Parentoffspring Conflict

1
Parent-offspring Conflict
Parents are only half related to their offspring,
but offspring are completely related to
themselves (r 1 with yourself) - offspring
mainly want their own needs met, but parents
must spread their efforts around to
multiple children - offspring are predicted
to demand an unequal share of parental
attention
2
Parent-offspring Conflict
Mothers ideally stop nursing when benefit-to-cost
ratio reaches 1 - at this point (P), offspring
can feed themselves, and she can put effort
into raising other young - otherwise, the cost
to her is more than the benefit to the kid A
kid ideally nurses until its costing mom twice
the payoff to the kid Past this point (O), the
kid will let mom put effort into other
siblings, since hes 1/2 related to them -
so her efforts go towards his indirect,
instead of direct, fitness
3
Parent-offspring Conflict
If mothers mate with multiple males and produce
mainly half-siblings, then offspring are only
1/4 related to littermates - in this case, kids
will push mom to nurse them until it costs her
4 times the payoff to themselves
4
Cooperation among non-relatives
How do you explain cooperation among
non-relatives, where there is no genetic
payoff (indirect fitness) for the performer?
- alleles causing this can spread if the
performer stands to benefit at a later
date by helping a non-relative right now Two
things are required for such reciprocal altruism
(1) the payoff to the receiver must be
larger than the cost to the
performer (2) recipients who dont return
the favor must be punished
5
The Prisoners Dilemma
Trust rewards, treachery and revenge!
6
Cooperation among non-relatives Vampire Bats
Vampire bats will starve to death if they go 3
nights w/out feeding Bats that roost together
will sometimes regurgitate blood meals to
their roost-mates Do bats regurgitate to (1)
relatives? (2) bats who they are just
around a lot?
7
Cooperation among non-relatives Vampire Bats
Blood sharing depends on both relatedness and
degree of association, which is how often they
hang together
relatedness is higher among regurgitators than
among all bats in a colony
association is higher among regurgitators than
among all bats in a colony
8
Cooperation among non-relatives Vampire Bats
Hypothesis Bats reciprocate blood sharing with
frequent roostmates Experiment 9
bats held in captivity for several weeks - each
night, food was withheld from a different bat -
so who fed whom? Result Bats fed other bats
who had previously fed them
9
Evolution at two loci Linkage Disequilibrium
So far, our population genetic models have
assumed that a trait is controlled by a single
locus (i.e., one gene product determines the
phenotype) Many traits are determined by the
products of multiple genes - termed multilocus
phenotypes What happens when you track not just
changing allele frequencies, but also the
actual chromosomes on which those alleles are
found? - does selection on one locus affect
inheritance of alleles at a different locus?
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Evolution at two loci Linkage Disequilibrium
Definitions haplotype the multilocus genotype
of a chromosome ? the alleles on one chromosome
form a haploid genotype ? commonly refers to the
genotype of haploid genomes, like
mitochondria or gametes linkage the physical
attachment of alleles that are located on the
same chromosome ? means that two alleles are not
inherited randomly tend to be passed on
together
A B
a b
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Evolution at two loci Linkage Disequilibrium
Linkage equilibrium the B allele occurs just as
frequently on chromosomes carrying the A allele
as on chromosomes carrying the a allele
12
Evolution at two loci Linkage Disequilibrium
Linkage disequilibrium the B allele occurs more
frequently on chromosomes carrying the a allele
(AB chromosomes occur less frequently than
youd predict from allele frequencies)
13
Evolution at two loci Linkage Disequilibrium
Definitions linkage equilibrium the genotype at
one locus is unrelated to the genotype at
another locus ? knowing which allele is present
at one locus tells you nothing about
which allele is likely to be present at another
locus linkage disequilibrium the genotype at
one locus is associated with a particular
genotype at another locus ? the alleles at two
different loci are not randomly distributed
certain pairs of alleles tend to be found
together
A B
A ?
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Evolution at two loci Linkage Disequilibrium
A B
A ?
