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The two-fold cost of sex

This diagram illustrates the two-fold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation.

In all sexual species, the population consists of two sexes, only one of which is capable of bearing young (with the exception of simultaneous hermaphrodites). In an asexual species, each member of the population is capable of bearing young. This implies that an asexual population has an intrinsic capacity to grow more rapidly each generation. The cost was first described in mathematical terms by John Maynard Smith ^[1]. He imagined an asexual mutant arising in a sexual population, half of which comprises males that cannot themselves produce offspring. With female-only offspring, the asexual lineage doubles its representation in the population each generation, all else being equal. Often all else is not equal, however, in which case the realized fitness cost to sex may be much less than this intrinsic two-fold cost of producing males. For example, an asexual mutant arising in a sexual population occupies a niche frozen to that of its parental genotype because the asexual descendants are genetically self-identical. Analysis of competitive Lotka-Volterra equations suggests that the asexual lineage may never realize its full two-fold advantage in population growth capacity, if the broader niche of the sexual population confers even a small competitive edge ^[2].

An additional cost is that males and females must search for each other in order to mate, and sexual selection often favours traits that reduce the survival of individuals ^[1].

Evidence that the cost is not insurmountable comes from George C. Williams, who noted the existence of species which are capable of both asexual and sexual reproduction. These species time their sexual reproduction with periods of environmental uncertainty, and reproduce asexually when conditions are more favourable. The important point is that these species are observed to reproduce sexually when they could choose not to, implying that there is a selective advantage to sexual reproduction ^[3].

Source from : http://en.wikipedia.org/wiki

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Early history of study

Modern philosophical-scientific thinking on the problem can be traced back to Erasmus Darwin in the 18th century; it also features in Aristotle's writings. The thread was later picked up by August Weismann in the 19th century, who understood that the purpose of sex was to generate genetic variation, as is detailed below.

Source from : http://en.wikipedia.org/wiki

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The evolution of sex is a major puzzle in modern evolutionary biology. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction. Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, and George C. Williams to explain how sexual reproduction is maintained in a vast array of different living organisms.

It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. There are three possible reasons this might happen. First, sexual reproduction can bring together mutations that are beneficial into the same individual (sex aids in the spread of advantageous traits). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (sex aids in the removal of deleterious genes). Last, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives.

These classes of hypotheses are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanism is acting in each species.

Source from : http://en.wikipedia.org/wiki

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Social and legal considerations

Main article: Sociology of gender

Forms of legal or social distinction or discrimination based on sex include sex segregation and sexism. Notably, some businesses, public institutions, and laws may provide privileges and services for one sex and not another, or they may require different sexes to be physically separated. Recently, western societies have moved towards greater sexual equality.

Source from : http://en.wikipedia.org/wiki

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Discordance

See also: transgender and third gender

Discordance is the term used to describe the extent to which people differ from the usual biological and psychosocial types described above. Some discordances are biological, such as when the sex of the chromosomes (genetic sex) does not match the sex of the external genitalia (anatomic sex) - this is known as intersex. Discordances between the biological and psychosocial levels (such as when the gender identity does not match the anatomic sex) or between the various psychosocial levels (such as when the gender role does not match the gender identity) are even more common, but less well understood.

In gender theory, the term "heteronormativity" refers to the idea that human beings fall into two distinct and complementary categories, male and female; that sexual and marital relations are normal only when between two people of different genders; and that people should follow roles determined by their gender. Instead, some people have sought to define their sexuality and sexual identity in non-polar terms, in the belief that the simple division of all humans into "males" and "females" does not fit their individual conditions. A proponent of this movement away from polar oppositions, Anne Fausto-Sterling, recognized five sexes: male, female, merm (male pseudohermaphrodite), ferm (female pseudohermaphrodite) and herm (true hermaphrodite). Although she was heavily criticized, her idea demonstrates the difficulty and imperfection of the current social responses to these variations.

