Transcript Document

Chapter 7: Migration,
genetic drift and nonrandom mating
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Migration: movement of alleles
between populations.
Migration can cause allele and
genotype frequencies to deviate from
Hardy-Weinberg equilibrium.
Migration
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Consider Continent-Island migration
model.
Migration from island to continent will
have no effect of continental allele
frequencies. Continental population
much larger than island.
However continent to island migration
can greatly alter allele frequencies.
Empirical example of
migration’s effects
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Lake Erie water snakes. Snakes range
in appearance from unbanded to
strongly banded.
Banding caused by single locus:
banded allele dominant over
unbanded.
http://animaldiversity.ummz.umich.edu/site/accounts/pictures/Nerodia_sipedon.html
Lake Erie water snakes
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Mainland: almost all snakes banded.
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Islands many snakes unbanded.
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Unbanded snakes have selective
advantage: better camouflage on
limestone rocks. Camouflage very
valuable when snake is young.
Fig 6.6
Lake Erie water snakes
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If selection favors unbanded snakes on
islands why aren’t all snakes
unbanded?
Migration introduces alleles for
banding.
Fig 6.7
A unbanded, B+C some banding, D strongly banded
Lake Erie water snakes
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Migration of snakes from mainland
makes island populations more like
mainland.
This is general effect of migration:
Homogenizes populations (making
them resemble each other).
Genetic Drift
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Genetic drift results from the influence
of chance. When population size is
small, chance events more likely to
have a strong effect.
Sampling errors are very likely when
small samples are taken from
populations.
Genetic Drift
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Assume gene pool where frequency A1
= 0.6, A2 = 0.4.
Produce 10 zygotes by drawing from
pool of alleles.
Repeat multiple times to generate
distribution of expected allele
frequencies in next generation.
Fig 6.11
Genetic Drift
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Allele frequencies much more likely to
change than stay the same.
If same experiment repeated but
number of zygotes increased to 250
the frequency of A1 settles close to
expected 0.6.
6.12c
Empirical examples of
sampling error: Founder
Effect
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Founder Effect: when population
founded by only a few individuals
allele frequencies likely to differ from
that of source population.
Only a subset of alleles likely to be
represented and rare alleles may be
over-represented.
Founder effect in
Silvereye populations.
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Silvereyes colonized South Island of
New Zealand from Tasmania in 1830.
Later spread to other islands.
http://www.derwenttraders.com.au/contents
/media/silvereye-460.jpg
6.13b
Founder effect in
Silvereyes
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Analysis of microsatellite DNA from
populations shows Founder effect on
populations.
Progressive decline in allele diversity
from one population to the next in
sequence of colonizations.
Fig 6.13 c
Founder effect in
Silvereyes
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Norfolk island Silvereye population has
only 60% of allelic diversity of
Tasmanian population.
Founder effect in human
populations
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Founder effect common in isolated
human populations.
E.g. Pingelapese people of Eastern
Caroline Islands are descendants of 20
survivors of a typhoon and famine that
occurred around 1775.
Pingelap Atoll
http://people.brandeis.edu/~msitzman/docs/pingelap_large.html
Founder effect in human
populations
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One survivor was heterozygous carrier
of a recessive loss of function allele of
CNGB3 gene.
That gene codes for protein in cone
cells of retina.
4 generations after typhoon
homozygotes for allele began to be
born.
Founder effect in human
populations
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People homozygous for the allele have
achromotopsia (complete color
blindness, extreme light sensitivity,
and poor visual acuity).
Achromotopsia is rare in most
populations (<1 in 20,000 people).
Among the 3,000 Pingelapese
islanders the frequency is 1 in 20.
Founder effect in human
populations
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High frequency of allele for
achromotopsia is not due to a selective
advantage, just a result of chance.
Founder effect followed by further
genetic drift resulted in current high
frequency.
Effects of genetic drift
over time
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Effects of genetic drift can be very
strong when compounded over many
generations.
Simulations of drift. Change in allele
frequencies over 100 generations.
Initial frequencies A1 = 0.6, A2 = 0.4.
Simulation run for different population
sizes.
6.15A
6.15B
6.15C
Conclusions from
simulations
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Populations follow unique paths
Genetic drift has strongest effects on small
populations.
Given enough time, even in large
populations genetic drift can have an effect.
Genetic drift leads to fixation or loss of
alleles, which increases homozygosity and
reduces heterozygosity.
6.15D
6.15E
6.15F
Conclusions from
simulations
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Genetic drift produces steady decline
in heterozygosity.
Frequency of heterozygotes is highest
at intermediate allele frequencies. As
one allele drifts to fixation the number
of heterozygotes inevitably declines.
Empirical studies on
fixation
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Buri (1956) established 107 Drosophila
populations.
All founders were heterozygotes for an
eye-color gene called brown. Neither
allele gives selective advantage.
Initial genotype bw75/bw
Initial frequency of bw75 = 0.5
Buri (1956) study
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Followed populations for 19
generations.
