The document discusses genetic divergence between populations, primarily using the statistic FST to measure genetic differences. It emphasizes the low genetic differentiation in humans, with only 12% of variation attributed to population differences, and illustrates various ecological and evolutionary factors, such as isolation-by-distance and the impact of selection versus gene flow on genetic variation. Additionally, it highlights examples from different species to demonstrate how environmental adaptation influences genetic divergence.

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Genetic Divergence
between Populations
P r e s e n t e d b y : I s k a k , M o h a i m i n S .

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Genetic Diveregence
Between Population
Genetic differences between populations can be described in
several ways. One of the most widely used approaches is
based on a statistic called FST, which measures the fraction of
the total genetic variance found across two or more populations
that results from genetic differences between them. A value of
FST = 0 means that the populations are genetically identical,
while a value of FST = 1 means that each population is fixed for
a different allele
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• FST is a statistic used to measure
genetic differences between two
or more populations. In this
schematic, two alleles at a locus
are represented by red and blue
circles. Top: FST = 0 when allele
frequencies are equal in the
populations. Middle: FST = 0.36
when allele frequencies are 0.2
and 0.8 in the two populations.
Bottom: FST = 1 when the
populations are fixed for different
alleles.

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• FST is often used to measure genetic
differences among human populations. The
International HapMap Project analyzed
differentiation in single nucleotide
polymorphisms (SNPs) across the genomes
of East Asians, Europeans, and Yoruba from
Nigeria. Across all the autosomes, FST is
0.12. This tells us that only 12 percent of all
the genetic variation in these populations is
caused by differences among them. A full 88
percent of all the variation in our species can
be found within a typical population. The
striking phenotypic differences we see
among human populations are therefore not
representative of the genome as a whole.
For a locus with two alleles, it is
calculated as where Var(p) is the
variance of the allele frequency among
populations, and p – is the mean allele
frequency across all the populations.

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• Isolation-by-distance in human
populations. The horizontal axis has
been corrected for large bodies of water
that could not be crossed when humans
first spread across Earth. The vertical
axis gives FST estimated from 783 loci.
Each dot represents a comparison
between a pair of populations from the
indicated region(s).
ISOLATION-BY-DISTANCE
In humans, it reflects the history of how we colonized different parts of Earth. A similar pattern can
also result from a balance between gene flow and random genetic drift. In other species, the
picture of genetic differentiation is quite different. Populations of the northern dusky salamander
(Desmognathus fuscus) living near New York City have FST values more than four times larger
than those in human populations spread across the entire planet [23]. The contrast between the
relatively low values of FST in humans and the high values in the salamander underlines the point
that simple measures of genetic similarity cannot be used to determine which populations do and
do not belong to the same species.

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• Divergence measured by FST between three
populations of three-spined sticklebacks
(Gasterosteus aculeatus) that independently
colonized freshwater and the marine
populations from which they evolved. One of
the strongest regions of differentiation in all
three comparisons is in the region on
chromosome 4 that carries the Eda locus.
This gene controls a major polymorphism in
bony plates along the side of the fish, which
differs between freshwater (low-plated) and
marine (completely plated) populations.
Marine populations of the three-spined stickleback (Gasterosteus aculeatus)
have invaded thousands of freshwater streams around the Northern Hemisphere.
Adapting to a freshwater environment involves many genetic changes. These
changes cause high FST between freshwater and marine populations to develop
in regions of the genome with loci that are locally adapted to those very different
environments

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Differences between populations can be caused by phenotypic plasticity as well as by genetic
divergence. A famous study by Clausen, Keck, and Hiesey concerns yarrow (Achillea millefolium), a
plant that grows from sea level up to tree line in the mountains of California. In their natural habitat,
plants at high elevations are much shorter than those at low elevations. This is likely adaptive
because tall plants attract more pollinators and have greater fecundity at low elevations, while short
plants are able to flower before winter arrives at high elevations. These results show that both
genetic variation and plasticity contribute to the differences seen among populations growing at
different elevations. In the yarrow, both genes and plasticity contribute to local adaptation.

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Gene Flow and Selection
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• When selection favors different alleles or phenotypes in different
places, a tension develops between local selection, which
enhances the genetic differences between populations, and gene
flow, which erodes them. Without gene flow, selection would
cause whatever alleles have highest fitness at any place to
become fixed there. Without selection (or genetic drift), gene flow
would make allele frequencies equal everywhere. The clines and
other spatial patterns seen in nature are compromises between
those extremes. The tug-of-war between gene flow and selection
plays out in the grass growing on the mine in Wales that was
discussed earlier.

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Rock Pocket Mouse
(Chaetodipus Intermedeus)
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• The rock pocket mouse
(Chaetodipus intermedius) lives
in the desert southwest of the
United States where the
landscape is a patchwork of dark
fields of lava (much like islands)
surrounded by light-colored
granite and sand. While most
populations of this mouse are
light colored, a dark form is
common on the lava, where it is
camouflaged from the owls that
prey on it.

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Gene Flow and Selection
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• The evolutionary tension between selection and gene flow plays out in
continuous habitats as well as on islands. Consider a grass growing in a
prairie that has two kinds of soil that meet at a sharp boundary. The soil
to the west of the boundary favors allele A1, while the soil to the east
favors allele A2. To be specific, say that allele A1 has a relative fitness
advantage of s in the west, while A2 has the same advantage in the east,
and there is no dominance. What happens? Without gene flow, A1 will
become fixed everywhere to the west of the boundary, and A2 will be
fixed everywhere to the east. But with gene flow, alleles move across the
boundary.

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The cline can be short or long, depending on the relative
strengths of gene flow and selection.
This is the width of region over which allele A2 increases from a low
frequency (p = 0.1) to a high frequency (p = 0.9).

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• Clines in allele frequencies predicted by a mathematical model. The horizontal axis
is distance along a transect. The vertical axis is the frequency of allele A2, which
has a relative fitness of 1 – s to the left (west) of x = 0, and fitness 1 + s to the right
(east). The four curves show the clines for different values of the ratio of the
migration variance to the selection coefficient, σm 2 /s. The clines become flatter as
the strength of gene flow increases relative to selection. (B) The cline width, wc, is
the distance over which the allele frequency changes from 0.1 to 0.9.
• Clines also develop when the transition between two types of habitats is gradual. In
that case, the shape of the cline is typically similar to those in Figure 8.11, even
though the clines in that figure result from an abrupt change in selection. The
shape of a cline therefore does not tell us much about whether selection varies
abruptly or in a smooth gradient

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Tension Zone
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• Tension zones are clines that form when there is
selection against heterozygotes. The
grasshopper Podisma pedestris is polymorphic
for a chromosome fusion. Where populations
with and without the fusion meet in the foothills
of the Alps, clines in the frequency of the
inversion form because heterozygotes have
reduced fertility. The filled part of each pie
diagram represents the frequency of the fusion.
The cline in the frequency of the fusion is only a
few dozen meters wide.
• These clines result from strong selection
against fusion heterozygotes, which have low
fertility. Clines that result from selection against
heterozygotes are called tension zones. They
often occur in areas where two species
hybridize

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