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Homosexuality:. A Darwinian. Paradox?. Matt Schechter 4/26/05. What Paradox?. Homosexuals, by definition, don’t reproduce Natural Selection selects for reproductive success How, if natural selection operates on humans, does homosexuality persist?. ?. Assumptions.

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slide2

Homosexuality:

A Darwinian

Paradox?

Matt Schechter

4/26/05

slide3

What Paradox?

  • Homosexuals, by definition, don’t reproduce
  • Natural Selection selects for reproductive success
  • How, if natural selection operates on humans, does homosexuality persist?

?

slide4

Assumptions

  • Homosexuality is heritable
  • Prevalence and history of homosexuality is constant
  • Homosexuals have decreased reproductive output (fitness)
slide5

The Nature of the Answers

  • Male Bias
    • Frequency
    • Exclusivity
  • Two Categories of Explanation
    • Direct benefit hypothesis
    • Indirect benefit hypothesis
slide6

Indirect Benefit Hypothesis

  • Homosexuality is a byproduct of a trait that has fitness benefits
    • Does this ever happen?
  • Selection acts on biology, so what have we discovered about the causes of homosexuality?
slide7

The Science of Homosexuality

  • The Difficulties
    • Political/Social
    • Scientific
  • Nature vs Nurture?
    • Genes vs Environment
    • The two in consort
slide8

Behavior

  • A blend of male and female
    • Female characteristics
      • Gender-atypical childhood behavior
    • Male characteristics
      • Promiscuity in male homosexuals
      • Male gender identification
  • Is there a biological basis for these observations?
slide9

Biology

  • Female brain characteristics present in male homosexuals
    • LH Response
    • SCN Nucleus
  • Cause of these characteristics
    • Brain development occurs in utero
    • Gender is defined in utero during sexual differentiation
      • Eve principle
slide10

Eve Principle

Either XX or XY Fetus

Female Gender “Seeds”

Male Gender “Seeds”

Brain

Body

slide11

Male Differentiation

XY Fetus – Heterosexual Development

Female Gender “Seeds”

Male Gender “Seeds”

Brain

TESTOSTERONE

MIH

Body

slide12

Male Differentiation

XY Fetus – Homosexual Development

Female Gender “Seeds”

Male Gender “Seeds”

Brain

TESTOSTERONE

MIH

Body

slide13

Biology

  • Homosexuality and birth order
    • Homosexuals tend have older male siblings
      • More older brothers means higher chances of homosexuality in younger siblings
      • Females are unaffected
    • The Mechanism
      • Maternal immune response provoked by H-Y antigens
      • One of the H-Y antigens is MIH
slide14

Why does this happen?

  • Maternal immune response to MIH is greater with each male child born
    • More femaleness left in brain) (defeminization is decreased) with each male child
  • Variations in sexual personality among male siblings are advantageous for survival
    • Don’t kill each other during childhood
    • Don’t compete for similar females
  • Homosexuality is this advantageous process gone too far
slide15

Wait just a minute…

  • But why does this process go too far?
  • And, if this physiological cause of homosexuality increases with the amount of sons born, why aren’t all males who have a given number of older brothers gay?
  • By the same token, why are some only-children gay?
    • An individual’s genes may determine the degree of susceptibility to these effects
slide16

Genetics

  • A “gay gene”
    • Maternal heritability
    • Xq28
      • 33 of 40 gay brothers shared the same gene in the same region on the X-chromosome
  • The X chromosome
    • Males get it only from their mothers
    • All X-linked traits are found twice as often in females, so the the selection of these traits depends more on their effect on females than on males
slide17

The Effect on Females

  • What might Xq28 or similar x-linked genes do to females?
    • The maternally related females of gay men have larger families than paternally related females
      • Ex. My mom’s sister has more kids than my dad’s sister
      • Ex. The daughters of my mom’s sister have bigger families than my do the daughters of my dad’s sister
    • This is clearly advantageous
slide18

Recall…

    • Homosexual male brains are more feminine than heterosexual male brains
    • The defeminization of developing male brains varies by birth order and is advantageous
    • This variation has a genetic component that may be due to a group of genes
  • AND…
    • There is at least one gene, located on the x-chromosome that influences homosexuality
    • An X-linked gene that is related to homosexuality also has a beneficial effect on the fertility of females related to homosexuals
slide19

A Possible Answer to the Paradox

  • In homosexual males, the same trait that increases fecundity in females causes too much susceptibility to the normally-advantageous defeminization-limiting immune response
  • The increase in the fertility of females with this trait is more than enough to keep the x-linked trait in the gene pool
  • It is likely that a group of genes control this trait, and different combinations produces more/less severe effects
slide20

Problems With the Answer

  • What about lesbians?
  • What about bisexuality?
  • What about first-born gay sons with straight younger brothers?
  • There’s no evidence for genes that increase female fecundity
  • There’s no evidence for genes other than Xq28 that affect homosexuality
slide21

Beyond The Science

  • Ethics of understanding the mechansim
    • Genetic/hormonal surgery
    • Prenatal screening
  • Implications of understand the evolutionary basis
    • Homosexuality as a product of success?
    • Choice in behavior?
  • Political implications
    • Don’t expect revolution
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