Download 1 Lecture 6 Migration, Genetic Drift and Nonrandom Mating I

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Lecture 6
Migration, Genetic Drift and Nonrandom Mating
I. Introduction –Greater Prairie Chicken Example (Tympanuchus cupido pinnatus)
A. Classic example of a species that uses a lek in mating.
1. A lek represents a polygynous mating system where males aggregate in a particular area
and exhibit their courtship displays to females that visit the group. One male may mate
with more than one female.
2. The decline in Greater Prairie Chicken populations can be attributed to the following 3
violations of H-W. They are 1) migration, 2) genetic drift and 3) nonrandom mating.
II. Migration – the movement of alleles between populations.
A. The one-island model of migration as an example.
B. Example of the impact of alleles entering a small population.
C. An example with water snakes (Nerodia sipedon)
1. Individuals vary in appearance ranging from strongly banded to undbanded.
a. Banding pattern is controlled (more or less) by a single locus with two alleles.
b. Banding is dominant over no bands.
2. On the mainland, virtually all of the snakes are banded, but on the island, the unbanded
variant predominates.
a. Unbanded pattern are more cryptic and presumably less vulnerable to predators.
3. If being unbanded should confer greater fitness, then why isn’t there a greater frequency
of unbanded snakes on islands?
a. Migration of individuals from the mainland.
b. The natural selection against banded individuals on the islands would be expected to
fix the allele frequency of being unbanded to 1. However, the effects of immigration
are opposing natural selection.
c. The result of constant introductions of alleles from the mainland is that this tends to
homogenize the allele frequencies on the island.
d. If natural selection did not oppose the effects of immigration, then the allele
frequency on the island would come to resemble that on the mainland.
III. Genetic Drift – Chance in a small population can lead to evolution.
1. Evolution is not a chance event.
2. Drift on the other hand, is a random process. Whether an individual has an appropriate
phenotype or not, it may be lost from the population.
a. Hence, genetic drift is a chance event that leads to changes in the allele frequency of a
population without leading to adaptation of the population.
A. A classroom example of genetic drift –red and black beans in a bag. Did your sampling
violate one of these conclusions of H-W?
1. The allele frequencies in a population will not change, generation after generation.
2. If the allele frequencies in a population are given by p and q, the genotype frequencies
will be given by (p2 + 2pq + q2).
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B. The source of small populations – the founder effect.
1. A population may be small because a portion of it was moved (or moved itself) to a new
location.
a. Original population was sampled, but not thoroughly. There has been sampling error.
b. Not all of the alleles have been sampled in the frequency that they occurred in the
original population.
c. As a result, the new allele frequency differs from the old allele frequency. This is
known as the founder effect.
2. Genetic drift can also occur when an existing population is severely reduced in size.
a. If the loss is due to an indiscriminate event, such as an environmental catastrophe,
then this too will result in a sampling error.
b. A reduction in the population size and the resulting change in allele frequency is
called a bottleneck effect.
c. It has been argued that only rare alleles are lost due to bottlenecking.
C. Genetic Drift over time – Random Fixation of Alleles and Loss of Heterozygosity.
1. Because the fluctuation in allele frequency is due to a sampling error, then each
generation will experience a different allele frequency due to chance.
2. Each generation, allele frequencies will randomly wander between 0 and 1. Over time,
this results in two major effects, one causing the other:
a. Eventually alleles drift to fixation (1) or loss (0).
b. The frequency of heterozygotes declines
D. Random Fixation of Alleles.
1. As the frequency of an allele fluctuates between 0 and 1, it will inevitably meet one of
two fates.
a. The frequency of the allele will reach 0, and the allele will be lost forever (unless it is
introduced through mutation or migration).
b. The frequency of the allele will reach 1, in which case, the allele is said to be fixed.
2. Is there anything about the an allele that will help us predict the probability that it will
drift towards fixation?
