Transcript The Evolution of Homosexuality

The Evolution of
Homosexuality
Carey LeClair
Introduction
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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
Theories
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Biological Theories: The “homosexual
gene”, Brain Mechanisms, Hormonal
Signals, and Hypervariability
Sociobiological Theories: Parental
Manipulation, and Kin Selection
The “homosexual gene”
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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
The “homosexual gene”…
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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.
Brain Mechanisms
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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
Hormonal Signals
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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
Hypervariability
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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
Parental Manipulation
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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
Kin Selection
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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
Criticisms
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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)
Conclusion
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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