Population size kept at 16 individuals.
What do we predict will occur in terms
of allele fixation and heterozygosity?
Buri (1956) study
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In each population expect one of the
two alleles to drift to fixation.
Expect heterozygosity to decline in
populations as allele fixation
approaches.
Buri (1956) study
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Distribution of frequencies of bw75
allele became increasingly U-shaped
over time.
By end of experiment, bw75 allele fixed
in 28 populations and lost from 30.
Fig 6.16
Buri (1956) study
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Frequency of heterozygotes declined
steadily over course of experiment.
Declined faster than expected because
effective population size was smaller than
initial population size of 16 (effective refers
to number of actual breeders; some flies
died, some did not get to mate).
Fig 6.17
Allele fixation in natural
populations
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Templeton et al. (1990) Studied
Collared Lizards in Ozarks of Missouri
Desert species occurs on remnant
pieces of desert-like habitat called
glades.
Templeton et al. (1990)
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Human fire suppression has resulted in
loss of glade habitat and loss of
crossable savannah habitat between
glades. Areas between glades
overgrown with trees.
Templeton et al. (1990)
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Based on small population sizes and
isolation of collared lizard populations
Templeton et al. (1990) predicted strong
effect of genetic drift on population
genetics.
Expected low genetic diversity within
populations, but high diversity between
populations.
Templeton et al. (1990)
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Found expected pattern. Genotype fixation
common within populations and different
genotypes were fixed in different
populations.
Lack of genetic diversity leaves populations
vulnerable to extinction.
Found >66% of glades contained no lizards.
Templeton et al. (1990)
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What conservation measures could be
taken to assist Collared Lizard
populations?
Templeton et al. (1990)
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Repopulate glades by introducing
lizards.
Burn oak-hickory forest between
glades to allow migration between
glades.
Non-Random mating
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The last of the five Hardy-Weinberg
assumptions is that random mating
takes place.
The most common form of nonrandom mating is inbreeding which
occurs when close relatives mate with
each other.
Inbreeding
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Most extreme form of inbreeding is self
fertilization.
In a population of self fertilizing organisms
all homozygotes will produce only
homozygous offspring. Heterozygotes will
produce offspring 50% of which will be
homozygous and 50% heterozygous.
How will this affect the frequency of
heterozygotes each generation?
Inbreeding
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In each generation the proportion of
heterozygous individuals in the
population will decline.
Inbreeding in California
Sea Otters
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Because inbreeding produces an
excess of homozygotes in a population
deviations from Hardy-Weinberg
expectations can be used to detect
such inbreeding in wild populations.
Inbreeding in California
Sea Otters
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Sea otters, once abundant along the
west coast of the U.S., were almost
wiped out by fur hunters in the 18th
and 19th centuries.
photo: www.turtletrack.org
Inbreeding in California
Sea Otters
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California population reached a low of
50 individuals (now over 1,500). As a
result of this bottleneck, the
population has less genetic diversity
than it once had.
Inbreeding in California
Sea Otters
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Population is still at a low density and
Lidicker and McCollum (1997)
investigated whether this resulted in
inbreeding.
They determined genotypes of 33
otters for PAP locus, which has two
alleles S (slow) and F (fast)
Inbreeding in California
Sea Otters
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The genotypes of the 33 otters were:
– SS 16
– SF 7
– FF 10
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This gives approximate allele
frequencies of S= 0.6 and F = 0.4
Inbreeding in California
Sea Otters
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If otter population in H-W equilibrium,
genotype frequencies should be
– SS = 0.6* 0.6 = 0.36
– SF =2*0.6*0.4 = 0.48
– FF = 0.4*0.4 = 0.16
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However actual frequencies were:
– SS= 0.485, SF= 0.212, FF =0.303
Inbreeding in California
Sea Otters
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There are more homozygotes and fewer
heterozygotes than expected for a random
mating population.
Having considered alternative explanations
for deficit of heterozygotes, Lidicker and
McCollum (1997) concluded that sea otter
populations show evidence of inbreedng.
General analysis of
inbreeding
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Self-fertilization and sibling mating are
the most extreme forms of inbreeding,
but matings between more distant
relatives (e.g. cousins) has the same
effect on the frequency of
homozygotes, but rate is slower.
General analysis of
inbreeding
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F = Coefficient of inbreeding:
probability that two alleles in an
individual are identical by descent
(this means both alleles are copies of
a particular ancestor’s allele in some
previous generation).
F increases as relatedness increases.
General analysis of
inbreeding
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If we compare heterozygosity of an inbred
population Hf with that of a random mating
population Ho the relationship is
Hf =Ho (1-F)
or expressed in H-W terms the expected frequency
of heterozygotes in an inbred population would be
Hf = 2pq (1-F)
Anytime F>0 frequency of heterozygotes is reduced
and frequency of homozygotes naturally increases.
General analysis of
inbreeding
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Calculating F. Need to use pedigree
diagrams.
