Download Population Genetics 2

Population Genetics 2
• Micro-evolution is changes in the genetic
structure of a population
• Last lecture described populations in HardyWeinberg equilibrium
• We will now consider the 4 main forces that
cause a population to evolve
• They are Migration, Mutation, Drift and
Selection
Mutation
• Mutation is the means by which new alleles are created
• Mutation rates are very low - about 10-6 (1 in a million) for
a gene, 10-9 (1 in a billion) for a particular basepair in DNA
• This can generate a lot of potential variation in a population
- 6,000,000,000 humans would produce about
12,000,000,000 new alleles per generation
• However in population genetics we usually study “old”
alleles that have been in the population for some time
• The probability of any particular allele mutating is very
low, so mutation does not usually make a detectable
difference to Hardy-Weinberg equilibrium
Migration
• If individuals migrate from a population into
another population, with different allele
frequencies, this will cause a deviation from H-W
equilibrium (until the migrants have randomly
interbred with the natives)
• Example: population 1 has p = q = 0.5, and
population 2 has p = 0.9, q = 0.1. Both populations
are individually in H-W equilibrium. Mix together
100 from each population…..
Migration (2)
Genotype From
popn 1
From
popn 2
Total
Expected 2
observed H-W
AA
25
81
106
98
64/98
Aa
50
18
68
84
256/84
aa
25
1
26
18
64/18
p1 = 0.5
q1 = 0.5
p2 = 0.9
q2 = 0.1
pt = 0.7
qt = 0.3
Total 2
= 7.25
This result is a statistically-significant deviation from H-W
Drift
• In a population, some individuals may by chance not pass on
their alleles to next generation, others by chance pass on
more than their “fair share”
• This effect causes changes in allele frequency between
generations - genetic drift
• The effect is particularly pronounced in small populations
• Given enough time, any allele frequency can drift to 1
(fixation) or 0 (extinction)
• Drift is the major cause of genetic differences between subpopulations
Drift (2)
• Many populations go through “bottlenecks” where the
population is reduced (migration, disease, famine, climate)
• See Fig 23.8 in Purves. A small sample from a population may
have a non-random distribution of alleles
• When the population grows, it will have different allele
frequencies from the population before bottleneck
• A few individuals colonising a new region can cause a
“founder effect” whereby some genes are more common in the
colony than the population they came from
• Example is myotonic dystrophy (inherited muscular disease) much more common in a region of French Canadian
immigrants than in Europe, because some of the original
settlers were carrying the gene
Selection
• Selection results when fitness (the probability that
an organism will pass on its genes) depends on
genotype
• Certain alleles can increase or decrease fitness
• These effects can be dominant, recessive, codominant or additive
• Whether an allele is selective will also depend on
the genetic background (other genes) and
environment
A Selection Experiment
• Bacteria are a good system to use because
of short generation time (30 minutes)
Selection in diploid organisms
• Selection on recessive
alleles is very
inefficient because
when the allele arises,
it is virtually always in
the heterozygotes
Heterozygote Advantage
• In some cases the fitness of the heterozygote can
be more than that of either homozygote
• Example - sickle cell anaemia, a lethal recessive
disorder of blood
• Homozygotes with SCA are less fit, but carriers
are more fit because of resistance to malaria
• This causes the SCA allele to be maintained in
populations where malaria is common
Distribution of SCA and malaria
Random Mating
AA (p2)
Aa (2pq)
aa (q2)
AA (p )
AA (p4)
AA (p2pq)
Aa (p2pq)
Aa (p2q2)
Aa (2pq)
AA (p2pq)
Aa (p2pq)
AA (p2q2)
Aa (2p2q2)
aa (p2q2)
Aa (q2pq)
aa (q2pq)
Aa (q2pq)
aa (q2pq)
2
2
aa (q )
Aa (p2q2)
aa (q4)
AA: p4 + 2p3q + p2q2 = p4 + 2p3(1-p) + p2(1-p)2
= p4 + 2p3 - 2p4 + p2 - 2p3 + p4 = p2
Assortative Mating - a type of selection
AA (p2)
AA
Aa (2pq) aa (q2)
AA (p2)
Aa
AA (pq/2)
Aa (pq)
aa (pq/2)
aa (q2)
aa
AA: p2 + pq/2
Aa: pq
aa: q2 + pq/2