Transcript Population Genetics

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Genes Within
Populations
Chapter 16
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1. The Gene Pool
•a. Members of a
species can
interbreed & produce
fertile offspring
b. Species have a
shared gene pool
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The Gene Pool
•2. Different
species do NOT
exchange genes by
interbreeding
Example: Different
species that can
interbreed =produce sterile or
less viable offspring
e.g. Mule
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Populations
•A group of the
same species living
in an area
No two individuals
are exactly alike
(variations)
More Fit
individuals survive &
pass on their traits
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Speciation
Definition:
Formation of new
species
a. One species may
split into 2 or more
species
b. species may
evolve into a new
species
Requires very long
periods of time
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Five Agents of Evolutionary Change
Selection pressures:
avoiding predators
matching climatic condition
pesticide resistance
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Modern
Evolutionary
Thought
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Modern Synthesis Theory
• Combines Darwinian
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selection and
Mendelian inheritance
Population genetics study of genetic
variation within a
population
Emphasis on
quantitative
characters
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Modern Synthesis Theory
• Today’s theory on evolution
• Recognizes that GENES are responsible
for the inheritance of characteristics
• Recognizes that POPULATIONS, not
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individuals, evolve due to natural
selection & genetic drift
Recognizes that SPECIATION usually is
due to the gradual accumulation of small
genetic changes
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Describing genetic structure
• genotype frequencies
• allele frequencies
rr = white
Rr = pink
RR = red
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Genetic variation in space and
time
Why is genetic variation important?
• adaptation to environmental change
- conservation
•divergence of populations
- biodiversity
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variation
no variation
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Why is genetic variation
important?
variation
global
warming
survival
EXTINCTION!!
no variation
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Why is genetic variation
important?
divergence
variation
no variation
NO DIVERGENCE!!
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Microevolution
• Changes occur in gene pools due to
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a.mutation, b.natural selection,
c.genetic drift.
Gene pool changes cause more
VARIATION in INDIVIDUALS in the
population
This process is called
MICROEVOLUTION
Example: Bacteria becoming unaffected
by antibiotics (resistant)
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Species & Populations
• Population - a localized group of
individuals of the same species.
• Species - a group of populations whose
individuals have the ability to breed and
produce fertile offspring.
Gene Pool is defined by TOTAL GENES
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Gene Pools
•A population’s gene pool composed
of the total # all genes in the
population at any time.
If all members of a population are
#11. )
homozygous for a particular allele,
then the allele is fixed in the gene
pool.
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The Hardy-Weinberg Theorem
•Used to describe a non-evolving
population.
•Shuffling of alleles by meiosis and
random fertilization have no
effect on the overall gene pool.
Natural populations are NOT
expected to actually be in HardyWeinberg equilibrium.
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Assumptions of the H-W Theorem
a. Large population size
- small populations have fluctuations in allele
frequencies (e.g., fire, storm).
b. No migration
- immigrants can change the frequency of an
allele by bringing in new alleles.
c. No net mutations
- if alleles change from one to another, this
will change the frequency of those alleles.
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Assumptions of the H-W Theorem
d. Random mating
- if certain traits are more desirable,
then individuals with those traits will be
selected
e. No natural selection
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Hardy-Weinberg Equilibrium
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
where p2 = frequency of AA genotype;
2pq = frequency of Aa
q2 = frequency of aa genotype
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Hardy-Weinberg Equilibrium
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Evolution within a species or a
population is microevolution.
Microevolution refers to
changes in allele frequencies
in a gene pool and represents
a change in a population.
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How does genetic structure change?
Causes of Microevolution
• a) Genetic Drift
• b) Gene flow
• c) Natural Selection
#18. Changes
in allele
frequencies
• d) Mutations
• non-random mating
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1) Genetic drift
Genetic drift = the alteration of the gene pool of a small
population due to chance.
Two factors may cause genetic drift:
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19 Bottleneck effect leads reduces
genetic variability following a large
disturbance such as an earthquake
that removes a large portion of
the population. The surviving
population often does not
represent the Original
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20) Founder effect may lead
to reduced variability
when a few individuals
from a large population
colonize an isolated
habitat.
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Genetic drift
Before:
8 RR
0.50 R
8 rr
0.50 r
After:
2 RR
6 rr
0.25 R
0.75 r
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Genetic Drift - Bottleneck Effect
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Natural selection
Resistance to antibacterial soap
Generation 1:
1.00 not resista
0.00 resistant
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Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resista
0.00 resistant
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Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resistant
0.00 resistant
Generation 2:
0.96 not resistant
0.04 resistant
mutation!
