Genes Within Populations
Chapter 20
1
Genetic Variation and Evolution
• Darwin: Evolution is descent with modification.
• Evolution: changes through time.
1. Species accumulate difference;
2. Descendants differ from their ancestors;
3. New species arise from existing ones.
2
Natural selection: mechanism of evolutionary
change
Natural selection: proposed by Darwin as the mechanism
of evolution.
• individuals have specific inherited characteristics;
• they produce more surviving offspring;
• the population includes more individuals with these specific
characteristics;
• the population evolves and is better adapted to its present
environment.
3
Darwin’s theory for
how long necks
evolved in giraffes
4
Natural selection: mechanism of evolutionary
change
Inheritance of acquired characteristics: Proposed by JeanBaptiste Lamarck.
• Individuals passed on physical and behavioral changes to
their offspring;
• Variation by experience…not genetic;
• Darwin’s natural selection: variation a result of preexisting
genetic differences.
5
Lamarck’s theory of how giraffes’ long necks evolved.
6
Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician.
Wilhelm Weinberg: German physician.
Concluded that:
The original proportions of the genotypes in a population will
remain constant from generation to generation as long as five
assumptions are met:
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Hardy-Weinberg Principle
Five assumptions :
1.
2.
3.
4.
5.
No mutation takes place
No genes are transferred to or from
sources
Random mating is occurring
The population size is very large
No selection occurs
other
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Hardy-Weinberg Principle
Calculate genotype frequencies with a binomial
expansion:
(p+q)2 = p2 + 2pq + q2
•
•
•
•
p = individuals homozygous for first allele;
2pq = individuals heterozygous for both alleles;
q = individuals homozygous for second allele;
because there are only two alleles:
p plus q must always equal 1 (that is the total population)
9
Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict frequencies in
subsequent generations.
10
Agents of Evolutionary Change
A population not in Hardy-Weinberg equilibrium indicates that
one or more of the five evolutionary agents are operating in a
population.
• 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.
11
Gene flow: A movement of
alleles from one population to
another.
Powerful agent of change;
Tends to homogenize allele
frequencies.
12
Agents of Evolutionary Change
• Nonrandom Mating: mating with specific genotypes:
– Shifts genotype frequencies
– Assortative Mating: does not change frequency of
individual alleles; increases the proportion of
homozygous individuals
– Disassortative Mating: phenotypically different
individuals mate; produce excess of heterozygotes
13
Genetic Drift
• Genetic drift: Random fluctuation in
allele frequencies over time by chance.
• important in small populations
– founder effect - few individuals
found new population (small
allelic pool)
– bottleneck effect - drastic
reduction in population, and gene
pool size
14
Genetic Drift: A bottleneck effect
15
Selection
• Artificial selection: a breeder selects for desired
characteristics.
16
Selection
• Natural selection: environmental conditions determine
which individuals in a population produce the most
offspring.
• 3 conditions for natural selection to occur:
– Variation must exist among individuals in a population;
– Variation among individuals must result in differences
in the number of offspring surviving;
– Variation must be genetically inherited.
17
Selection
Pocket mice from the Tularosa Basin
18
Selection to match climatic conditions
Selection for pesticide resistance
19
Fitness and Its Measurement
• 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
Body size and egg-laying in water striders.
20
Interactions Among Evolutionary Forces
• Mutation and genetic drift may counter selection.
• The magnitude of drift is inversely related to population
size.
21
Interactions Among Evolutionary Forces
• Gene flow may promote or constrain evolutionary change:
– Spread a beneficial mutation;
– Impede adaptation by continual flow of inferior alleles
from other populations.
• Extent to which gene flow can hinder the effects of natural
selection depends on the relative strengths of gene flow:
– High in birds & wind-pollinated plants;
– Low in sedentary species.
22
Maintenance of Variation
• Frequency-dependent selection: depends on how
frequently or infrequently a phenotype occurs in a
population.
– Negative frequency-dependent selection: rare
phenotypes are favored by selection.
– Positive frequency-dependent selection: common
phenotypes are favored; variation is eliminated from
the population.
• Strength of selection changes through time.
23
Maintenance of Variation
Negative frequency dependent selection
Positive frequencydependent selection.
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Maintenance of Variation
• 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;
Heterozygotes may exhibit greater fitness than
homozygotes.
25
Maintenance of Variation
• Homozygous
dominant phenotype:
no anemia; susceptible
to malaria.
• Heterozygous
phenotype: no
anemia; less
susceptible to malaria
Frequency of sickle cell allele.
26
Maintenance of Variation
Disruptive selection: acts to eliminate intermediate
types.
Disruptive selection for large and small beaks in blackbellied seedcracker finch of west Africa.
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Maintenance of Variation
Directional selection: acts to eliminate one extreme from
an array of phenotypes
Directional selection for negative phototropism in
Drosophila.
28
Maintenance of Variation
Stabilizing selection: acts to eliminate both extremes.
Stabilizing selection for birth weight in humans.
29
Experimental Studies of Natural Selection
• In some cases, evolutionary change can occur rapidly;
• Evolutionary studies can be devised to test evolutionary
hypotheses;
• Guppy studies (Poecilia reticulata) in the lab and field
– Populations above the waterfalls: low predation
– Populations below the waterfalls: high predation
30
Experimental Studies
• High predation environment - Males exhibit drab
coloration and tend to be relatively small and reproduce at
a younger age.
• Low predation environment - Males display bright
coloration, a larger number of spots, and tend to be more
successful at defending territories.
31
The Limits of Selection
• Genes have multiple effects:
– Pleiotropy: sets limits on how much a phenotype
can be altered.
• Evolution requires genetic variation
– Thoroughbred horse speed
– Compound eyes of insects: same genes affect
both eyes
– Control of ommatidia number in left and right eye
32
Experimental Studies
Selection for increased speed in racehorses is no longer
effective.
33
Experimental Studies
Phenotypic variation in insect ommatidia.
34