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Homosexuality - Biological Hypotheses
If we assume that historically
homosexual individuals have had
lower reproductive success than
heterosexual individuals, how is that
homosexuality is present as a
surprisingly common (1-10%?) trait in
the human population? Difficult to
explain as an evolutionary adaptation.
Homosexuality - Biological Hypotheses
1. Kin selection hypothesis (Wilson
1978)
2. Pleiotropy hypothesis (Zeitsch et al
2008 – our seminar reading – is a
good example, but see also
Iemmola & Ciani 2009 – slide 7)
3. Biological (non-adaptive) side effect
Example: mother immune response
(fraternal birth order) hypothesis
4. Alternative strategy – pursued when
prospects of RS by conventional
strategy is low.
5. Reciprocity in same-sex alliances
(Kirkpatrick 2000)*
* In reading list
Kin Selection Hypothesis
E. O. Wilson, the author of
Sociobiology, noted that homosexuality
seems to be a universal in human
societies, found throughout history and
in all cultures, albeit at relatively low
levels (~ 1 to 10%). Working from
animal analogies – the sterile castes of
eusocial insects (honeybees et al) and
bird “helpers at the nest” – Wilson
proposed that homosexual individuals
help collateral relatives (siblings,
nephews and nieces, cousins) so that
“homosexual genes proliferate through
collateral lines of descent, even if the
homosexuals themselves do not have
children”.
Kin Selection Hypothesis
Wilson then argued that “this kinselection hypothesis would be
substantially supported if some amount
of predisposition to homosexuality were
shown to be inherited” (1978). He then
cites evidence from twin studies which
suggests the existence of genes for
homosexuality (e.g., monozygotic twins
are more likely to be concordant for the
trait than are dizygotic twins).
But there are problems with this
hypothesis…
Problems with Wilson Theory
1. Wilson’s human model does not fit
the animal models which suggested the
kin-selection hypothesis! In the eusocial
insects and the birds and mammals
with ‘helpers’, the non-reproductive
helper is not genetically different from
the reproductives s/he helps! Helping
(or sterility) is a facultative trait in these
species – it has zero heritability. For
example, Queen and worker are
different not because of different genes
they have but because of different
developmental histories.
2. Wilson does not indicate why same
sex sexual orientation needs to go
along with helping (it doesn’t, for
example, in the animal models). Why
not just help and skip sex altogether?
Pleiotropy
single gene affects multiple traits
Epistasis
• multiple genes interact to affect a
trait
• multiple traits interact to produce
fitness
• therefore, natural selection for gene
combinations
Maternal line and
Class or relatives:
paternal line fecundity
• Mothers (1)
of the two sexual
• Mothers of first borns (1)
orientation groups
• Maternal aunts (0.75)
(likelihood of sharing
• Maternal uncles (0.25)
• Maternal grandparents (0.5) X-chromosomes)
•
•
•
•
•
Sons & daughters of maternal grandparents (0.25-1)
Paternal aunts (0)
Paternal uncles (0)
Paternal grandparents (0)
Sons & daughters of paternal grandparents (0)
1. Mothers (1)
2. Mothers 1st (1)
3. Mat aunts (0.75)
4. Mat uncs (0.25)
5. Mat grdps (0.5)
6. s&d of mat grps
7. Pat aunts (0)
8. Pat uncs (0)
9. Pat grdps (0)
10.s&d of mat grps
Iemmola, F. & Ciani, A. C. (2009)
Immune Response Theory of
Male Homosexuality
Bogaert, A. F. (2006). Biological versus nonbiological older
brothers and men’s sexual orientation. Proceedings of the
National Academy of Sciences, 103, 10771-10774.
Immune Response Theory of
Male Homosexuality
Note: This is from original paper (Blanchard & Bogaert 1996).
Subsequent research has removed the large-family confound and
shown that older brothers but not older sisters have this effect.
Immune Response Theory of
Male Homosexuality
G estimates the likelihood that if you are a
homosexual male, it can be attributed to your
having had older brothers. And the more older
brothers you had, the more likely it is.
Overall they have estimated that as a
statistical predictor of homosexuality in males,
the older brother factor accounts for 14%
(1/7th) of the variance.
This is certainly compatible with the immune
response theory, for other factors could protect
the mother or the female fetus against the
immune reaction.