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Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resist
0.00 resistant
Generation 2:
0.96 not resist
0.04 resistant
Generation 3:
0.76 not resist
0.24 resistant
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Natural
selection
Resistance to antibacterial soap
Generation 1:
1.00 not resist
0.00 resistant
Generation 2:
0.96 not resist
0.04 resistant
Generation 3:
0.76 not resist
0.24 resistant
Generation 4:
0.12 not resist
0.88 resistant
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*Yes, I realize that this is not really a cheetah.
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2) Natural selection
As previously stated, differential success in reproduction
based on heritable traits results in selected alleles being
passed to relatively more offspring (Darwinian
inheritance).
The only agent that results in adaptation to environment.
3) Gene flow
-is genetic exchange due to the migration of fertile
individuals or gametes between populations.
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4) Mutation
Mutation is a change in an organism’s DNA and is
represented by changing alleles.
a. Mutations can be transmitted in gametes to offspring,
and immediately affect the composition of the gene pool.
b. The original source of variation and the driving force
for Natural Selection
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Genetic Variation, the Substrate for Natural Selection
Genetic (heritable) variation within and between
populations: exists both as what we can see (e.g., eye
color) and what we cannot see (e.g., blood type).
Not all variation is heritable.
Environment also can alter an individual’s phenotype.
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Industrial
Melanism
of
Butterfly
Population
Industrial
Melanism
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Variation between populations
Geographic variations are differences between gene pools
due to differences in environmental factors.
It often occurs when populations are located in different
areas, but may also occur in populations with isolated
individuals.
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Mutation and sexual recombination generate genetic
variation
a. New alleles originate only by mutations (heritable only in
gametes; many kinds of mutations; mutations in functional
gene products most important).
- Mutations are more beneficial (rare) in changing
environments. (Example: HIV resistance to antiviral drugs.)
b. Sexual recombination is the source of most genetic
differences between individuals in a population.
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Diploidy and balanced polymorphism preserve variation
a. Diploidy often hides genetic variation from selection in
the form of recessive alleles.
Dominant alleles “hide” recessive alleles in heterozygotes.
b. Balanced polymorphism is the ability of natural
selection to maintain stable frequencies of at least two
phenotypes.
Heterozygote advantage is one example of a balanced
polymorphism, where the heterozygote has greater
survival and reproductive success than either homozygote
(Example: Sickle cell anemia where heterozygotes are
resistant to malaria).
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Diversifying selection
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Sexual selection leads to differences between sexes
a. Sexual dimorphism is the difference in appearance
between males and females of a species.
-Intrasexual selection is the direct competition between
members of the same sex for mates of the opposite sex.
-This gives rise to males most often having secondary
sexual equipment such as antlers that are used in
competing for females.
-In intersexual selection (mate choice), one sex is choosy
when selecting a mate of the opposite sex.
-This gives rise to often amazingly sophisticated
secondary sexual characteristics; e.g., peacock feathers.
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Sickle Cell and Malaria
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Evolutionary Change in Spot Number
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Population genetics – Outline
 What is population genetics?
 Calculate
- genotype frequencies
- allele frequencies
Why is genetic variation important?
How does genetic structure change?
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Example use of H-W theorem
1000-head sheep flock. No selection for color. Closed
to outside breeding.
910 white (B_)
90 black (bb)
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Start with known: f(black) = f(bb) = .09 =q2
q  q  .09  .3  f (b)
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Then, p = 1 – q = .7 = f(B)
f(BB) = p2 = .49
f(Bb) = 2pq = .42
f(bb) = q2 = .09
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In summary:
Allele freq.
f(B) = p = .7 (est.)
f(b) = q = .3 (est.)
Genotypic freq.
f(BB) = p2 = .49 (est.)
f(Bb) = 2pq = .42 (est.)
f(bb) = q2 = .09 (actual)
Phenotypic freq.
f(white) = .91 (actual)
f(black) = .09 (actual)
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Mink example using H-W
Group of 2000homo (1920 brown, 80 platinum) in
equilibrium. We know f(bb) = 80/2000 = .04 =
q2
f(b) = (q2) = .04 = .2
f(B) = p = 1- q = .8
f(BB) = p2 = .64
f(Bb) = 2pq = .32
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