Download File - Honors Biology 16-17

Population Genetics
Chapter 16
Populations and Gene Pools
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Evolution is the change of genetic
composition of populations over time.
Microevolution is change within species
which can occur over dozens of
generations
examples: antibiotic resistant bacteria,
mosquitoes evolving resistance to DDT,
HIV strains evolving resistance to antiviral
medicines
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Macroevolution involves longer periods
of time and formations of new species
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Population Genetics is the field of
biology that studies microevolution
Macroevolution refers to evolution of groups
larger than an individual species.
Gene Pool
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A gene pool consists of all
the genes of a local
population of organisms
A gene pool only refers to
one population not an
entire species
Organisms in the same
geographical region make
up a local population and
the unit of evolution
Genetic Variation
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Differences between
individuals in a population
Mutations and crossingover are two sources of
genetic variation
Variation is the “Raw
material for evolution”
Why is genetic variation
important?
variation
global
warming
survival
EXTINCTION!!
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no variation
Why do populations change?
Hardy and Weinberg
The
Hardy-Weinberg Principal describes a
hypothetical situation where there is no change
in the gene pool…SO no evolution.
An
important way of discovering if real
populations are changing with time is to
construct a model of a population that does not
change to use as a comparison
Can
be used to calculate the genetic variation
of a population at equilibrium
.
https://www.youtube.com/watch?v=oG7ob-MtO8c
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Hardy-Weinberg Model
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Hardy & Weinberg developed this
mathematical model to study
populations
Used to describe a non-evolving
population under certain conditions
Natural populations are not expected
to be in Hardy-Weinberg equilibrium
Understanding the conditions
necessary for consistent allele
frequencies helps us understand why
populations change
Hardy-Weinberg Assumptions
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The Hardy-Weinberg
principle states that allele
and genotype frequencies
remain stable in a population
over generations if certain
conditions are met:
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Population size is very
large
No Mutations
Mating is random
No Migration
Natural Selection does
not operate
Hardy-Weinberg conditions
Condition 1 – Large Population size
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The larger the population
the less likely they will be
affected by chance
fluctuations in allele
frequencies (eg. Hurricane,
firestorm)
Hardy Weinberg Conditions
Condition 2 – Random mating
Individuals do not choose
mates based on appearance
or other traits
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Assortative mating reduces the
number of heterozygous
individuals (change in gene
pool)
Sexual selection would steer
the gene pool toward the
desired phenotypes and
genotypes. (change in gene
pool)
Hardy Weinberg Conditions
Condition 3 – No Mutations
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Changes in allelic
frequency due to mutation
are negligible
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Any mutation in a
particular gene would
change the balance of
alleles in the gene pool.
Hardy Weinberg Conditions
Condition 4 - No Migration
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No new alleles can come
into the population, and
no alleles can be lost.
Both immigration and
emigration can alter allelic
frequency.
Hardy Weinberg Conditions
Condition 5 – No Natural
Selection
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Natural Selection does not
operate
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No alleles are selected
over other alleles
If selection occurs, those
alleles that are selected
for will become more
common. (change in gene
pool)
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Hardy Weinberg Conditions:
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These conditions remove all things that can cause evolution
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Allele frequencies predicted for each generation with the
following equations:
p = occurrence of dominant allele, q = occurrence of recessive
allele
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If all conditions are present, the gene pool frequencies will remain
unchanged from generation to generation
All five conditions occurring is highly unlikely
1=p+q (allele frequencies)
1=p2+2pq+q2 (Genotype frequencies)
Deviation from the conditions, changes allele frequencies and
usually results in Evolution
Hardy Weinberg Equilibrium
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Allele frequencies of nonevolving populations are
stable over generations
The Equation for allele
frequencies:
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1.0 = p + q
p = occurrence of
dominant allele
q = occurrence of
recessive allele
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Values always recorded as
decimals
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Genotype frequencies of nonevolving populations are stable
over generations
The Equation for genotype
frequencies:
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1.0 = p2 + 2pq + q2
p2 = occurrence of
homozygous dominant
genotype (AA)
2pq = occurrence of
heterozygous genotype (Aa)
q2 = occurrence of
homozygous recessive
genotype (aa)
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Values always recorded as
decimals
Example use of H-W theorem
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1000-head sheep flock. No selection for color. Closed to outside
breeding.
