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Class 4: Oct 4, 2010 - Sexual selection I : Intrasexual competition
A brief history of the concept of sexual selection
Intrasexual competition and mate choice
How and why the sexes differ : Bateman’s principle
Effective polygamy
Vestiges of effective polygyny in Homo sapiens
Genghis Khan’s Y-chromosome
Homicide rates as an assay of male-male competition
Natural Selection is the differential reproduction of types as a
consequence of the differences among them.
Sexual Selection is the component of natural selection that derives
from differential access to mates or gametes, as a result of either
(1) vanquishing same-sex rivals (intrasexual selection), or
(2) being chosen by the opposite sex (intersexual selection).
A brief history of the concept of
Sexual Selection
1858 - Darwin & Wallace jointly present the
theory of natural selection to a meeting of
the Linnaean Society
1859 - Publication of “The origin of species ...”
in which Darwin presents the evidence for
natural selection and the germ of the idea
of sexual selection
1871 - Publication of “The descent of man and selection in relation
to sex ”, where Darwin develops the idea of sexual selection as
that part of selection (differential reproduction of types) that is
attributable to differential access to mates , and divides it into ...
(1) selection for attributes that help individuals vanquish sam e-sex
rivals in competition for mating opportunities
and
(2) selection for attributes that “charm” the opposite sex
A brief history of the concept of Sexual Selection (continued)
1871-1971 - natural selection becomes generally accepted as the
unifying theory of biology (especially after the “modern synthes is”
of Darwinism + Mendelism in the early 1930s) ...
but sexual selection is mostly ignored and sometimes dismissed,
especially Darwin’ s 2nd proposed process, mate choice.
1971 - A symposium commemorating the
centenary of “The descent of man...”
revives interest in sexual selection.
The resultant volume includes
Trivers’s (1972) influential analysis of
sexual selection and parental investment
... and sexual selection (especially mate
choice) becomes a major focus of
theory and research.
Darwin initially treated “sexual selection” as a distinct process
from the “natural selection” he had posited 12 years earlier.
Why? Because sexual selection can favour traits that natural
selection (e.g. by predation) opposes.
(e.g. antlers for fighting, peacocks’ tails for courting).
But antagonistic selective pressures aren’t peculiar to sexual
selection. Any trait under selection has both costs & benefits,
and will proliferate if and only if its benefits exceed its costs.
Traits that facilitate growth and early reproduction also hasten
senescence. Greater allocation of sensory apparatus or
neural tissue to the solution of one adaptive problem requires
sacrificing something else. Everything entails “trade-offs”.
Modern Darwinians now consider sexual selection to be
one component of natural selection.
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What defines female versus male?
Anisogamy : an ovum is much
larger than a sperm.
(The photo is of human gametes)
Darwin noted that aggressive mating competition occurs
mainly among males, whereas mate choice is exercised
mainly by females. But he had no explanation.
Bateman (1948) was the first to clearly recognize that males
gain fitness from additional mates in a way that females don’t.
He showed this in experiments with fruit flies, and explained
his results by the fact that one mating can fertilize all of a
female’s eggs, whereas males typically produce many more
sperm than will ever be incorporated into zygotes.
Bateman drew general implications
from his fruitflies:
Male fitness often depends more on N
of mates than female fitness, so sexual
selection will often favour “eager”
males and “discriminating” females.
Williams (1966) and Trivers (1972) expanded on Bateman’s argument:
the sex difference in parental investment is not just a matter of
anisogamy. Anisogamy “set the stage” for the evolution of further sex
differences (e.g. in pregnancy and lactation), and thus for distinct male
and female “sexual strategies” of competition and mate choice, because
of the differential impact of sexual selection on the sexes.
The sex that makes the lesser parental investment (usually males)
has a higher ceiling on fitness, and a higher fitness variance.
Bateman’s Principle
Bateman put 3 -5 fruit flies of each sex in a jar, watched matings, and
used genetic markers to assess each fly’s reproductive success (RS)
In 64 replicate
experiments, male
RS was always
more variable
than female RS.
Male RS increased
with each mate,
whereas females
did not gain from
polyandry.
Mean
RS
120
Males
90
o
o
o
o
o
Females
60
o
30
0
o
N of mates 0
1
2
3
“the greater dependence of males for their fertility on frequenc y of
insemination [is] an almost universal attribute of sexual reprod uction”
Bateman (1948) Heredity 2 : 349-368
The Bateman-Williams-Trivers theory of sexual selection suggests
not only that females will be more “discriminating” in courtship and
males more “eager”, but also that
polygamy (multiple mating partners) will appeal more to males
Because women make the greater parental investment and are thus the
“slow sex”, a limiting factor for male fitness is N of wives. I n societies
with polygynous marriage, male fitness tends to increase with each wife.
An example is provided by the
polygynous Temne people of
Sierra Leone
Dorjahn (1958) American
Anthropologist 60: 838- 860
Another example is provided
by polygynous U.S. Mormons :
Church elders typically had the most wives and often had hundred s of
grandchildren.
Smith & Kunz (1976) Population Studies 30: 465-480
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Effective Polygamy
In the majority
of 849 human
societies
(including all
face-to-face
hunter-gatherer
societies),
polygyny is a
legitimate
option for men
who can
attract / afford
more than one
wife
The “effective polygamy” of a breeding system
is the ratio of
male fitness variance / female fitness variance
In a strictly monogamous species, this ratio = 1.0
If the ratio < 1.0, the breeding system is “effectively
polyandrous”.
In many species, including fruit flies and most mammals,
the ratio > 1.0 and the system is “effectively polygynous”.