Two alleles A and B are in linkage equilibrium
if (1) the frequency of B on chromosomes
carrying A is the same as the frequency
of B on chromosomes carrying a (or any
other allele at the A locus) (2) the frequency
of any chromosomal haplotype can be
calculated by multiplying the individual allele
frequencies ? the odds of a chromosome being aB
is equal to the frequency of a times the
frequency of B
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Evolution at two loci Linkage Disequilibrium
What creates linkage disequilibrium ? (1)
selection on multilocus phenotypes (2) genetic
drift (chance events, especially in small
populations) (3) mixing of populations
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(1) Selection on multilocus phenotypes
Consider our old friend, the long-necked,
strong-hearted antelope
normal normal neck heart
long strong neck heart
If a favorable combination confers a strong
fitness advantage, then selection can keep
two alleles in linkage disequilibrium
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(2) Genetic drift
- start with a population that only has the A
allele - now, a mutation happens on a b
chromosome that converts an A a - if
the a allele confers a fitness advantage, it
will quickly increase in frequency... -- but
it only exists on chromosomes that carry
the b allele
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Linkage disequilibrium versus sex
Linkage disequilibrium will always be eroded by
recombination
normal normal neck heart
normal strong neck heart
long strong neck heart
long normal neck heart
Crossing over may put new allele combinations
together in the first place, but will also
inevitably tend to separate them

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Linkage disequilibrium versus recombination
Linkage disequilibrium will ways be eroded by
recombination Sex thus presents the opportunity
to put new allele combinations together in the
first place, but will inevitably take them
apart - only strong selection will maintain
linkage disequilibrium for loci that are
not extremely close together
20
Genetic drift, HIV and the CCR5-?32 allele
The HIV virus docks on two cell-surface receptors
to gain entry to helper T cells the CD4
receptor, and the CCR5 co-receptor Certain
individuals never become HIV despite long-term
exposure to the virus Studies revealed that
these individuals carried a mutant allele of the
CCR5 receptor carrying a 32-base deletion, thus
called ?32
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The ?32 allele is fairly common in Caucasian
populations, at an average frequency of about
9 Rare or absent from most Asian and African
populations May have spread through a selection
event in Europe, possibly black plague in the
14th century
22
Origins of the CCR5-?32 allele
Why is this allele prevalent only in European
populations? (1) Look for linkage
disequilibrium, evidence of an origin via a
chance event in the past (i.e., the result of
genetic drift) (2) Postulate a selective
advantage such a mutation would have had,
that would have prevented the loss of the new
allele most new mutations are lost by drift
in large populations
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Linkage and microsatellites
Microsatellites are widely used for evolutionary
analysis, because they are usually non-coding
regions of DNA - since these short
repeats dont code for anything, different
alleles have no selective advantage -
allows you to study the rate of evolution without
confounding effects of selection on
protein function
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Linkage and microsatellites
Stephens et al. examined two neighboring
microsatellites, loci that have short, tandem
repeats - our genome is riddled with
these short repeats, which can grow
longer through mistakes in replication crossing
over ------------ATGT-ATGT-ATGT----------
normal alignment ------------ATGT-ATGT-ATGT-----
----- ------------ATGT-ATGT-ATGT---
25
Linkage and microsatellites
Stephens et al. examined two neighboring
microsatellites, loci that have short, tandem
repeats - our genome is riddled with
these short repeats, which can grow
longer through mistakes in replication crossing
over ---ATGT-ATGT-ATGT-------------------
mis-alignment ------------ATGT-ATGT-ATGT---------
- ---ATGT-ATGT-ATGT-ATGT--- 3-peat expands to
a 4-peat
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Linkage and microsatellites
Stephens et al. examined two microsatellites near
the CCR5 gene called GAAT and AFMB -
found that nearly all ?32 alleles were on
chromosomes with GAAT allele 197, and AFMB
allele 215 CCR5-?32 chromosomes 85
?32 - 197 - 215 15 ?32 - other -
other This strongly suggests that the ?32
mutation arose once, on a chromosome that
happened to be GAAT-197 and AFMB-215
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Linkage and microsatellites
CCR5-?32 chromosomes 85 ?32 - 197 -
215 15 ?32 - other - other
Linkage disequilibrium has slowly been eroded by
recombination, however
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Linkage and microsatellites
Using recombination rate and distance between
CCR5 gene and both markers, age of mutation
was estimated at 700 ago -
consistent with theory that black plague selected
against the normal CCR5 allele, so
individuals carrying the ?32 had a
survival advantage
215
197
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Linkage and microsatellites
However --- 1) later work showed the age
estimates were wrong because the distance
estimates along the chromosome were off -
revised estimate mutation occurred 5,000 years
ago - researchers argued drift alone could have
produced the high frequency of the
?32 mutation 2) in 2005, researchers sequenced
DNA from 17 corpses 3,000 years old, found
?32 at a frequency of 12 (same as now) -
argued theres no way drift alone could have
gotten a mutation to 12 frequency
in 5000-3000 2,000 years - modeling studies
indicate some selection must have been
involved
30
Linkage disequilibrium versus recombination
Linkage disequilibrium is Created by
(1) selection on multi-locus phenotypes (2)
genetic drift - chance events not resulting from
selection Eliminated by (1)
recombination (over time), which happens during
sex
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Sample protein polymorphism questions
How many alleles are present in this sample?
What are the allele frequencies? genotype
frequencies? If the population was in
Hardy-Weinberg eq, what would the expected
genotype frequencies be? Is this population
close to, or far from, H-W expectations? What
forces could produce the observed pattern?
32
Effects of monogamy
In bat species where females are promiscuous
(mate with multiple males), males evolved larger
testicles (advantage in sperm competition)
- monogamous species testes up to 1.4 of male
body weight - promiscuous species testes
up to 8.5 of male body weight but
advantage in sperm competition comes at a price
males of promiscuous species also have
smaller brains Promiscuous species male body
uses more energy to enhance testes --
trade-off lack energy to further develop the
brain In species where females are faithful to
one partner, males have smaller testes and bigger
brains
Pitnick et al. 2006, Proc Roy Soc B
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In summary think about sexual conflict in terms
of (1) Male-advantage alleles (i.e., bright
color) - are good when expressed in males
attract mates - are bad when expressed in
females attract predators resolution
confinement to Y chromosome (females are safe)
result eventual deterioration of the Y
(Mullers ratchet) (2) Sexually antagonistic
alleles (i.e., toxic seminal fluid) - help male
assert his reproductive agenda over that of the
females he mates with, to their harm
resolution females constantly counter-adapt
(open-ended) result arms race each sex
limits others adaptive evolution