910 white (BB or Bb)
90 black (bb)
Occurrence of Allele B = p, occurrence of Allele b = q
p2 = occurrence of BB, 2pq = occurrence of Bb, q2 = occurrence of
bb
Start with known: 90bb/1000 = .09=q2
q= √.09 = .3 (allele frequency)
p+q = 1 so p = 1 – q = .7 = p
p2+2pq+q2 to figure out genotypic frequencies:
Frequency of (BB) = p2 = (.7)2 = .49 or 49% BB
Frequency of (Bb) = 2pq = 2(.7)(.3) =.42 or 42% Bb
Frequency of (bb) = q2 = (.3)2 = .09 or 9% bb
Phenotypic frequency = .09/9% black & .91/91% white
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Hardy-Weinberg Equilibrium
Hardy-Weinberg Applied…
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The allele for black coat is recessive
to the allele for white coat. Can you
count the number of recessive alleles
in this population?
p = the frequency of the dominant
allele (represented here by A)
q = the frequency of the recessive
allele (represented here by a)
For a population in genetic
equilibrium:
p + q = 1.0 (The sum of the
frequencies of both alleles is 100%.)
Genotypic frequencies represented
by:
p2 + 2pq + q2 = 1
p2 = frequency of AA (homozygous
dominant)
2pq = frequency of Aa (heterozygous)
q2 = frequency of aa (homozygous
recessive)
How To Solve The Problems???
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Calculate q2
 Count the individuals that are homozygous recessive in
the illustration above. Calculate the percent of the total
population they represent. This is q2
 Answer:
Four of the sixteen individuals show the recessive
phenotype, so the correct answer is 25% or 0.25.
Find q.
 Take the square root of q2 to obtain q, the frequency of
the recessive allele.
 Answer:
q = 0.5
Find p.
 The sum of the frequencies of both alleles = 100%, p + q
= l.You know q, so what is p, the frequency of the
dominant allele?
 Answer:
p = 1 - q, so p = 0.5
Find 2pq.
 The frequency of the heterozygotes is represented by
2pq. This gives you the percent of the population that is
heterozygous for white coat:
 Answer:
2pq = 2(0.5) (0.5) = 0.5 , so 50% of the population is
heterozygous.
Hardy-Weinberg calculations
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Allele frequencies
p+q=1
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p=dominant ALLELE frequencies
q=recessive ALLELE frequencies
Genotype frequencies
p2+2pq+q2=1
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p2= Homozygous Dominant
GENOTYPE frequency
2pq=Heterozygous GENOTYPE
frequency
q2=Homozygous recessive
GENOTYPE frequency
Using HWE equation
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Assuming a population is at HWE means that you can
predict the allelic and genotypic frequencies for the next
generation
Let’s go back to the sheep
.7 = p and .3 =q
Frequency of (BB) = p2 = (.7)2 = .49 or 49% BB
Frequency of (Bb) = 2pq = 2(.7)(.3) =.42 or 42% Bb
Frequency of (bb) = q2 = (.3)2 = .09 or 9% bb
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If the next generation has statistically different p and q
values, then biologists know some form of evolution is
occurring in the population
HWE is used as a null hypothesis for comparison
Main Factors that Affect Gene Pools
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Natural Selection
Gene Flow
Mutations
Genetic Drift
Natural Selection English Peppered Moth
(Biston bitularia)
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In pre-industrial England;
birches had white bark (due
to white lichens)
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In industrial England; birch
bark was coated with soot
(lichens died)
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White moths predominated
over black moths
95% white, 5% black
Black moths predominated
over white moths
Percentages almost reversed
Natural Selection changed
the frequencies in the gene
pool.
SHOW MOVIE
Natural selection: Sickle Cell Anemia
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Sickle cell anemia is a disease in
which your body produces
abnormally shaped red blood
cells.
Hemoglobin is the protein in
red blood cell that carries
oxygen throughout the body
Most people have normal
hemoglobin Allele (A)
One amino acid difference
caused by a change in one
nucleotide in the DNA
produces and alternative
defective sickle allele (S)
https://www.youtube.com/watch?v=mWU22DGOO2c
Sickle Cell Advantage
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(SS) individuals have Sickle cells anemia
and serious health problems
Sickle Cell disease is most common in regions with high
incidence of Malaria
Sickled cells provide resistance to malaria!
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(SS)-have Sickle cell Anemia but are resistant to Malaria
(AA)- are healthy but vulnerable to Malaria
(AS)- Enough healthy blood cells to not have true symptoms
and enough sickled cells to be resistant to Malaria
Natural Selection favors Heterozygotes where there is
Malaria maintaining both alleles in the population
Malaria
Three Modes of Natural Selection:
Directional Selection occurs when selection favors one
extreme trait value over the other extreme.
Result= a change in the mean value of the trait under selection.
 Disruptive Selection occurs when selection favors the extreme
trait values over the intermediate trait values.
