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The Evolution of Homosexuality

Explore the persistence of homosexuality and its evolutionary implications through empirical studies and theories, including the "homosexual gene," brain mechanisms, hormonal signals, hypervariability, parental manipulation, and kin selection.

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The Evolution of Homosexuality

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  1. The Evolution of Homosexuality Carey LeClair

  2. Introduction • The persistence of a trait that drastically lowers reproductive fitness deserves examination. • Empirical studies have shown a relatively low rate of direct reproduction by homosexual men • Exclusive homosexuality, not bisexuality • There are biological, social, and sociobiological theories

  3. Theories • Biological Theories: The “homosexual gene”, Brain Mechanisms, Hormonal Signals, and Hypervariability • Sociobiological Theories: Parental Manipulation, and Kin Selection

  4. The “homosexual gene” • Two homosexual alleles • B = heterosexuality • b = homosexuality • Basically, bb is required for the expression of homosexual orientation • How did the homosexual gene survive? • A variation of a gene that is opposed to reproductive success must also contribute to reproductive success when in other individuals

  5. The “homosexual gene”… • The b allele has a desirable effect in heterosexuals. • Feminine traits such as sensitivity, kindness, empathy, etc. (these traits would often make a better father and provider, therefore females are more attracted to and want to reproduce with those type of men. • So: Bb is better than BB (too masculine for optimal reproductive success)Therefore, the b allele has survived.

  6. Brain Mechanisms • Brain mechanism for attraction to males • Normally turned on only in females • Purpose: to cause mature females to be attracted to mature males to produce offspring • Homosexuality occurs with the random activation of this mechanism in the males that become homosexuals

  7. Hormonal Signals • Sexual development is controlled by hormonal signals (or the absence of them), and once the signal is given, it affects a wide range of development conditions from the genitals to the brain • All or most of the sex-specific traits are activated (or turned off) by this hormonal mechanism • For homosexuality, this mechanism is only partially activated in males

  8. Hypervariability • Basically, the “homosexual gene” stays in the gene pool because of a high rate of mutation • Hypervariable DNA sequences can change their structure at rates hundreds to thousands of times higher than the normal rate of spontaneous mutation • Suggests the “homosexual gene” mutations could “piggy-back” on the hypervariability of DNA sequences

  9. Parental Manipulation • Parental Manipulation “mechanism”; parents have an interest in their children and grandchildren passing on copies of their genes • From the parents’ point of view, it doesn’t matter which of their children pass on gene copies, as long as the number of gene copies transmitted are maximized • Parents decide to allocate their resources to their children on the basis of their reproductive efforts – because they will have the beneficial evolutionary effect for the parents of maximizing transmission of parental gene copies

  10. Kin Selection • Kind of similar to the parental manipulation “mechanism” • Homosexual men reproduce “indirectly” through aid given to kin • Seen in insects and other species • It’s in the “genetic transmission” interest of the individual to reduce their reproductive efforts if they can therefore increase a kin’s reproduction by over 100% through some type of altruistic mechanism • Also, if they are reproductively unsuccessful

  11. Criticisms • No scientific evidence mentioned for brain mechanisms, hormonal signals, hypervariability, and parental manipulation related to homosexuality • Evidence doesn’t support Kin Selection (Bobrow & Bailey, 2001; Rahman & Hull, 2005) • The “homosexual gene” has had proof for its existence (Hamer et al., 1993; Pool, 1993; Risch et al., 1993; Hamer & Copeland, 1994; Hu et al., 1995)

  12. Conclusion • This is all really strange to think about… • The “homosexual gene” sounds most sensible as it does have support • Research is relatively new in looking at homosexuality in animals (reported to be present in more than 200 species of mammals – not exclusive homosexuality though) • Could be one or a combination of theories to explain the evolution of homosexuality

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