Sexual Behavior

41 Pages·2007·0.52 MB·English

by  Ray Hames

Recent research reveals
In an astonishing study recently undertaken in Western
Europe, the following facts emerged: Married females
choose to have affairs with males who are dominant, older,
more physically attractive, more symmetrical in
appearance, and married; females are much more likely to
have an affair if their mates are subordinate, younger,
physically unattractive, or have asymmetrical features;
cosmetic surgery to improve a male's looks doubles his
chances of having an adulterous affair; the more attractive
a male, the less attentive he is as a father; roughly one in
three of the babies born in Western Europe is the product
.
of an adulterous affair
Ridley, M The Red Queen
WWhhyy HHaavvee SSeexx??
IIff ffiittnneessss iiss mmeeaassuurreedd bbyy yyoouurr
(cid:139)(cid:139)
rreepprroodduuccttiivvee oouuttppuutt ((nnuummbbeerr ooff ggeenneess
ccoonnttrriibbuutteedd ttoo tthhee nneexxtt ggeenneerraattiioonn)),,
wwhhyy hhaavvee sseexx ggiivveenn oonnee’’ss rreepprroodduuccttiivvee
oouuttppuutt iiss hhaallff ooff aann aasseexxuuaall oorrggaanniissmm??
TThhiiss iiss kknnoowwnn aass tthhee ccoosstt ooff mmeeiioossiiss..
(cid:139)(cid:139)
WWee nneeeedd ttoo uunnddeerrssttaanndd hhooww tthhee
(cid:139)(cid:139)
bbeenneeffiittss ooff sseexx oouuttwweeiigghh tthhee ccoossttss ooff
mmeeiioossiiss..
WWhhyy HHaavvee SSeexx?? ((II)) WWiinnnniinngg
tthhee AArrmmss RRaaccee
We are involved in an “evolutionary arms race” against
(cid:139)
pathogens who reproduce faster than we do and
therefore can modify their genetic structure to overcome
our immune system.
Therefore, sex allows us to modify our genetic structure,
(cid:139)
through our offspring, to win (for a time) this battle.
By mixing the genes of two individuals, sex creates a
(cid:139)
third - the offspring - with a new, unique combination.
This maintains genetic variation, fuelling evolution
and keeping one step ahead of agents of infectious
disease
As evolutionary biologist Jonathan Howard put it: “If sex
(cid:139)
causes disease, then it might also be true that disease
causes sex. (Nature 296-299 (2000)
WWhhyy HHaavvee SSeexx?? ((IIII)) RReeppaaiirriinngg
EErrrroorrss
(cid:131)
Muller’s ratchet and Kondrashov’s solution
(cid:131)
Muller noted that deleterious mutations increase simply as the
number of genes in an organism increases. (Think of a photocopy
of a photocopy of a photocopy, etc.) More complex organisms
have more genes and therefore are more likely to produce
offspring with genetic defects. How do complex organisms deal
with this problem?
(cid:131) Kondrashov theorized that sexual reproduction is the costly
solution.
(cid:131) Organisms “shed” deleterious genes by producing offspring that
have them and these offspring, because they are less fit, will not
pass on the deleterious genes to future generations.
(cid:131) Offspring that lack the deleterious genes have higher fitness and
only pass “good” genes to future generations.
SSeexxuuaall SSeelleeccttiioonn
DDeeaallss wwiitthh ""TThhee aaddvvaannttaaggee wwhhiicchh cceerrttaaiinn iinnddiivviidduuaallss hhaavvee
oovveerr ootthheerrss ooff tthhee ssaammee sseexx aanndd ssppeecciieess ssoolleellyy iinn rreessppeecctt ttoo rreepprroodduuccttiioonn ......””**
nnoott ffrroomm bbeeiinngg bbeetttteerr aabbllee ttoo ssuurrvviivvee iinn tthhee ssttrruuggggllee ffoorr eexxiisstteennccee bbuutt ffrroomm
hhaavviinngg ggaaiinneedd aann aaddvvaannttaaggee oovveerr ootthheerr mmaalleess..""
OOrr,, tthhee ddiiffffeerreennttiiaall aabbiilliittyy ooff iinnddiivviidduuaallss ooff ddiiffffeerreenntt
ggeenneettiicc ttyyppeess ttoo aaccqquuiirree mmaatteess..