Result = the variance increases as the population is divided into
two distinct groups. Disruptive selection plays an important role
in speciation.
 Stabilizing Selection occurs when selection favors the
intermediate trait value over the extreme values.
Result= a decrease in the amount of genetic variation for the
trait under selection.
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When Populations change…
Gene Flow
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Gene Flow Refers to
migration of individuals
between populations.
Potentially introduces new
alleles into a population.
Mutations
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Mutations are changes in
genetic information
Usually result from a slight
error in DNA replication
Some are beneficial to the
species and favored by
natural selection causing
them to increase in
frequency over
generations.
Mutations in Bacteria
https://www.youtube.com/watch?v=znnp-Ivj2ek
https://www.youtube.com/watch?v=znnp-Ivj2ek
Genetic Drift
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Two factors cause Genetic
Drift
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Founder Effect
Bottleneck Effect
Random changes in allele
frequencies in small
populations
Substantial effect on small
populations but little
effect on large populations
Genetic Drift due to Founder Effect
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Genetic drift that follows
the colonization of a new
habitat.
Who settles the new
population has great effect
on the following genetic
diversity and allele
frequencies.
Genetic Drift due to Bottleneck Populations
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Drastic reduction in
populations for a few
generations.
Ex: American Bison
experienced a severe
population bottleneck in
the19th century due to
over hunting
Genetic drift >>>Inbreeding
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Genetic Drift can also lead to
loss of genetic variation.
Gradual increase in
homozygosity is called
inbreeding.
Inbreeding depression
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Small populations have
problems maintaining stable
numbers
Humans, plants, and animals
have recessive alleles that
would be lethal if homozygous
http://www.biotecharticles.com/G
enetics-Article/Lethal-Alleles-ItsInstances-in-Humans-Plants-andAnimals-2144.html
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Evolution
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One of the most important ideas in biology.
It’s an area of active research.
Has been well tested scientifically and is supported by a
large body of evidence.
Fossil Evidence-Paleontology
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Fossils offer physical
records of organisms not
found on Earth today.
Microfossils – species
recorded in fossil record
that are related to modern
species.
Cyanobacteria, Diatoms
-Bones and shells survive decay, minerals in rock
replace the original substance.
-Softer tissue can leave a fossil in soft mud that
hardens.
-Insects can get trapped in tree sap that becomes
amber.
https://www.youtube.com/watch?v=XRW-ATOUJus
Evidence in Ecology & Homologies
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Differences among closely related species reflect
adaptations to different environments.
Any organisms anatomical structure, its behavior, and the
structure of its DNA and proteins can be compared to
other species in search for homologies.
Homologous Structures (BONES in the FORELIMBS) shows
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Similarities in mammals.
Homologous bones from human (I), dog (II), pig
(III), cow (IV), tapir (V) and horse (VI):
Similarities in Vertebrate
Embryological development
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Genetic and Molecular Evidence
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Comparison of the amino acid sequences of homologous
proteins in different species.
Nucleotide sequences of homologous genes
Speciation
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The formation of new
species
Happens when two
populations become so
different in their genetic
makeup that they can no
longer interbreed.
Often a small population
that is isolated from the
rest of its species develops
into a new species.
Speciation
These happy face spiders look different, but
since they can interbreed, they are considered
the same species: Theridion grallator.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_42
Mechanisms of speciation
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When 2 populations become so different in their genetic
makeup that they can no longer interbreed.
Geographic isolation occurs when 2 groups are separated by
water, mountains, canyon
ex: squirrels
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Ecological Isolation- when 2 populations adapt to
different habitats
Behavioral Isolation- If the mating pattern of a group of
organisms becomes different from the main group,
they can become Reproductively isolated
Adaptive Radiation…rapid speciation
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When a pop enters an env. with few competing species it often
divides into several smaller populations.
Happens when populations avoid competing with each other
by adapting to different habitats/using different resources. The
adaptations keep populations separate leading to a new
species
http://speciationsimulator.com/?p=526
Artificial Selection
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Involves Breeders who select
only plants and animals with
desired traits for breading
This process allows breeders
to manipulate animal
populations over generations
Natural selection vs. artificial
selection:
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Nature provides the genetic
variation but…
In Natural Selection nature
selects which individuals
reproduce
In Artificial Selection the
breeder (people) selects which
individuals reproduce
Crash courses
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https://www.youtube.com/watch?v=WhFKPaRnTdQ –
pop genetics
https://www.youtube.com/watch?v=aTftyFboC_M- natural
selection
https://www.youtube.com/watch?v=P3GagfbA2vo –
evidence of evolution
https://www.youtube.com/watch?v=2oKlKmrbLoU speciation