Download Understanding Our Environment

Genes Within Populations
Chapter 15
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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Gene Variation is Raw Material
•
Evolution is change over time (through
several generations).
 Darwin
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Individuals with desirable characteristics
survive and pass on their traits to the next
generation
Population gradually includes more individuals
with advantageous characteristics.
Species accumulate differences so that
ancestral and descendent species are not
identical.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Gene Variation is Raw Material
•
Natural Selection and Evolutionary Change
 Some individuals in a population possess
certain inherited characteristics that play a
role in producing more surviving offspring
than individuals without those
characteristics.
- Population gradually includes more
individuals with advantageous
characteristics.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Gene Variation In Nature
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Polymorphism
 A gene with more than one allele is
polymorphic.
- Natural populations tend to have more
polymorphic genes than can be
accounted for by mutation.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Hardy-Weinberg Principle
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Population Genetics - Study of properties of
genes in populations.
 Blending Inheritance (Phenotypic
Inheritance) was widely accepted.
- New genetic variants would quickly be
diluted.
 Meiosis had not yet been discovered.
SCIENTISTS COULD NOT ANSWER:
 Why don’t recessive traits disappear after
a while?
Hardy-Weinberg Principle
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Hardy-Weinberg – Two scientists came up with an
explanation at the same time
 In 1908, G. H. Hardy and Wilhelm Weinberg
independently described a basic principle of
population genetics, which is now named the
Hardy-Weinberg equation.

Original proportions of genotypes in a
population will remain constant from generation
to generation.
Hardy-Weinberg Principle
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Five Assumptions
 Population is very large
 Mating is random
 No mutations
 No migration
 No selection occurring
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
A herd of horses has one dominant male. All young
males are forced to leave the herd. Which of the 5
assumptions is being violated?
A) Population is large
 B) Mating is random
 C) No mutations
 D) No migration
 E) No selection

A dandelion head full of seeds blows to a
secluded valley where there are no others of its
kind. The seeds begin a new population.
Which of the 5 assumptions is being violated?
 A) Population is large
 B) Mating is random
 C) No mutations
 D) No migration
 E) No selection
A small pride of lions moves into a territory
where the dominant male has died. This pride
takes in the females of the original pride.
Which of the 5 assumptions is being violated?
A) Population is large
 B) Mating is random
 C) No mutations
 D) No migration
 E) No selection

In a population of rabbits, there are white and
brown phenotypes. The white rabbits are
easily seen by predators in the landscape.
Which of the 5 assumptions is being violated?
A) Population is large
 B) Mating is random
 C) No mutations
 D) No migration
 E) No selection

Hardy-Weinberg Principle
•
Calculate Genotype
Frequencies
To find allele frequencies,let:
T=frequency of dominant allele
Frequency of homozygous
recessive individuals (tt) in
the population is:
t x t = t2
Frequency of recessive allele
(t) in the population is:
t=frequency of recessive allele
T + t = 1 (100% of alleles in
population)
t = t2
Frequency of dominant allele
(T) in the population is:
T=1-t
Hardy-Weinberg Equilibrium
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Five Agents of Evolutionary Change
1. Mutation
 Mutation rates are generally so low they
have little effect on Hardy-Weinberg
proportions of common alleles.
- Ultimate source of genetic variation.
2. Gene Flow
 Movement of alleles from one population to
another.
- Tend to homologize allele frequencies.
Five Agents of Evolutionary Change
3. Genetic Drift
 Frequencies of particular alleles may
change by chance alone.
- Important in small populations.
 Founder Effect - Few individuals found
new population (small allelic pool).
 Bottleneck Effect - Drastic reduction in
population, and gene pool size.
Five Agents of Evolutionary Change
4. Selection
 Artificial - Breeders exert selection.
 Natural - Nature exerts selection.
- Variation must exist among individuals.
- Variation must result in differences in
numbers of viable offspring produced.
- Variation must be genetically inherited.
 Natural Selection is a process, and
Evolution is an outcome.
Five Agents of Evolutionary Change
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Selection Pressures:
 Avoiding Predators
 Matching Climatic Condition
 Pesticide Resistance
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Evolutionary Forces Maintaining Polymorphism
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Adaptive Selection Theory
 Heterogeneous environments produce
many alleles.
Neutral Theory
 Balance between mutation and genetic
drift.
Gene Flow versus Natural Selection
 Genetic drift may decrease frequency of an
allele favored by selection.
Heterozygote Advantage
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Heterozygote Advantage will favor
heterozygotes, and maintain both alleles
instead of removing less successful alleles
from a population.
 Sickle Cell Anemia
- Homozygotes exhibit severe anemia,
have abnormal blood cells, and usually
die before reproductive age.
- Heterozygotes are less susceptible to
malaria.
Forms of Selection
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•
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Disruptive Selection
 Selection eliminates intermediate types.
Directional Selection
 Selection eliminates one extreme from a
phenotypic array.
Stabilizing Selection
 Selection acts to eliminate both extremes.
- Fitness - Number of surviving offspring
passed to the next generation.
Types of Natural Selection
In a population of lizards, there are small,
medium, and large sized individuals. A food
shortage occurs and all of the large sized
lizards die of starvation. Which type of
selection is occurring?
A.
B.
C.
Stabilizing selection
Directional selection
Disruptive selection
Limits to Selection
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Evolution Requires Genetic Variation
 Intense selection may remove variation
from a population at a rate greater than
mutation can replenish.
- Thoroughbred Horses are highly inbred
Limits to Selection
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Selection Against Rare Alleles
 As selection only operates on phenotypes,
rare recessives cannot be effectively
selected against except in homozygote
form.
- Hardy-Weinberg can still apply even
though mutations occur
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