Genes Within
Populations
Chapter 20
1
Genetic Variation and Evolution
• Evolution: change in a species through time
1. Species accumulate differences
2. Descendants differ from their ancestors
3. New species arise from existing ones
2
Mechanism of evolutionary change
Lamarck’s theory of evolution
Inheritance of acquired characteristics:
• Individuals passed on physical and
behavioral changes to their offspring
• Variation by experience…not genetic
3
Lamarck’s theory of how giraffes’ long necks
4
evolved
Darwin’s Theory of Evolution
•
•
Observed land & organisms
everywhere they went
Began to notice connections
between species
Charles Darwin
Served as naturalist on 5 year mapping
expedition(HMS BEAGLE) around
coastal South America.
Used many observations to develop his
ideas
Proposed that evolution occurs by
natural selection
6
Voyage of the Beagle
7
http://etext.library.adelaide.edu.au/d/darwin/charles/
Charles Darwin
evolution: modification of a species over
generations
-“descent with modification”
natural selection: individuals with superior
characteristics are more likely to survive
and reproduce than those without such
characteristics
9
Tortoises on different
islands…
“normal” tortoise
Saddleback allows tortoise to reach
higher leaves on drier islands
Darwin’s Evidence
Similarity of related species
- Darwin noticed variations in related
species living in different locations
11
Most famous for his observations of Galapagos
finches
• Some islands much drier than others
• Different islands had their own, slightly
different varieties of animals
• Darwin hypothesized that new species
could gradually appear, much like animal
breeders can artificially develop new
varieties through selective breeding
Explaining his observations: natural selection
1. Variation: individuals in a population differ
from one another
2. Heritability: variations are inherited from
parents
3. Overproduction: organisms produce more
offspring than can survive (survival of the
fittest)
4. Reproductive advantage: some variations
allow the organism that possesses them to
have more offspring – those variations
become more common, and the population
changes over time
Darwin’s
theory for
how long
necks
evolved in
giraffes
15
Post-Darwin Evolution Evidence
Fossil record
Mechanisms of heredity
Comparative anatomy
Molecular evidence
16
Why are albinos rare?
Why does a peacock have a large tail?
Hardy-Weinberg Principle
-Genetic equilibrium: Allele (and genotype)
frequencies in a population will remain
constant from generation to generation
-if equilibrium is upset  evolution
(punctuated equilibrium chp 22)
18
Hardy-Weinberg Principle
Requirements to maintain genetic equilibrium:
1. No mutation
2. No genes are transferred to or from other
sources
3. Random mating
4. Very large population
5. No selection
19
Hardy-Weinberg Principle
Calculate genotype frequencies
(p+q)2 = p2 + 2pq + q2
• p = frequency of the 1st allele
• q = frequency of the 2nd allele
• p2 = individuals homozygous for 1st allele
• 2pq = heterozygous individuals
• q2 = individuals homozygous for 2nd allele
• because there are only two alleles:
p plus q must always equal 1
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Hardy-Weinberg Principle
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Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
22
A population not in Hardy-Weinberg
equilibrium indicates an agent of
evolutionary change is operating in a
population
(one or more of the 5 conditions are
not being met)
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Agents of Evolutionary Change
1. Mutation: A change in a cell’s
DNA
– Mutation rates are generally
so low they have little effect
on Hardy-Weinberg
proportions of common
alleles.
– Ultimate source of genetic
variation
24
Agents of Evolutionary Change
2. Gene flow: A movement of
alleles from one population
to another
-Powerful agent of change
-Tends to homogenize allele
frequencies between
populations
25
Agents of Evolutionary Change
3. Nonrandom Mating: mating with specific
genotypes
– E.G. Sexual Selection – “Peacocks”
– Shifts genotype frequencies
26
3a. Assortative Mating:
•mates that are phenotypically similar
•does not change frequency of individual
alleles
•Disruptive selection: forms at both ends of the
range of variation are favored over
intermediate forms
27
3b. Disassortative Mating:
•phenotypically different individuals mate
•Stabilizing selection: intermediate
(heterozygous) forms are favored and
extremes are eliminated
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Genetic Drift
4. Small populations
Genetic drift: Random fluctuation in allele
frequencies over time by chance
• important in small populations
–founder effect – When a few individuals
start a population (small allelic pool)
»Amish
–bottleneck effect - drastic reduction in
population, and gene pool size
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Founder Effect
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Genetic Drift: A bottleneck effect
31
Selection
5. Natural selection: environmental
conditions determine which individuals in
a population produce the most offspring
• This is the only agent that produces
adaptive evolutionary change (selects
individuals that are more fit)
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Selection
33
http://www.pbs.org/wgbh/evolution/library/01/1/l_011_03.html
Evolution of the eye: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html
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Fitness and Its Measurement
• Fitness: A phenotype with greater
fitness usually increases in frequency
• Fitness is a combination of:
–Survival: how long does an
organism live
–Mating success: how often it mates
–Number of offspring per mating that
survive
35
Body size and egg-laying in water striders
36
• Oscillating selection: selection favors one
phenotype at one time, and a different
phenotype at another time
• Galápagos Islands ground finches
– Wet conditions favor big bills (abundant
seeds)
– Dry conditions favor small bills
37
Maintenance of Variation
• Heterozygotes may exhibit greater fitness
than homozygotes
• Heterozygote advantage: keep
deleterious alleles in a population
38
Maintenance of Variation
Example: Sickle cell anemia
•Homozygous recessive phenotype:
exhibit severe anemia
• Homozygous dominant phenotype:
no anemia; susceptible to malaria
• Heterozygous phenotype: no anemia;
less susceptible to malaria
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Maintenance of Variation
Frequency of sickle cell allele
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Maintenance of Variation
Disruptive selection acts to eliminate
intermediate types
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Maintenance of Variation
Disruptive selection for large and small
beaks in black-bellied seedcracker finch of
west Africa
42
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
43
44
Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
45
Maintenance of Variation
Stabilizing selection for birth weight in
humans
46