Download Biology 4974/5974 Evolution

Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
Biology 4974/5974
Evolution
Gene Flow, Genetic
Drift, and the Shifting
Balance Theory
Figures from Hall and Hallgrimsson, 2014, Strickberger’s Evolution
Learning goals
Understand how the following processes change allele
frequencies within a population:
• Mutation
• Neutral selection
• Balanced polymorphism
• Gene flow (migration)
• Natural selection
• Genetic drift
Know and understand:
• Why local populations may differ from one another. How
to tell genetic differentiation from environmental effects.
• How genetic drift alters allele frequencies.
• How the Shifting Balance Theory maintains alleles within
a population. How a deme moves from peak to peak.
Demes
Individuals of a species share a gene pool by
definition.
• Demes and local populations may differ from one
another because of genetic differences due to natural
selection, mutation, and genetic drift.
• Gene flow reduces these differences.
• Variation in phenotype among populations of a species
is known as geographic variation.
Example: Populations of yarrow (Achillea lanulosa and
Achillea borealis) vary in height and many leaf and flower
traits.
How can we tell whether this variation is genetically
based or in response to environmental differences?
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Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
Different
populations of
yarrow in a
transect across
California.
Fig. 19.1
Timberline = 3,050 m
Mather = 1,400 m
Stanford = sea level
Fig. 19.2
Mutation
One explanation for the origin and continued existence
of genetic variation is gene mutation.
• The rates of forward mutation and back mutation are
usually not equal.
• Mutation rates are low: one mutation per 100,000 copies
of a gene.
• Each egg and sperm carries less than one new mutation
on average.
Assuming no difference in selection between alleles.
p = frequency of A
A
q = frequency of a
a, mutation rate is u; a
A, mutation rate is v
Δq = u p0 – v q0
An equilibrium is reached between the two gene frequencies, based on
the respective mutation rates:
^q = u/(u + v) ^p = v/(u + v) and ^p/^q = v/u
Gene flow (migration)
Alleles may enter a population through gene flow or
gene migration. This will change allele frequencies.
Factors that influence the recipient population, given one
allele in a population:
• The difference in gene frequency between recipient (q0)
and migrant (Q) populations for that allele.
• The proportion of migrating genes incorporated each
generation into the recipient population (m).
• Whether gene flow occurs once or repeatedly. If
repeatedly, the gene frequency of the recipient
population become qn.
• (1 – m)n = qn- Q/ q0 - Q
Gene flow can oppose selection (oppose adaptation), for
example. In other cases it can add important genetic
variation or reduce inbreeding effects.
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Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
Random genetic drift
Random genetic drift usually results in the loss of
allelic variation. Random events (random variation) in
populations can alter allele frequencies at a gene locus.
• The smaller the population, the more likely that genetic
drift will cause changes in allele frequencies.
• Assume a diploid population, with two alleles for a gene,
indicated by the frequencies p and q.
• The Standard Deviation in p and q caused by random
variation each generation is calculated as:
σ = √pq/2N
• N is the number of parents; 2N is the total genes for that
locus.
• Gene frequencies would assume a normal distribution
with p and q the mean frequency and σ the variance
around the mean due to random events.
Drift when population is large
N = 5,000 parents, and 2N = 10,000 genes examined.
p = q = 0.5 are initial allele frequencies (p + q = 1).
• After one generation, the range of drift possible:
σ = √(0.5)(0.5)/10,000 = √0.000025 = 0.005
• This means that the values for p and q will vary around
0.5 ± 0.005,that is between 0.495 to 0.505.
• For a population with the extreme values for p and q,
next generation:
σ = √(0.495)(0.505)/10,000 = 0.0049.
• Thus, p and q could become even more different by
chance: ~ 0.490 and 0.510.
The likelihood is that drift will fluctuate around 0.5 for each
allele, back and forth over time.
100 populations, each starting with p = q = 0.5
T=0
N = 5,000
T=1
T=2
0
0.490
0.495
0.5
0.505
0.510
1.0
P
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Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
Drift when population is small
In contrast, small populations: N =2 parents
σ = √pq/2N =√(0.5)(0.5)/4 √0.0625 = σ = 0.25
• After one generation, p and q may vary in frequency:
0.5 ± 0.25 by chance.