Example: Female is daughter of two halfsiblings.
There are two ways the female could
receive alleles that are identical by descent.
Calculating probability that two alleles in an inbred individual are identical by descent
Male
Female
Male
Half-sibling mating
Female
Male
Fig 6.27a
Fig 6.27b
General analysis of
inbreeding
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Total probability of scenario is 1/16 +
1/16 = 1/8.
Inbreeding depression
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Inbreeding increases the frequency of
homozygotes and thus the probability that
deleterious alleles are visible to selection
because an individual will receive two copies
of the deleterious allele.
In humans, children of first cousins have
higher mortality rates than children of
unrelated individuals.
Fig 6.28
Each dot on graph
represents mortality
rates for a human
population.
Mortality rate for
children of cousins
consistently about 4%
higher than rate for
children of
non-relatives.
Inbreeding in humans
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Royal families have been particularly
prone to inbreeding.
In Ancient Egypt because royal women
were considered to carry the royal
bloodline the pharaoh routinely was
married to a sister or half-sister.
Inbreeding in humans
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The most famous example of a genetic
disorder exacerbated by inbreeding is
the Hapsburg jaw or Hapsburg lip
[severe lower jaw protrusion] .
(Hapsburgs were the ruling family of
Austria and Spain for much of the
1400’s-1700’s)
Inbreeding in humans
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Extensive intermarriage of close Hapsburg relatives
occurred.
The last of the Spanish Hapsburgs, Charles II
(1661-1700) had such severe jaw protrusion he
could not chew his food properly.
Charles II also had a large number of other
recessively inherited genetic problems that caused
physical, mental, sexual and other problems.
Charles was infertile and the last of the Spanish
Hapsburg kings.
http://en.wikipedia.org/wiki/Charles_II_of_Spain
Inbreeding depression
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Inbreeding depression (reduction in fitness
caused by inbreeding) also documented in
studies of wild animals.
E.g. Great Tit. Two studies show that
survival of inbred nestlings is lower than
that of outbred individuals and that hatching
success of inbred eggs is lower than that of
outbred eggs.
Fig. 6.30
Inbreeding depression in
plants
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Inbreeding depression best studied in
plants.
Can experimentally produce inbred
and outbred plants easily.
Inbreeding depression in
plants
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Patterns to emerge from studies:
– Inbreeding effects are clearest when plants are
stressed (competition, under pest attack, grown
outdoors).
– Inbreeding effects most often show up later in
life cycle. (Appears maternal effects i.e.
contributions from the mother to the offspring
[e.g. provisioning of seed] mask effect initially).
– Inbreeding depression varies among family
lineages.
Fig 6.29
Open bars first year data. Filled
bars second year data.
Coefficient of inbreeding
depression is measure of how
much inbreeding reduces
values for various parameters.
Waterleaf (a biennial plant)
Inbreeding avoidance
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Many mechanisms to avoid inbreeding
have evolved. Include:
– Dispersal.
– Genetically controlled self-incompatibility.
– Mate choice.
Small populations and
inbreeding
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In small populations inbreeding may be
unavoidable.
Even with random mating, a small
population that stays small and receives no
immigrants will become inbred.
Major problem for rare species such as
California sea otters.
Population genetics and
conservation of Prairie
Chickens
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Two hundred years ago Illinois
covered with prairie and home to
millions of Greater Prairie Chickens.
Steel plough allowed farmers to farm
the prairie. Acreage of prairie
plummeted and so did Prairie Chicken
numbers.
Lesser Prairie Chicken
Conservation of Prairie
Chickens
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In 1960’s habitat protection measures
introduced and population increased
until mid 1970’s.
Then population collapsed. By 1994
<50 birds in two populations in Illinois.
Fig 6.3
Conservation of Prairie
Chickens
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Why did prairie chicken populations
decline even though available habitat
was increasing?
Prairie destruction reduced numbers of
birds and isolated the populations
from each other.
Conservation of Prairie
Chickens
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No migration between populations.
Small populations vulnerable to genetic drift
and inbreeding depression.
Accumulation of deleterious recessive alleles
(genetic load) can lead to extinction of small
populations.
Conservation of Prairie
Chickens
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Problem exacerbated when exposure of
deleterious mutations further reduces
population size and increases effectiveness of
drift. “Extinction vortex”.
Prairie chickens showed clear evidence of
inbreeding depression. Egg hatching rates
had declined dramatically by 1990 < 40%
hatch rate.
FIG 6.31
Conservation of Prairie
Chickens
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Illinois Prairie chicken populations showed
less genetic diversity than other populations
and less genetic diversity than they had in
the past.
Illinois birds 3.67 alleles per locus rather
than 5.33-5.83 alleles of other populations
and 5.12 of Illinois museum specimens.
Conservation of Prairie
Chickens
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Conservation strategy?
Conservation of Prairie
Chickens
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In 1992 prairie chickens introduced
from other populations to increase
genetic diversity.
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Hatching rates increased to >90%.
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Population increased.