Source from : http://en.wikipedia.org/wiki

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Humans

Human Male and Female - Anatomical Features pointed out

See Human sexuality for information about sexual activities, sexual sensation, sexual gratification, and sexual intimacy between human beings

In humans, "sex" is often perceived as a dichotomous state or identity for most biological and social purposes - such that a person can only be female or male. But many factors, including one's biology, environment, psychology and social context, have a role in determining how a particular person, and those around them, view their sex. Although the table below shows common differences between males and females, many people do not correspond to "male" or "female" with regard to every criterion.

"Primary" sexual characteristics are typically present at birth and directly involved in reproduction. "Secondary" sexual characteristics typically develop later in life (usually during puberty) and are not directly involved in reproduction. Differences between the sexes are known as sexual dimorphism. At the biological level these differences are usually:

Level	Characteristics	Female	Male
Primary
	Sex chromosomes	XX	XY
	Gametes	Ova	Spermatozoon
	Sex organs	Ovaries	Testes
	Predominant Sex hormones	Estrogen and Progesterone	Testosterone
	Anatomy of internal genitalia	clitoral crura, vagina, uterus, fallopian tubes	corpora cavernosa, urethra, prostate, seminal vesicles
	Anatomy of external genitalia	glans clitoris, labia, vulva, clitoral hood, perineal urethra	penis, scrotum, foreskin fused perineum
Secondary
	Skeletal Structure	Relatively shorter, wider in hips	Relatively taller, wider in shoulders, bigger chest
	Face	None	Bigger nose, brow bone, squarer jaw, facial hair
	Body fat and muscles	Relatively more fat	Relatively more muscles[10]
	Fat Distribution	More in buttocks, hips and thighs	More in abdomen
	Body form development	"Hourglass" shape: 8	"Triangular" shape: ▼
	Other	Breasts and menstrual cycle[citation needed]	Adam's apple and body hair

The relationship between the various levels of biological sexual differentiation is fairly well understood. Many of the biological levels are said to cause, or at least shape, the next level. For example, in most people, the presence of a Y chromosome causes the gonads to become testes, which produce hormones that cause the internal and external genitalia to become male, which in turn lead parents to assign 'male' as the sex of their child (assigned sex), and raise the child as a boy (gender of rearing). However, the degree to which biological and environmental factors contribute to the psychosocial aspects of sexual differentiation, and even the interrelationships between the various psychosocial aspects of differentiation, is less well understood as illustrated by the ongoing nature versus nurture debate.

Source from : http://en.wikipedia.org/wiki

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Animal species

Main articles: Animal sexuality and Mating system

Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, rape, necrophilia, sexual orientation (heterosexuality, homosexuality, bisexuality and situational sexual behaviour) and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals.

The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear to both masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been observed among 1,500 species, and in 500 of those it is well documented. There are species that have 3, 4 or 5 sexes. For example:

• the clam shrimp Eulimnadia texana has three sexes, that is, it is a triploid organism.^[1]
• harvester ant genus Pogonomyrmex has three sexes^[2] or possibly four sexes^[3]
• the reptile tuatara might have four sexes^[4]
• Coprinus macrorhizus (Pers.) Rea might have three or four sexes^[5]
• Coprinus lagopus has four sexes^[6]
• Fausto-Sterling proposes that there are five sexes of humans ^[7][8]
• Other species have exhibited evidence of 5 sexes^[9]

Source from : http://en.wikipedia.org/wiki

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Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, sexed male and female pairs have the ability to produce offspring through meiosis and fertilization. The two sexes attract one another and communicate their readiness to procreate through differences in their biology.

An organism's sex reflects its biological function in reproduction, not its sexuality or other behavior. The female sex is defined as the one which produces the larger gamete and which typically bears the offspring. In contrast, the male sex has a smaller gamete and rarely bears offspring. In some animals, sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites.

Source from : http://en.wikipedia.org/wiki