1. Biology Sylvia S. Mader Michael Windelspecht
Chapter 16
How Populations Evolve
Lecture Outline
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2. Biology, 9th ed,Sylvia Mader
Chapter 18
Outline
Processes of Evolution
16.1 Genes, Populations, and Evolution
16.2 Natural Selection
16.3 Maintenance of Diversity

3. MRSA, a Common Skin Bacterium Resistant to Antibiotics
Biology, 9th ed,Sylvia Mader
Chapter 18
MRSA, a Common Skin Bacterium Resistant to Antibiotics
Processes of Evolution
Methicillin-resistant Staphlococcus aureus is a strain of a common skin bacterium that can be deadly.
It has become resistant to antibiotics.
Overuse of antibiotics resulted in the evolution of resistant bacterial strains like MRSA.
These strains are also known as “superbugs.”
Understanding of evolutionary biology is helping fight the superbugs.
It is also an example of why evolution is important in our lives.

4. 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 a group of organisms 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.
It evaluates the diversity of a population by studying genotype and phenotype frequencies over time.

5. 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 populations.
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

6. The Genetic Basis of Body Color in the Peppered Moth
Biology, 9th ed,Sylvia Mader
The Genetic Basis of Body Color in the Peppered Moth
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phenotype:
Genotype: DD Dd dd
homozygous
heterozygous
homozygous
Alleles: D D D d d d
chromosome

7. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Allele Frequencies
It is 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.

8. 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

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

10. Hardy-Weinberg Equilibrium
Biology, 9th ed,Sylvia Mader
Hardy-Weinberg Equilibrium
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
eggs
0.20 D
0.80 d
0.20 D
sperm
0.04 DD
0.16 Dd
0.80 d
0.16 Dd
0.64 dd 10
Offspring

11. 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.

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

13. Mechanisms of Microevolution
Biology, 9th ed,Sylvia Mader
Mechanisms of Microevolution
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
F1 generation
F2 generation
Allele
frequencies:
Genotype
frequencies:
Allele
frequencies:
Genotype
frequencies:
Conclusion: DD DD D D Dd Dd dd d
If... dd d
Random mating
No selection
No migration
No mutation
No change in allele frequencies
No change in genotype frequencies
Evolution has not occurred
p2 + 2pq + q2 = 1
p2 + 2pq + q2 = 1
0.20 D
0.20 D
0.80 d
DD = 0.04
...then we expect
0.80 d
DD = 0.04
Dd = 0.32
Dd = 0.32
dd = 0.64
dd = 0.64 DD
If...
Nonrandom mating D
...then we observe Dd d dd
No change in allele frequencies
Genotype frequencies change
Evolution has not occurred X
p2 + 2pq + q2 = 1 DD X Dd
0.20 D
0.80 d
DD = 0.10 or
Dd = 0.20 Dd X DD
dd = 0.70 dd
If...
Selection
...then we observe d Dd D DD
Allele frequencies change
Genotype frequencies change
Evolution has occurred
p2 + 2pq + q2 = 1
0.80 D
0.20 d
DD = 0.64
Dd = 0.32
dd = 0.04

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

15. 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.
If migration between populations doesn’t occur, the gene pools become more different over time.
The differences in the genetic makeup of these populations become so large that they become reproductively isolated and incapable of interbreeding.
This is the first step in species formation.

16. Biology, 9th ed,Sylvia Mader
Chapter 18
Gene Flow
Processes of Evolution

17. 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

18. 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

19. 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.
It is mostly due to natural disasters or habitat loss.

20. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Founder Effect – a type of genetic drift
A new population is started from just a few individuals.
The alleles carried by population founders are dictated by chance.
The gene pool of the small population is much different from the gene pool of the original population.

21. Genes, Populations, and Evolution
Biology, 9th ed,Sylvia Mader
Chapter 18
Genes, Populations, and Evolution
Processes of Evolution
Founder Effect – a type of genetic drift (cont’d)
The greater the reduction in population size, the greater the effects on allele frequencies.
Formerly rare alleles will either:
Occur at a higher frequency in the new population, or
Be absent in new population
Higher-than-normal occurrence of inbreeding (mating between relatives) occurs.
Rare recessive disorders emerge more frequently in inbred populations.

22. Consequences of Bottleneck and Founder Effects
Biology, 9th ed,Sylvia Mader
Consequences of Bottleneck and Founder Effects
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

23. 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.
It increases the frequency of homozygotes for certain loci.
It can play an important role in the evolution of a population.
Example: On island of Pingelap, nonrandom mating resulted in an increased frequency of colorblindness.

24. Biology, 9th ed,Sylvia Mader
Chapter 18
16.2 Natural Selection
Processes of Evolution
Natural selection is the 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.
Natural selection favors the variant that is most adaptive to the present environmental conditions.

25. 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 25

26. 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
An extreme phenotype is favored.
The curve shifts in one direction.
Bacteria become resistant to antibiotics.
Body size increases in horse evolution. 26

27. Biology, 9th ed,Sylvia Mader
Chapter 18
Natural Selection
Processes of Evolution
Three types of selection occur (cont’d):
(2) Stabilizing Selection
An intermediate phenotype is the most adaptive for the given environmental conditions.
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
Two or more extreme phenotypes are favored over the intermediate phenotype.
The curve has two peaks.
Example: British land snails vary because a wide geographic range causes selection to vary. 27

28. 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. 28

29. 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) 29

30. 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. 30

31. Biology, 9th ed,Sylvia Mader
Disruptive Selection
Chapter 18
Processes of Evolution 31

32. 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.
With males there is an 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). 32

33. Biology, 9th ed,Sylvia Mader
Natural Selection
Chapter 18
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.
Sexual dimorphism: Males and females differ in size and other traits.
Female choice can explain why male birds are more ornate than females.
Remarkable plumes of males may signify health and vigor. 33

34. Biology, 9th ed,Sylvia Mader
Natural Selection
Chapter 18
Processes of Evolution
Male Competition
Males can father many offspring because they continuously produce sperm in great quantity.
They compete to inseminate as many females as possible.
Cost-benefit analysis: Is access to mating worth the competition? 34

35. Biology, 9th ed,Sylvia Mader
Chapter 18
Dimorphism
Processes of Evolution
The drab females tend to choose flamboyant males as mates.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 35

36. Sexual Selection: Competition
Biology, 9th ed,Sylvia Mader
Chapter 18
Sexual Selection: Competition
Processes of Evolution 36

37. Sexual Selection: Competition Between Male Red Deer
Biology, 9th ed,Sylvia Mader
Chapter 18
Sexual Selection: Competition Between Male Red Deer
Processes of Evolution 37

38. 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. 38

39. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Maintenance of Diversity
Chapter 18
Processes of Evolution
Natural selection causes imperfect adaptations.
It depends on evolutionary history.
Imperfections are common because of necessary compromises.
Evolution doesn’t start from scratch.
The processes of development restrict the emergence of novel features.
Imperfections are common.
The benefit of freeing our hands outweighed the cost of spinal injuries from assuming an erect posture. 39

40. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Maintenance of Diversity
Chapter 18
Processes of Evolution
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.
Maintenance of variation among a population’s members has survival value for the species.
Example: Average beak size of Galápagos finches depended on the available food supply. 40

41. Subspecies Help Maintain Diversity
Biology, 9th ed,Sylvia Mader
Subspecies Help Maintain Diversity
Chapter 18
Processes of Evolution 41

42. 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. 42

43. Maintenance of Diversity
Biology, 9th ed,Sylvia Mader
Maintenance of Diversity
Chapter 18
Processes of Evolution
Heterozygote Advantage
It assists the maintenance of genetic, and therefore phenotypic, variations in future generations.
Sometimes recessive alleles confer an advantage to heterozygotes.
In sickle-cell disease heterozygous individuals don’t die from sickle-cell disease, and they don’t die from malaria.
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.
Malaria is caused by a protozoan parasite that lives in and destroys red blood cells of a homozygous normal individual (no sickle-cell allele).
Heterozygotes (one sickle-cell allele) have an advantage.
This is a form of stabilizing selection. 43

44. Biology, 9th ed,Sylvia Mader
Sickle-Cell Disease
Chapter 18
Processes of Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
malaria
sickle-cell
overlap of both 44