1. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow
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2. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture
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3. Genetic Drift
The smaller a sample, the greater the chance of deviation from a predicted result
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next (chance)
Genetic drift tends to reduce genetic variation through losses of alleles
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4. Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CRCR
Figure
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCW
Figure 23.9 Genetic drift.
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

5. 5 plants leave off- spring
Figure 5
plants
leave
off-
spring
CRCR
CRCR
CWCW
CRCR
CRCW
CRCW
CWCW
CRCR
CRCR
CWCW
CRCW
CRCW
CRCR
CRCW
CWCW
CRCR
Figure 23.9 Genetic drift.
CRCR
CRCW
CRCW
CRCW
Generation 1
Generation 2
p (frequency of CR) = 0.7
p = 0.5
q (frequency of CW) = 0.3
q = 0.5

6. 5 plants leave off- spring 2 plants leave off- spring
Figure 5
plants
leave
off-
spring 2
plants
leave
off-
spring
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CWCW
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
Figure 23.9 Genetic drift.
CRCR
CRCW
CRCW
CRCW
CRCR
CRCR
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

7. The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population
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8. The Bottleneck Effect
sudden reduction in population size due to a change in the environment
New gene pool may not reflect original
If the population remains small, it may be further affected by genetic drift
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9. Original population Bottlenecking event
Figure
Figure The bottleneck effect.
Original
population
Bottlenecking
event

10. Original population Bottlenecking event Surviving population
Figure
Figure The bottleneck effect.
Original
population
Bottlenecking
event
Surviving
population

11. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat = reduction of population
The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched
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12. Greater prairie chicken
Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
1930–1960s
1993
1,000–25,000
<50
5.2
3.7 93
<50
Figure Genetic drift and loss of genetic variation.
Kansas, 1998
(no bottleneck)
750,000
5.8 99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8 96
(b)

13. The results showed a loss of alleles at several loci
Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck
The results showed a loss of alleles at several loci
Researchers introduced greater prairie chickens from populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%
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14. Effects of Genetic Drift: A Summary
significant in small populations
causes allele frequencies to change at random
can lead to a loss of genetic variation within populations
can cause harmful alleles to become fixed
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15. 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 variation among populations over time
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16. Gene flow can decrease the fitness of a population
Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland
Mating causes gene flow between the central and eastern populations
Immigration from the mainland introduces alleles that decrease fitness
Natural selection selects for alleles that increase fitness
Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness
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17. Population in which the surviving females eventually bred 60 Central
Figure 23.12
Population in which the
surviving females
eventually bred 60
Central
population
Central 50
NORTH SEA
Eastern
population
Eastern
Vlieland,
the Netherlands 40
2 km
Survival rate (%) 30 20 10
Figure Gene flow and local adaptation.
Females born
in central
population
Females born
in eastern
population
Parus major

18. Gene flow can increase the fitness of a population
Consider, for example, the spread of alleles for resistance to insecticides
Insecticides have been used to target mosquitoes that carry West Nile virus and malaria
Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes
The flow of insecticide resistance alleles into a population can cause an increase in fitness
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19. Gene flow is an important agent of evolutionary change in human populations
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