SSeexxuuaall sseelleeccttiioonn lleeaaddss ttoo sseexxuuaall ddiimmoorrpphhiissmm..
The Two Forms of Sexual Selection
Inter-sexual (epigamic): based on female choice of
l
certain qualities possessed by males
Intra-sexual: based on male-male competition for
l
mating rights (development of fighting weapons)
*The Descent of Man and Selection in Relation to Sex (1879), page 209.
Charles Darwin
SSeexxuuaall aanndd NNaattuurraall
SSeelleeccttiioonn
IInn nnaattuurraall sseelleeccttiioonn,, tthhee
eennvviirroonnmmeenntt ddooeess tthhee sseelleeccttiinngg.. WWiitthh
sseexxuuaall sseelleeccttiioonn iitt iiss mmeemmbbeerrss ooff
oonnee''ss oowwnn ssppeecciieess tthhaatt aacctt aass tthhee
sseelleeccttiivvee aaggeennttss.. WWiitthh sseexxuuaall
sseelleeccttiioonn ""ggeenneess aacctt aass bbootthh ffaasshhiioonn
mmooddeellss aanndd ffaasshhiioonn ccrriittiiccss"",,ccrreeaattiinngg
ffeeeeddbbaacckk llooooppss tthhaatt ccaann rreessuulltt iinn
eexxttrraaoorrddiinnaarryy aanndd rraappiidd eevvoolluuttiioonn..**
*Paraphrased from Geoffrey Miller’s The Mating Mind, 2000
Outcomes of Sexual Selection
SSeexxuuaall DDiimmoorrpphhiissmm
(cid:139)(cid:139)
–– FFeemmaalleess lliitteerraallllyy mmoolldd mmaallee ffeeaattuurreess ((eeppiiggaammiicc oorr
iinntteerrsseexxuuaall oonnllyy))..
HHiigghh rreepprroodduuccttiivvee vvaarriiaannccee ((ggeenneerraallllyy mmaallee))
(cid:139)(cid:139)
TTrriivveerrss aanndd SSeexxuuaall CChhooiiccee::
(cid:139)(cid:139)
–– TThhee sseexx tthhaatt iinnvveessttss mmoosstt iinn rreepprroodduuccttiioonn wwiillll
eexxeerrcciissee cchhooiiccee iinn tthhee sseelleeccttiioonn ooff aa mmaattee ((ggeenneerraallllyy,,
ffeemmaallee cchhooiiccee))
HHuummaann mmaattiinngg ssyysstteemmss,, ttoo ssoommee ddeeggrreeee,, aarree
(cid:139)(cid:139)
bbaasseedd oonn sseexxuuaall sseelleeccttiioonn::
–– UUssuuaallllyy ppoollyyggyynnoouuss aanndd bbaasseedd oonn mmaallee wweeaalltthh ((rreessoouurrccee
ddeeffeennssee ppoollyyggyynnyy)),, pphhyyssiiccaall qquuaalliittiieess,, oorr ssttaattuuss..
MMaallee && FFeemmaallee DDiiffffeerreenncceess iinn FFeerrttiilliittyy aanndd
SSeexxuuaall BBeehhaavviioorr -- hhiigghh mmaallee vvaarriiaannccee
12
for Males ( ) and Females ( )
9
Female Male
t 10.20 10.20
n Mean
u
o 6 S.D 3.25 11.28
C
Min 1.00 1.00
Max 16.00 44.00
2
0
1 6 11 16 21 26 31 36 41 46 51
Number of Offspring or Lifetime Sexual Partners
TTrriivveerrss'' PPaarreennttaall IInnvveessttmmeenntt aanndd
SSeexxuuaall SSeelleeccttiioonn::
The sex that invests most in reproduction will be
the choosier sex.
Trivers generalizes that the sex that invest most in reproduction will have
a different sexual strategies from the sex that invests less. It is clear that
females, most of the time (through anisogamy) invest more. As a result,
the following can be deduced:
(cid:206) female RS is limited by either male quality (good genes) or
male parental investment
(cid:206) females will be highly selective because both the costs of error
in mate selection and the benefit of a correct choice powerfully
affect fitness
(cid:206) males reproduction will be more strongly limited by the number
of copulations they can attain but this will have no effect on
female RS (but consider partible paternity)
(cid:206) males will compete with one another for the right to mate with
other females
Methodological Problems in the Study
of Human Sexuality
(cid:190)(cid:190)
pprriivvaatteellyy ppeerrffoorrmmeedd
(cid:190)(cid:190)
iinnffoorrmmaanntt rreeppoorrttss aarree ddiissttoorrtteedd
(cid:190)(cid:190)
rreesseeaarrcchheerrss hhaavvee ccuullttuurraall bbiiaasseess
Basic Cross-cultural Research
(cid:190)
Symons The Evolution of Human Sexuality
(Oxford, 1979)
(cid:190)
Buss, D. The Evolution of Desire
(Basic Books, 1993)