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a. Each allele’s chance of becoming fixed is
2Ν
3. The initial frequency of an allele is equal to its chance of being fixed.
a. If there are x copies of A1 and y copies of A2 and z copies of A3
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b. And each allele has a
chance of being the one that drifts to fixation.
2Ν
1
x
=
c. Then the probability that A1 drifts to selection is x x
2Ν 2Ν
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E. Loss of Heterozygosity.
1. As any allele drifts towards 0 or 1, the frequency of heterozygotes declines.
2. Sewell Wright predicted heterozygosity by
x
Hg+1 = Hg[1]
2Ν
x
is always between 0.5 and 1.0, so the value for heterozygosity
2Ν
must always decline.
a. The value of 1-
3. Example of Buri’s experiment with 107 populations of Drosophila.
a. The rate of decline in heterozygosity should follow Sewell Wright’s equation.
4. At the end of the experiment, the fit between the loss of heterozygosity and what was
expected under theory was not perfect.
a. The data followed the expectation for a population of size 9, not 16.
b. This is the effective population size.
c. The population has become so small that not all of the members participate equally in
the mating, effectively reducing its size.
5. The effective population size is sensitive to imbalances in the 1:1 ration of reproducing
females to reproducing males.
a. Effective population size can be calculated by:
Ne =
4N m N f
(N
m
+Nf
)
F. Rates of Evolution by Genetic Drift. Mutation rate = rate of evolution.
1. When selection is absent, what is the probability that a new allele created by mutation
will become fixed?
a. When an allele created by mutation becomes fixed, that is known as substitution.
2. The rate of evolution by genetic drift = the mutation rate (Box 6.5)
a. A diploid population of N has 2N alleles.
b. v = rate of neutral mutations, per allele, per generation. (the allele is selectively
neutral)
c. Each generation, there will be 2Nv new alleles created by mutation that have not
previously existed.
d. Because these alleles are selectively neutral, genetic drift is the only force at work.
e. As each allele fluctuates, it has an equal chance of arriving at fixation as any other
allele.
f. It’s chance of fixation is 1/2N
g. The number of alleles that are created by fixation and that are destined to become
fixed can be expressed by multiplying the two probabilities.
2Nv x 1/2N = v which is equal to the mutation rate.
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3. This rate holds no matter the size of the population.
a. More mutations occur in a larger population.
b. But this is offset by the lower probability that any one allele will drift to fixation.
(1/2N becomes smaller as N increases.)
4. What is the importance of drift on mutations versus natural selection?
a. There are 3 types of mutations – d,deleterious, a,selectively advantageous, and f,
selectively neutral.
b. If d , a , f each represent the fraction of mutations at a locus, then d + a+ f =1.
c. If we consider a gene encoding a protein that is L amino acids long, then the mutation
rate is
u= uL(d + a+ f )
This can be rewritten as:
u = uLd + uLa + uLf
5. In demonstrating that the rate of substitution is equal to the mutation rate, v.
a. Assume that deleterious alleles are disadvantageous and quickly eliminated.
b. Advantageous alleles arising by mutation are extremely rare.
c. This leaves u = uLf – the rate of neural mutations.
d. AND this is the same thing as the substitution rate v.
6. Neutral theory versus selectionist theory.
a. Proponents of selectionist theory maintain that advantageous mutations are
common.
b. In addition, they would predict that for many genes, the rate of substitution
reflects the act of natural selection on such advantageous mutations.
c. The Neutral Theory of Molecular Evolution holds that the great majority of
alleles are effectively neutral with respect to fitness and are fixed by genetic drift.
d. Because neutral mutations and subsequent substitutions proceed at roughly a
constant rate, they can be used as a Molecular Clock to determine the divergence
time of species.
e. The neutral theory does not deny the operation of natural selection on some base
pair or amino acid differences.
f. It holds thought that most of the variation we observe at the molecular level, both
within and among species, has little effect on fitness.