the ratio of sexually competing male to females that are ready to mate Sex allocation is the allocation of resources to male versus female reproduction in sexual species (Charnov 1982 Parents frequently prefer their offspring to be of a particular sex for a variety of reasons Sex-selective abortion is the targeted Abortion of a Fetus based upon its Sex. The Demographic transition model (DTM is a model used to explain the process of shift from high Birth rates and high Death rates to low birth rates and low death rates A population pyramid, also called age-sex pyramid and age structure diagram, is a graphical illustration that shows the distribution of various age groups in a The XY sex-determination system is the Sex-determination system found in Humans most other Mammals some insects ( Drosophila) and some Bride kidnapping, also known as marriage by abduction or marriage by capture, is a form of Marriage practiced in some traditional cultures in countries Coney and W. C. Mackey. 1998. "The woman as final arbiter: a case for the facultative character of the human sex ratio," Journal of Sex Research 35 (May): 169-175. The Journal of Sex Research (or JSR, ISSN 0022-4499 is published by the Society for the Scientific Study of Sexuality registered in New York City
  2. ^ Eric L. Charnov. Eric L Charnov (born Oct 29 1947) is an American theoretical ecologist at the University of New Mexico. (1982) Sex allocation. Princeton University Press, Princeton, New Jersey. ISBN 0-691-08312-6
  3. ^ Hamilton, W. D. 1967 Extraordinary sex ratios Science 156: 477-488
  4. ^ Maynard Smith, J & Price, G. R. 1973 The logic of animal conflict Nature 246: 15-18
  5. ^ [Kiørboe, T. 2006. Sex, sex-ratios, and the dynamics of pelagic copepod populations. Oecologia 148(1):40-50]http://dx.doi.org/10.1007/s00442-005-0346-3
  6. ^ Ferguson, Mark W. J. , and Joanen, Ted (1982) Temperature of egg incubation determines sex in Alligator mississippiensis Nature 296, 850 - 853 (29 April 1982)
  7. ^ Соrrеns С. (1922) Geschlechtsbestimmung und Zahlenverhaltnis der Geschlechter beim Sauerampfer (Rumex acetosa). “Biol. Zbl. ” 42, 465-480.
  8. ^ Rychlewski J. , Kazlmierez Z. (1975) Sex ratio in seeds of Rumex acetosa L. as a result of sparse or abundant pollination. “Acta Biol. Cracov” Scr. Bot. , 18, 101-114.
  9. ^ Correns C. (1928) Bestimmung, Vererbung und Verteilung des Geschlechter bei den hoheren Pflanzen. Handb. Vererbungswiss. , 2, 1-138.
  10. ^ Mulcahy D. L. (1967) Optimal sex ratio in Silene alba. “Heredity” 22 № 3, 41.
  11. ^ Riede W. (1925) Beitrage zum Geschlechts- und Anpassungs-problem. “Flora” 18/19
  12. ^ 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).
  13. ^ Pike, T. W. , and M. Petrie (2005). Maternal body condition and plasma hormones affect offspring sex ration in peafowl. Animal Behaviour 70(October), pp. 745-751; cited in http://www.sciencenews.org/articles/20070512/bob9.asp

References


External links

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
 Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic