1. Chapter 16 How Populations Evolve

2. 16.1 Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
16.1 Genes, Populations, and Evolution
Processes of Evolution
A population is all the members of a single species occupying a particular area at the same time.
Diversity exists among members of a population.
Population genetics is the study of this diversity in terms of allele differences.
Evaluates the diversity of a population by studying genotype and phenotype frequencies over time

3. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
In the 1930s, population geneticists began to describe variations in a population in terms of alleles
Microevolution pertains to evolutionary changes within a population.
Various alleles at all the gene loci in all individuals make up the gene pool of the population.
The gene pool of a population can be described in terms of:
Genotype frequencies
Allele frequencies

4. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Allele Frequencies
The proportion of each allele within a population’s gene pool.
Frequencies of the dominant and recessive allele must add up to 1.
This relationship is described by the expression p + q = 1
p is the frequency of one allele and q is the frequency of the other
Microevolution involves a change in these allele frequencies within populations over time.
If the gene frequencies do not change over time, microevolution has not occurred.

5. How Hardy-Weinberg Equilibrium Is Estimated
Biology, 9th ed,Sylvia Mader
How Hardy-Weinberg Equilibrium Is Estimated
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Equilibrium
genotype frequencies:
Population = 25 moths, 50 alleles
Gene pool:
Allele frequencies:
D = 10
d = 40 DD DD Dd Dd Dd
D D D D D d d d D Dd Dd Dd Dd Dd
D D D D d d d d d dd
p + q = 1
P2 + 2pq + q2 = 1 Dd
D d
p = frequency of D
p = frequency of D
q = frequency of d
q = frequency of d dd dd dd dd
d d d d d d d d
Frequency of D
Frequency of DD
p = 10/50 alleles
= 0.20
p2 = freq D2
= (0.2)(0.2)
= 0.04 dd dd dd dd
d d d d d d d d
Frequency of d
Frequency of Dd
q = 40/50 alleles
= 0.80
2pq = 2(freq D x freq d)
= 2(0.20 x 0.80)
= 0.32 dd dd dd dd
d d d d d d d d
Frequency of dd
q2 = freq d 2
= (0.08)(0.80)
= 0.64 dd dd dd dd
d d d d d d d d
1.00
1.00

6. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
The Hardy-Weinberg Equalibrium states that:
Allele frequencies in a population will remain constant assuming:
No Mutations
No Gene Flow
Random Mating
No Genetic Drift
No Selection

7. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Hardy-Weinberg Equilibrium:
Required conditions are rarely (if ever) met
Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place
Analysis of allele changes in populations over time determines the extent to which evolution has occurred.

8. Biology, 9th ed,Sylvia Mader
Chapter 18
Processes of Evolution

9. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Causes of Microevolution
Genetic mutations
The raw material for evolutionary change
Provide new alleles
Some mutations might be more adaptive than others
Ex: Genetic mutations affecting pigment color in peppered moths have provided the variation needed for natural selection to occur

10. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Causes of Microevolution
Gene Flow (gene migration)
Movement of alleles between populations when:
Gametes or seeds (in plants) are carried into another population
Breeding individuals migrate into or out of population
Continual gene flow reduces genetic divergence between populations

11. Biology, 9th ed,Sylvia Mader
Chapter 18
Processes of Evolution
Gene Flow
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
gene flow
Pisum arvense
Pisum sativum

12. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Causes of Microevolution
Genetic Drift
Changes in the allele frequencies of a population due to change rather than selection by the environment
Does not necessarily lead to adaptation to the environment
Occurs by disproportionate random sampling from population
Can cause the gene pools of two isolated populations to become dissimilar
Some alleles are lost and others become fixed (unopposed)
Likely to occur:
After a bottleneck
When severe inbreeding occurs, or
When founders start a new population
Stronger effect in small populations

13. Biology, 9th ed,Sylvia Mader
Chapter 18
Processes of Evolution
Genetic Drift
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10% of
population
natural disaster kills
five green frogs
20% of
population

14. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Bottleneck Effect
A random event prevents a majority of individuals from entering the next generation
The next generation is composed of alleles that just happened to make it

15. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Founder Effect
When a new population is started from just a few individuals
The alleles carried by population founders are dictated by chance
Formerly rare alleles will either:
Occur at a higher frequency in the new population, or
Be absent in new population

16. Bottleneck and Founder Effects Change Allele Frequencies
Biology, 9th ed,Sylvia Mader
Bottleneck and Founder Effects Change Allele Frequencies
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. c. d.
Original population
gene pool = 3,800 alleles*
Remnant population
gene pool = 90 alleles*
13%
11% 8%
26%
44%
45%
53%
*1 marble = 10 alleles

17. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Nonrandom Mating
When individuals do not choose mates randomly
Assortative mating:
Individuals select mates with the same phenotype with respect to a certain characteristic
Individuals reject mates with differing phenotype
Increases the frequency of homozygotes for certain loci

