Download lecture 04 - selection, mutation and drift

Selection and Genetic Variation
1) selection against
recessive alleles
If alleles are recessive lethal, then
selection can only act on them
when they are homozygous
consider Dawson’s flour beetles:
started with population of all
heterozygotes, + / l
l / l is lethal, but + / l is same
as wildtype +/+
Selection and Genetic Variation
1) selection against
recessive alleles
Although selection initially
removed the l allele from
population at a rapid rate,
with each generation the
frequency of l declined
more slowly
Selection and Genetic Variation
2) selection against homozygotes
This population was started with 100% heterozygotes for a
viable allele V, and an allele L that is lethal when homozygous
although selection rapidly
caused the V allele to
increase in frequency,
the L allele never
disappeared
in fact, the frequency
of L stabilized at 0.21
Selection and Genetic Variation
2) selection against homozygotes
1/5th of the population carried the lethal allele at equilibrium
(the point where the population ceased to evolve)
Why?
Selection and Genetic Variation
3) selection against heterozygotes
consider the case of flies with compound chromosomes
normal pair of
homologous
chromosomes
compound chromosomes: arms swapped
- one ends up with both left halves
- other ends up with both right halves
when these flies make sperm/eggs, meiosis gets screwed up...
they make 4 kinds of gametes
Selection and Genetic Variation
3) selection against heterozygotes
C and N flies can’t
make viable zygotes
together
- Flies can be homozygous for C (compound) or N (normal) allele
- two N/N flies can reproduce; all zygotes are viable (fitness =1)
- two C/C flies can reproduce; 1/4th of zygotes viable (fitness = 0.25)
- C/N flies don’t exist; they never develop (fitness = 0)
Selection and Genetic Variation
3) selection against heterozygotes
one or the other allele quickly becomes fixed in a mixed population
Selection and Genetic Variation
3) selection against heterozygotes
one or the other allele quickly becomes fixed in a mixed population
- why? if there are few N/N flies, the odds of 2 mating are low
- most N/N flies will not produce viable offspring
- the allele will vanish
- if there are many N/N flies, they quickly out-breed C/C flies,
due to their 4-fold advantage in producing viable offspring
this is underdominance:
Models of heterozygote superiority and inferiority
- in overdominance (heterozygote fitness > homozygote fitness),
population fitness is maximized at its stable internal equilibrium,
the point to which the population naturally returns
Models of heterozygote superiority and inferiority
- in underdominance (homozygote fitness > heterozygote fitness),
the population fitness is minimized at the unstable internal
equilibrium, the point from which the population naturally diverges
Frequency-dependent selection
Attack other fish by sneaking up,
rushing them, biting off a mouthful
of scales
- Those with mouths that curve to
the right attack the left side of
victims, and vice-versa
- Handedness of mouth is
determined by a single locus
with 2 alleles (simplest case!)
- Right-handedness is dominant
scale-eating fish of
Lake Tanganyika
Frequency-dependent selection
- victims come to expect attacks from the direction that the
majority of the scale-eaters attack from, at that particular time
- when right-handed fish are more common, victims pay less
attention to their right side (where few attacks come from);
this gives left-handed fish the edge
- as left-handers get more food, they survive and reproduce better
- then, when left-handed offspring are the majority, the situation
reverses
proportion of
left-handers
Frequency-dependent selection
- squares = proportion of successful breeding adults
proportion of
left-handers
Frequency-dependent selection
Frequency-dependent selection
The equilibrium point should be 50/50 of each phenotype…
…so what are the expected allele & genotype frequencies?
Alleles:
Allele frequencies
Possible genotypes:
R
0.3
RR
Hardy-Weinberg predicts: R2
Genotype frequencies:
0.09
L
0.7
RL
+
2RL
0.42
LL
+
L2
0.49
Frequency-dependent selection 2
Another case: pea aphid Acyrthosiphon pisum occurs in
green and red color morphs
- what maintains polymorphism,
the occurrence of both phenotypes
in the population?
Differential vulnerability to predation versus parasitism,
depending on color
- green aphids are more parasitized by wasps that lay
their eggs inside aphids
- red aphids get eaten more by ladybugs (they’re more obvious
sitting there on green plants)
Mutation as an evolutionary force
Mutation is ultimately responsible for creating new alleles and
genes, but..
- can mutation also represent an evolutionary force, by
changing allele frequencies?
- can mutation affect the predictions of Hardy-Weinberg
equilibrium?
Mutation as an evolutionary force
Consider a population where allele frequencies are:
A
0.9
a (a recessive, loss-of-function allele)
0.1
In the ordinary Hardy-Weinberg state, adult genotypes will be:
AA
0.81
Aa
0.18
aa
0.01
Mutation as an evolutionary force
Now assume A mutates to a at a rate of 1 per 10,000 genes
each generation
due to mutation, the allelic makeup of gametes will be:
A
0.9 – (0.9)(0.0001)
= 0.899991
a
0.1 + (0.9)(0.0001)
= 0.10009
Mutation as an evolutionary force
When gametes randomly fuse to form zygotes, the genotype
frequencies will be:
AA
0.80998
Aa
0.18016
aa
0.01002
Hardly any change; mutation had little effect over one generation
Over thousands of generations, mutation can affect allele
frequencies