Murdock (1967) Ethnographic Atlas
Effective
polygyny in a
natural-fertility
human
population,
the !Kung San,
traditional
hunter-gatherers
in the Kalahari
desert of
Botswana.
Effective Polygamy
The “effective polygamy” of a breeding system is the ratio of
male fitness variance / female fitness variance
There are many reasons to conclude that the breeding
system of Homo sapiens has been effectively polygynous,
but only mildly so, for a long time, including
Effective
polygamy = 1.42
Lifetime reproduction for birth cohorts of 100 individuals of each sex
- the fact that the MRCA of human Y chromosomes lived
more recently than the MRCA of human mtDNA
- some polygyny in all known hunter-gatherers
- sex differences in polygamous desires
- sexual size dimorphism
- sexual bimaturism
- sex differential senescence
N. Howell (1979) Demography of the Dobe !Kung
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Genghis Khan (1162 - 1227), portrayed here on a contemporary
Mongolian banknote, was the greatest conqueror in history,
amassing an
empire much
bigger than
Alexander
the Great’s.
Genghis Khan died in 1227, but his many sons expanded the empire,
taking Baghdad in 1258. Grandson Kublai Khan conquered south China,
but failed to annex Japan despite invading with a fleet of 4000 ships.
A commoner
by birth, he
was a military
officer (named Temüjin) who defeated the Tatars in 1202, killed all
the men, and distributed the women among his supporters.
The Mongol Khan (ruler) then made Temüjin his adopted heir, and
he became Genghis Khan (meaning Universal Ruler) in 1206.
In 1211, he conquered Beijing, in 1219 Samarkand & Bukhara,
then Persia, Azerbaijan, Armenia, Georgia and, in 1223, Kiev.
Zerjal et al. (2003) Amer J Human Genetics 7 : 717-721
reported that they had found Genghis Khan’s Y-chromosome
… all over Asia!
8% of 2123 men from 16 Asian populations have closely related
Y -chromosomes (the “star cluster”) whose Most Recent Common
Ancestor lived 600 – 1300 (best guess = 860 – 1000) yrs ago.
“A single male line, probably originating in Mongolia,
has spread in the last ∼ 1000 years to represent ∼ 8%
of the males in a region stretching from northeast
China to Uzbekistan. If this spread were due to a
general population expansion, we would expect to find
multiple lineages with the same characteristics of high
frequency and presence in multiple populations, but we
do not. The star cluster pattern is unique.”
Maximum extent of the Mongol empire in the 13th and 14th centuries
Y -chromosome non-coding
haplotypes of 2123 men in
16 Asian populations show
this network of lineage
clustering.
Colours: populations of origin.
Circle size: haplotypefrequency.
Lines: mutational steps.
8% of 2123 men have
the “star cluster” Y. If the
sample is representative,
then ∼ 16 million Asian
men’s Y-chromosomes
had a common ancestor
about 1000 yrs ago !
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Homicides in which Victim and Killer were
Unrelated Same -Sex Adults
Canada 1974-1990
England & Wales 1977-1990
Iceland 1946-1970
Chicago 1965-1989
Detroit 1972
Miami 1980
California 1987-2003
The highest concentrations of the star -cluster were found among
Mongolians and Hazaras of Pakistan, whose oral history claims they are
descendents of Genghis Khan (1162- 1227). Best bet: the last common
ancestor of the star cluster was a recent ancestor of Genghis Khan.
!Kung San 1920-1955
Tiv 1931-1949
Tzeltal 1938-1965
Men
Women
3881
3087
10
94
108
0
% men
98
97
100
9761
282
359
22,873
229
9
0
334
98
97
100
99
19
93
33
0
1
0
100
99
100
Zerjal et al. (2003) Amer J Human Genetics 7 : 717- 721.
Why do men kill one another ?
Acquaintance cases and
stranger cases are similar.
Rates at which Men Killed Unrelated Men in Relation to
the Killer’s Age and Employment Status (Detroit 1972)
Context # 1. Social competition
- a status challenge.
- a public insult, “dissing”,
failure to show deference.
- rivalry over a woman.
Knife fight - José Francisco Borges
Context # 2. Material competition
- robbery
Macho disputes in bars, robbery, etc. are high-risk ways of
competing with other men. So who is undeterred by risk?
Answer : young men with little to lose.
Wilson & Daly (1985) Ethology & Sociobiology 6: 59 -73
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Rates at which Men Killed Unrelated Men in Relation to
the Killer’s Age and Marital Status (Canada 1974-1990)
Homicides per Million per Annum
160
Age-specific Rates of Killing
same-sex unrelated persons
(per million populace per year )
in 3 data sets.
140
Homicide rates vary widely
but
Sex differences and age
patterns are consistent
Single
Married
120
Divorced
Widowed
100
80
60
40
20
0
15-29
30-44
45-59
> 59
Age
Daly & Wilson (2001) Nebraska Symposium on Motivation 47: 1 -36
Gini coefficient of income inequality = Area 1 / (Area 1 + Area 2)
Homicide rates in 50 US states (1990) and 10 Canadian provinces
(1988-92) as a function of the Gini coefficient of income inequality
(based on 1990 gross pre -tax household income)
1.0
Line of equal income
Cumulative
Proportion
of Income
Area 1
Lorenz curve
Area 2
Income Units (e.g. Households)
150
Wealthiest
USA
Canada
120
90
60
30
0
0.35
0
Poorest
Homicides per Million Persons per Annum
180
0.38
0.41
0.44
0.47
Gini (total household income)
Daly, Wilson & Vasdev (2001) Canadian Journal of Criminology 43: 219-236
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