1. 21
The Evolution of Populations

2. Terminology Alleles: different forms of a gene; A or a
Genotype: the genetic make-up of an organism; AA or Aa, or aa
Phenotype: the observable trait; purple flower color
Species: a group of individuals that can exchange genetic material through interbreeding to produce viable, fertile offspring (i.e.share alleles through reproduction). Genetically distinctive.
Populations: an interbreeding group of organisms of the same species living in the same geographical area
Gene pool: all the alleles present in all individuals in a species in a given population
In order to understand evolution, we need to first understand some terminology that will be used.
A species is a group of individuals that can exchange genetic material through interbreeding, or share alleles through reproduction.
The alleles that are shared are from a single gene pool for that species. In other words, the gene pool consists of all the alleles present in all individuals in a species.
A ppulation is an interbreeding group of organisms of the same species living in the same geographical area. 2

3. Evolution
Evolution is a change in the frequency of alleles or genotypes over time
Changes in a population’s genetics over time
By changing allele or genotype frequency
Natural selection works on the individual, but it is the population that evolves
NOTE: Population is the smallest unit to evolve. Individuals do not evolve.
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4. Concept 21.1: Genetic variation makes evolution possible
Genetic variation is the result of
differences in DNA sequences among individuals in a population
2 causes of genetic variation:
Mutation
Sexual recombination
Genetic variation: differences in genotype among individuals in a population. In spite of a high degree of phenotypic variation, humans actually rank low in terms of overall genetic variation. We have found that the fruit fly is about 10 times more variable than we are in terms of the number of DNA bases that differ from one individual to the next. And even though the Adelie penguins are uniform in appearance, they are actually 2−3 times more genetically variable than we are.

5. WHAT CAUSES GENETIC VARIATION?
Mutations:create new variations
Somatic
Germ-line…can be
passed on to next generation
Deleterious
Neutral
Advantageous…to be best
adapted to their environment
Sexual Recombinations:shufflee
Mutations in new combinations
Genetic variation: 2 causes; (1)mutation and (2) recombination. Mutations generate new variation, and recombination shuffles mutations to create new combinations of mutations—both of which result in new alleles being formed.
Mutations can be somatic (in the body’s tissues) or germ-line (in the reproductive cells that are passed to the next generation).Only germ-line is passed on to next generation.
Mutations can be harmful (deleterious), neutral (meaning that they are neither good nor bad), or advantageous (meaning they improve the carrier’s chances of survival). The advantageous ones improve the organisms chances of survival or reproduction and are thus the ones that result in a species that is best adapted to its environment.

6. Allele Frequencies
Information about allele frequencies is key to understanding patterns
of genetic variation
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 1: The genotypes for pea color in Mendel’s pea plants are AA – yellow pea plant Aa – yellow pea plant aa – green pea plant What are the frequencies of the a and A alleles if every plant is green?
In order to measure genetic variation in a population you must know the rates of occurrence of the alleles in the population or allele frequencies: The frequency of an allele is simply the number of that particular allele present in a population divided by the total number of alleles.
In this example, if every plant is green (meaning aa), only the a allele is present in the population. The allele frequency of a is therefore 100% and the allele frequency of A is 0%.

7. ANSWER
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 1:If every plant is green, then they are all aa genotypes Only the a allele is present in the population Allele frequency of a is 100% and for A is 0%
In order to measure genetic variation in a population you must know the rates of occurrence of the alleles in the population or allele frequencies: The frequency of an allele is simply the number of that particular allele present in a population divided by the total number of alleles.
In this example, if every plant is green (meaning aa), only the a allele is present in the population. The allele frequency of a is therefore 100% and the allele frequency of A is 0%.
Population is FIXED for that allele when it exhibits only one allele at a given gene 7

8. Allele Frequencies
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 2: A population of 100 pea plants has the following genotype frequencies: 50% aa, 25% Aa, 25% AA= 100% What is the frequency of a?
In this example, we have a population of 100 pea plants, and we are given the frequency of each genotype in the population. What is the frequency of a?
First, how many copies of a are present in this population? Each plant that is homozygous for a has two copies of a, and each heterozygous plant has one copy of a. 50 aa individuals and 25 Aa individuals means (50 x 2) + 25 copies of a, or 125 copies of a. Now you must divide by the total number of alleles in the population. In this case we have 100 diploid plants, meaning there are 200 alleles in the population.
The resulting frequency for a is 62.5%.
To determine the frequency for A: 100% − 62.5% = 37.5 %.
Here, we were given the genotype frequencies, but how are they determined?

9. ANSWER
Allele Frequency =
# of copies of an allele
total # of alleles in population
Example 2: How many copies of a allele are present? There are 50 aa plants and 25 Aa plants (50 x 2) +25 = 125 copies of a 100 diploid plants = 200 alleles in population Therefore, 125/200 = 62.5% a
In this example, we have a population of 100 pea plants, and we are given the frequency of each genotype in the population. What is the frequency of a?
First, how many copies of a are present in this population? Each plant that is homozygous for a has two copies of a, and each heterozygous plant has one copy of a. 50 aa individuals and 25 Aa individuals means (50 x 2) + 25 copies of a, or 125 copies of a. Now you must divide by the total number of alleles in the population. In this case we have 100 diploid plants, meaning there are 200 alleles in the population.
The resulting frequency for a is 62.5%.
To determine the frequency for A: 100% − 62.5% = 37.5 %.
Here, we were given the genotype frequencies, but how are they determined? 9

10. Hardy-Weinberg Conditions
Hardy-Weinberg Equilibrium: situation in which evolution does not occur
There can be no differences in the survival and reproductive success of individuals.
Where p=%A and q=%a
Eq for allele frequencies: p (%A) + q (%a) = 1
Eq for genotype freq: p2 (AA)+2pq(Aa)+q2 (aa) = 1
The Hardy–Weinberg equilibrium specifies the relationship between allele frequencies and genotype frequencies when a number of key conditions are met. When these conditions are met—and the Hardy–Weinberg equilibrium holds—we can conclude that evolutionary forces are not acting on the gene in the population we are studying. In many ways, then, the Hardy–Weinberg equilibrium is most interesting when we find instances in which allele or genotype frequencies depart from expectations. This finding implies that one or more of the conditions are not met and that evolutionary forces are at work.

11. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg equation describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time
5 conditions for a population to be in Hardy-Weinberg equilibrium (not evolving)
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
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12. Applying the Hardy-Weinberg Equation
We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that
The PKU gene mutation rate is low
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
The population is large
Migration has no effect, as many other populations have similar allele frequencies
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13. The occurrence of PKU is 1 per 10,000 births
Hardy-Weinberg Example
The occurrence of PKU is 1 per 10,000 births
q2 =
q = 0.01
The frequency of normal alleles is
p = 1 – q = 1 – 0.01 = 0.99
The frequency of carriers is
2pq = 2  0.99  0.01 =
or approximately 2% of the U.S. population
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14. Only natural selection consistently causes adaptive evolution
Microevolution
Microevolution: change in allele frequencies in a population over generations
3 main mechanisms cause allele (variations in a trait) frequency change
Natural selection
Genetic drift
Gene flow
Only natural selection consistently causes adaptive evolution
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15. Concept 21.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
3 major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow
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16. Natural Selection
Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the proportion of individuals in a population that have traits favorable for the particular environment increases
Because the environment can change, adaptive evolution is a continuous, dynamic process
NOTE: Genetic drift and gene flow do not consistently lead to adaptive evolution, as they can increase or decrease the match between an organism and its environment
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17. Figure 21.5
Natural selection can only act on phenotypic variation that has a genetic component
(a)
(b)
Figure 21.5 Nonheritable variation
(a) Caterpillars raised on a diet of oak flowers
(b) Caterpillars raised on a diet of oak leaves
Phenotype is the product of both inherited genotype and environmental influences
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18. Figure 21.9
Genetic Drift
Highlighted plants
leave offspring.
CRCR
CRCW
Only 5 of
10 plants
leave
offspring.
Only 2 of
10 plants
leave
offspring.
CRCR
CWCW
CRCR
CRCW
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCR
CRCR
CWCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
CRCR
Figure 21.9 Genetic drift
Generation 1
Generation 2
Generation 3
p (frequency of CR ) = 0.7
p = 0.5
p = 1.0
q (frequency of CW ) = 0.3
q = 0.5
q = 0.0
The smaller a sample, the more likely it is that chance alone will cause deviation from a predicted result
Genetic drift: describes how allele frequencies fluctuate in a small population due to chance alone
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19. 2 Types of Genetic Drift 2 types of genetic drift:
Founder effect
Bottleneck effect
Founder effect: occurs when a few individuals become isolated from a larger population
Bottleneck effect: can result from a drastic reduction in population size due to a sudden environmental change (Mother Nature)
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20. Bottleneck Effect of Genetic Drift
Figure 21.10
Bottleneck Effect of Genetic Drift
Original
population
Bottlenecking
event
Surviving
population
(a) By chance, blue marbles are overrepresented in the surviving population, and gold marbles are absent.
Figure The bottleneck effect
(b) Florida panther (Puma concolor coryi)
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21. Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed
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22. Gene Flow
Gene flow consists of the movement of alleles among populations
Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce genetic variation among populations over time
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23. Concept 21.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Evolution by natural selection involves both chance and “sorting”
New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness
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24. Relative Fitness
The phrases “struggle for existence” and “survival of the fittest” are misleading, as they imply direct competition among individuals
Natural selection or differential reproductive success is generally more subtle and depends on many factors
Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
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25. Directional, Disruptive, and Stabilizing Selection
There are 3 modes of natural selection
Directional selection: occurs when conditions favor individuals at one end of the phenotypic range
Disruptive selection occurs when conditions favor individuals at both extremes of the phenotypic range
Stabilizing selection occurs when conditions favor intermediate variants and act against extreme phenotypes
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26. 3 Modes of Natural Selection
Figure 21.13
3 Modes of Natural Selection
Frequency of
individuals
Original
population
Original
population
Evolved
population
Phenotypes (fur color)
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection
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27. Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
Balancing selection includes
Heterozygote advantage
Frequency-dependent selection
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28. Heterozygote Advantage
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance
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29. (does not aggregate into fibers)
Figure
Make Connections: The Sickle-Cell Allele
Template strand
Sickle-cell allele
on chromosome
Low-
oxygen
condi-
tions
Fiber
An adenine
replaces a thymine.
Sickle-
cell
hemoglobin
Wild-type
allele
Sickled red
blood cell
Figure Make connections: the sickle-cell allele (part 1)
Normal hemoglobin
(does not aggregate into fibers)
Normal red
blood cell
Events at the
Molecular Level
Consequences
for Cells
Effects on
Individual Organisms
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30. Frequency-Dependent Selection
Frequency-dependent selection occurs when the fitness of a phenotype depends on how common it is in the population
For example, when “right-mouthed” individuals become more common in a population of scale-eating fish, prey are more likely to guard their left side, giving an advantage to “left-mouthed” fish
When “left-mouthed” fish become more common, the advantage is reversed
Balancing selection (due to frequency dependence) keeps the frequency of each phenotype close to 50%
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31. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
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32. Sexual Selection Continued
Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival
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33. Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact
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