18. Biology, 9th ed,Sylvia Mader
Chapter 18
16.2 Natural Selection
Processes of Evolution
Adaptation of a population to the biotic and abiotic environment
Requires:
Variation - The members of a population differ from one another
Inheritance - Many differences are heritable genetic differences
Differential Adaptiveness - Some differences affect survivability
Differential Reproduction – Some differences affect the likelihood of successful reproduction

19. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Results in:
A change in allele frequencies of the gene pool
Improved fitness of the population
Major cause of microevolution 19

20. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Most traits are polygenic - variations in the trait result in a bell-shaped curve
Three types of selection occur:
(1) Directional Selection
The curve shifts in one direction
Bacteria become resistant to antibiotics
Increasing body size in horse evolution 20

21. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Three types of selection occur (cont):
(2) Stabilizing Selection
The peak of the curve increases and tails decrease
Example - human babies with low or high birth weight are less likely to survive
(3) Disruptive Selection
The curve has two peaks
Example – British land snails vary because a wide geographic range causes selection to vary 21

22. Three Types of Natural Selection
Biology, 9th ed,Sylvia Mader
Three Types of Natural Selection
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Number of Individuals
Phenotype Range
Phenotype Range
Phenotype Range
stabilizing selection
directional selection
disruptive selection
Peak narrows.
Peak shifts.
Two peaks result.
Number of Individuals a. b. c. 22

23. Human Birth Weight (stabilizing selection)
Biology, 9th ed,Sylvia Mader
Chapter 18
Human Birth Weight (stabilizing selection)
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 20
100 70 15 50 30 20
Percent of Births in Population
Percent Infant Mortality 10 10 7 5 5 3 2 .9
1.4
1.8
2.3
2.7
3.2
3.6
4.1
4.5
Birth Weight (in kilograms) 23

24. Directional Selection
Biology, 9th ed,Sylvia Mader
Directional Selection
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Initial Distribution
After Time
After More Time
Number of
Individuals
Number of
Individuals
Number of
Individuals
Body Size
Body Size
Body Size a.
Hyracotherium
Merychippus
Equus b. 24

25. Biology, 9th ed,Sylvia Mader
Disruptive Selection
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Number of
Individuals
Initial
Distribution
Banding Pattern
After
Time
Number of
Individuals
Banding Pattern
After
More Time
Number of
Individuals
Banding Pattern a. b.
b: © Bob Evans/Peter Arnold/Photolibrary 25

26. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Sexual selection - adaptive changes in males and females lead to an increased ability to secure a mate.
Males - increased ability to compete with other males for a mate
Females choose to select a male with the best fitness (ability to produce surviving offspring). 26

27. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Female Choice
Choice of a mate is a serious consideration because females produce few eggs
Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival.
Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance.
Male Competition
Males can father many offspring because they continuously produce sperm in great quantity.
Compete to inseminate as many females as possible. 27

28. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
A study of sexual selection among humans shows that female choice and male competition apply to humans too
Women must invest more in having a child than men.
Men need only contribute sperm
Generally more available for mating than are women.
More available men results in competition
Men also have a choice
Prefer women who are most likely to present them with children. 28

29. 16.3 Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Chapter 18
16.3 Maintenance of Diversity
Processes of Evolution
Genetic Variability
Populations with limited variation may not be able to adapt to new conditions
Maintenance of variability is advantageous to the population
Only exposed alleles are subject to natural selection 29

30. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Chapter 18
Maintenance of Diversity
Processes of Evolution
Natural selection causes imperfect adaptations
Depends on evolutionary history
Imperfections are common because of necessary compromises
The environment plays a role in maintaining diversity
Disruptive selection due to environmental differences promotes polymorphisms in a population
If a population occupies a wide range, it may have several subpopulations designated as subspecies
The environment includes selecting agents that help maintain diversity 30

31. Subspecies Help Maintain Diversity
Biology, 9th ed,Sylvia Mader
Subspecies Help Maintain Diversity
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pantheropsis obsoleta obsoleta
Pantheropsis obsoleta quadrivittata
Pantheropsis obsoleta lindheimeri
Pantheropsis obsoleta rossalleni
Pantheropsis obsoleta spiloides
(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.;
(E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited 31

32. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Chapter 18
Maintenance of Diversity
Processes of Evolution
Recessive alleles:
Heterozygotes shelter recessive alleles from selection
Heterozygotes allow even lethal alleles to remain in the population at low frequencies virtually forever
Sometimes recessive alleles confer an advantage to heterozygotes
The sickle-cell anemia allele is detrimental in homozygote
However, heterozygotes are more likely to survive malaria
The sickle-cell allele occurs at a higher frequency in malaria prone areas 32

33. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Chapter 18
Maintenance of Diversity
Processes of Evolution
Heterozygote Advantage
Assists the maintenance of genetic, and therefore phenotypic, variations in future generations.
In sickle cell disease heterozygous individuals don’t die from sickle-cell disease, and they don’t die from malaria. 33