• p or q may end up as low as 0.25 with the other allele as
high as 0.75 in frequency.
• After the second generation, they could shift more:
σ = √(0.25)(0.75)/4 = 0.22.
• Here, p ranges from 0.03 to 0.47 and q varies from 0.53
to 0.97.
Thus, in the extreme, p = 0.03, and q = 0.97 after two
generations of genetic drift. Another generation and p
could be lost and q = 1.0.
Imagine 100 populations, each starting with p=q=0.5
T=0
N=2
T=1
If p = 0, q = 1.0
If p = 1, q = 0
Allelic fixation
T=2
0.03
0.25
0.5
0.75
0.97
P
The outcome of random genetic drift
Random genetic drift may drastically alter gene
frequencies in small populations.
• Small populations may achieve allelic fixation: where
the value for p and q becomes 0 and 1.0.
• Loss of alleles causes loss of heterozygosity. In small
populations, this process is inevitable.
• The “rate of fixation” or probability of fixation is
considered 1/2N, which gives the proportion of
populations that eventually attain fixation.
• For the first example: 1/10,000 is very small; but for the
second ¼ is very large.
Genetic drift in small populations is strong enough to
counter the effects of natural selection, i.e., the more fit
allele may be lost by chance.
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Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
The effects of random genetic drift on the
distribution of equilibrium gene frequencies, when
selection is zero. Fig. 19-3
Laboratory study of random genetic
drift: Buri’s work on genetic drift in
Drosophila at the bw locus
• For each breeding line, Buri began with
p = bw = 0.5 and q = bw75 = 0.5
• He selected 16 parents (8 males, 8
females) at random each generation to
continue each line.
• Set up 107 different breeding lines.
• Continued all lines for 19 generations.
• By generation 19, the bw75 allele was
eliminated completely from 30 lines
(q = 0), and bw75 was fixed at 28 lines
(p = 1).
Fig. 19.4
The Shifting Balance Theory
Proposed by Sewall Wright to explain allelic variation at a
particular gene locus. Demes occupy an adaptive
landscape of peaks and valleys.
• Different genotypes occupy different adaptive peaks.
• Each peak has the potential to be the highest under the
right environmental conditions.
Movement of a deme from peak to peak is initiated by
genetic drift, which changes allele frequencies.
• Drift moves a deme down a peak and maybe into a valley
• Gene migration into the deme changes allele frequencies
further, and the deme crosses the valley.
• Selection drives the deme up the nearest peak.
• Environmental change alters the landscape, so peaks
change in height (new genotypes may be best adapted.)
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Gene flow and genetic drift
Evolution Biology 4971/5974
D F Tomback
Adaptive Landscape illustrating the Shifting Balance Theory
End Box 19.1
In this scenario there are four gene loci, each intended to be homozygous.
Having capital letter alleles at two loci are required for a deme to be on the top
of a peak.
Study questions
• How does mutation maintain genetic variation?
• How does gene flow (migration) change allele
frequencies? What are the important variables to know to
calculate the new frequency (qn).
• How does random genetic drift affect allele frequencies
for a gene over time? Are frequency changes always
unidirectional for an allele?
• Explain the implications of the Standard Deviation σ =
√pq/2N for allele frequencies p and q after one
generation. Over more time? How does population size
N affect drifting allele frequencies?
• Explain why genetic drift results in loss of heterozygosity?
What other genetic mechanism reduces heterozygosity?
Study questions, p. 2
• Fig. 19.3. Can you explain what is happening to a
population of N = 100,000, N = 10,000, N = 5,000, and N =
1,000 with respect to allele frequency q?
• Fig. 19.4. Explain the final distribution of alleles in the Buri
Drosophila experiment based on what you known about
genetic drift and its effects?
• How does the Shifting Balance Theory explain the
maintenance of different alleles for a gene?
• How does a deme shift from one peak to another? How
does an adaptive peak change height (fitness)?
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