Transcript Chapter 16 - Central Magnet School

Chapter 16
Population Genetics and Speciation
Mrs. Stewart
Honors Biology
Central Magnet School
Bell Work

List the evidence (at least 3) that supports the
theory of evolution.
Standard / Objective

CLE 3210.5.3 Explain how genetic
variation in a population and changing
environmental conditions are associated
with adaptation and the emergence of
new species.
Variation of Traits Within a
Population

Variations in the genotypes of a population arise
by:
 mutation – changes in genes that occur either
naturally or influenced by environment
 Passed to offspring if occurs in gametes
 Recombination – reshuffling of alleles
(chromosomes) and crossing over during
meiosis
 random pairing of gametes – organisms
produce large numbers of gametes, so the union
of a particular pair is strictly by chance.
The Gene Pool

The total genetic information available in a
population is called the gene pool.
Allele Frequency
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
Allele frequency is
the number of times
an allele occurs in
the gene pool
This is in
comparison to how
often the other
alleles occur too
Relative Allele Frequencies

determined by dividing the total number of a
certain allele by the total number of alleles of
all types in the population

Expressed as a percentage or a decimal.
Example: I do
B = Black
b = brown
What are the allele
frequencies?
B = 20
b = 30
Total = 50
B = 20/50 = .40 or 40%
b = 30/50 = .60 or 60%
Example: We do
B = black
b = white
What is the allele
frequency of B?
.60
12
How many B? _________
How many b? _________
8
Total # of alleles for fur
20
color? _________________
What is the allele
frequency of b?
.40
Example: You do

Half of the population of four o’clocks has red
flowers, and half has white flowers. What is
the frequency of “r” allele?
Predicting Phenotype

Phenotype frequency is equal to the number
of individuals with a particular phenotype
divided by the total number of individuals in
the population.
Phenotype Frequency

Evolution is any change in the
relative frequency of alleles in a
population.

Populations, not individual
organisms, can evolve over time.
Hardy Weinberg Genetic
Equilibrium



Due to sexual reproduction, phenotypic
frequencies may change over time.
Does that mean the allele frequencies change
too?
Unless acted upon by an outside force (perhaps
a changing environment), the answer is no.
The Hardy-Weinberg Genetic
Equilibrium

Allele frequencies in the gene pool do not
change unless acted upon by certain forces.

Hardy-Weinberg genetic equilibrium is a
theoretical model of a population in which no
evolution occurs and the gene pool of the
population is stable.
What factors affect the allele
frequencies in a gene pool?
Factors to keep H.W. equilibrium:
1. Lack of mutations
2. No immigration or emigration
3. Ideally large population size
4. Individuals mate randomly
5. Selection does not occur
Calculating using the Hardy
Weinberg equation
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Dominant allele frequency = p
Recessive allele frequency = q
p+q=1
p2 +2pq+ q2 = 1
Exit Ticket



M.socrative.com
Room: stewart348
Final question: How does immigration or
emigration affect allele frequencies in a gene
pool?
Darwin’s
Finches
http://people.rit.edu/rhrsbi/GalapagosPages/Pictures/LandBirds/FinchType
http://www.biology-online.org/images/darwin_finches.jpg
Five conditions under which
evolution may take place

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Genetic mutations
Gene flow
Genetic drift
Nonrandom mating
Natural selection.
Mutation

Mutations are changes in the DNA.
Gene Flow

Emigration and immigration cause
gene flow between populations and
can thus affect gene frequencies.
Genetic Drift

Genetic drift is a change in allele
frequencies due to random events.

Genetic drift operates most strongly
in small populations.
Nonrandom Mating

Mating is nonrandom whenever
individuals may choose partners.
Sexual selection occurs when certain traits
increase an individual’s success at mating.
 Sexual selection explains the development
of traits that improve reproductive success
but that may harm the individual.
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Natural Selection

Three general patterns
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Stabilizing Selection
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Disruptive Selection
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favors the formation of average traits.
favors extreme traits rather than average traits.
Directional Selection
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favors the formation of more-extreme traits.
The Concept of Species

Biological species concept

a species is a population of organisms
that can successfully interbreed but
cannot breed with other groups
Isolation and Speciation

Geographic Isolation
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
Allopatric Speciation
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Speciation due to separation of subgroups of a population
Reproductive Isolation
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Results from the separation of population subgroups by
geographic barriers.
Results from the separation of population subgroups by
barriers to successful breeding.
Sympatric Speciation

Reproductive isolation within the same geographic area
Allopatry vs Sympatry
http://deltabiology.com/wp-content/uploads/2012/02/Sympatry.jpg
http://scienceblogs.com/evolvingthoughts/allopatry.jpg
Reproductive isolation:
Monkeyflower
http://faculty.washington.edu/toby/images/mim29%20Nature.jpg
Reproductive isolation
http://evolution.berkeley.edu/evolibrary/images/evo/drosophila_scene7.g
Rates of Speciation
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Gradualism


species undergo small changes at a constant rate.
Punctuated equilibrium

new species arise abruptly, differ greatly from their
ancestors, and then change little over long periods.