1. AP Biology
Chapter 23~
The Evolution of Populations

2. Overview: The Smallest Unit of Evolution
One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only populations evolve
Genetic variations in populations contribute to evolution
Microevolution is a change in allele frequencies in a population over generations

3. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
Variation in individual genotype leads to variation in individual phenotype
Not all phenotypic variation is heritable
Natural selection can only act on variation with a genetic component

4. Fig. 23-2
(a)
(b)
Figure 23.2 Nonheritable variation- Catepillars of the moth Nemoria arizonaria owe their difference in appearances to chemicals in their diets, not their genotypes.

5. Variation Between Populations
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups
Figure 23.3 Geographic variation in isolated mouse populations on Madeira

6. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

7. Ldh-B b allele frequency
Fig. 23-4
1.0
0.8
0.6
Ldh-B b allele frequency
0.4
0.2
Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30
Latitude (°N)
Maine
Cold (6°C)
Georgia
Warm (21°C)

8. Animation: Genetic Variation from Sexual Recombination
Mutation
Mutations are changes in the nucleotide sequence of DNA
Mutations cause new genes and alleles to arise
Only mutations in cells that produce gametes can be passed to offspring
Animation: Genetic Variation from Sexual Recombination

9. Point Mutations A point mutation is a change in one base in a gene
The effects of point mutations can vary:
Mutations in noncoding regions of DNA are often harmless
Mutations in a gene might not affect protein production because of redundancy in the genetic code
Mutations that result in a change in protein production are often harmful
Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

10. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of large chromosome segments is usually harmful
Duplication of small pieces of DNA is sometimes less harmful and increases the genome size
Duplicated genes can take on new functions by further mutation

11. Mutation Rates
Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutations rates are often lower in prokaryotes and higher in viruses

12. Sexual Reproduction
Sexual reproduction can shuffle existing alleles into new combinations
In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

13. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
A population is a localized group of individuals capable of interbreeding and producing fertile offspring

14. Gene Pools and Allele Frequencies
A gene pool consists of all the alleles for all loci in a population
If only one allele exists for a particular locus in a population, that allele is said to be fixed. For loci that are fixed, all individuals in a population are homozygous for the same allele.
Population genetics: the study of genetic changes in populations
Individuals are selected, but populations evolve.”

15. Porcupine herd Porcupine herd range Fortymile herd range
Fig. 23-5
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON
Fortymile herd

16. The frequency of an allele in a population can be calculated
For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2
The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

17. The frequency of all alleles in a population will add up to 1
By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies
The frequency of all alleles in a population will add up to 1
For example, p + q = 1

18. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving.
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

19. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population

20. Alleles in the population Frequencies of alleles
Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg:
Each sperm:
CR allele = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele = 0.2
Figure 23.6 Selecting alleles at random from a gene pool

21. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2 + 2pq + q2 = 1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

22. 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) Eggs CW (20%) CR CW
Fig
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Figure 23.7 The Hardy-Weinberg principle
Eggs
64% ( p2)
CRCR
16% ( pq)
CRCW
4% (q2)
CW CW
16% (qp)
CRCW CW
(20%)

23. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
Figure 23.7 The Hardy-Weinberg principle
4% CW     +    16% CW   =   20% CW = 0.2 = q

24. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
Genotypes in the next generation:
Figure 23.7 The Hardy-Weinberg principle
64% CRCR, 32% CRCW, and 4% CWCW plants

25. Figure 23.7 The Hardy-Weinberg principle
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Eggs
64% ( p2)
CR CR
16% ( pq)
CR CW
4% (q2)
CW CW CW
(20%)
16% (qp)
CR CW
64% CR CR, 32% CR CW, and 4% CW CW
Figure 23.7 The Hardy-Weinberg principle
Gametes of this generation:
64% CR    +     16% CR    =   80% CR  = 0.8 = p
4% CW      +    16% CW    =  20% CW = 0.2 = q
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants

26. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time
The five conditions for nonevolving populations are rarely met in nature:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow

27. Applying the Hardy-Weinberg Principle
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

28. 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 x 0.99 x 0.01 =
or approximately 2% of the U.S. population

29. 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

30. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

31. Animation: Causes of Evolutionary Change
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
Genetic drift tends to reduce genetic variation through losses of alleles
Animation: Causes of Evolutionary Change

32. Generation 1 p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3
Fig
CR CR
CR CR
CR CW
CW CW
CR CR
CR CW
CR CR
CR CW
Figure 23.8 Genetic drift
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3

33. Generation 1 Generation 2 p (frequency of CR) = 0.7 p = 0.5
Fig
CR CR
CR CR
CW CW
CR CR
CR CW
CR CW
CW CW
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CR CW
CW CW
CR CR
Figure 23.8 Genetic drift
CR CR
CR CW
CR CW
CR CW
Generation 1
Generation 2
p (frequency of CR) = 0.7
p = 0.5
q (frequency of CW ) = 0.3
q = 0.5

34. Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7
Fig
CR CR
CR CR
CW CW
CR CR
CR CR
CR CW
CR CW
CR CR
CR CR
CW CW
CR CR
CR CR
CW CW
CR CR
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CW CW
CR CR
Figure 23.8 Genetic drift
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
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

35. 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

36. The Bottleneck Effect
The bottleneck effect is a sudden reduction in population size due to a change in the environment
The resulting gene pool may no longer be reflective of the original population’s gene pool
If the population remains small, it may be further affected by genetic drift
Understanding the bottleneck effect can increase understanding of how human activity affects other species

37. Original population Bottlenecking event Surviving population
Fig. 23-9
Figure 23.9 The bottleneck effect
Original
population
Bottlenecking
event
Surviving
population

38. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

39. Fig
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
5.2
3.7 93
<50
1930–1960s
1,000–25,000
1993
<50
Figure Bottleneck effect and reduction of genetic variation
Kansas, 1998
    (no bottleneck)
750,000
5.8 99
Nebraska, 1998
    (no bottleneck)
75,000–
200,000
5.8 96
Minnesota, 1998
    (no bottleneck)
4,000
5.3 85
(b)

40. 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 population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

41. Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
Genetic drift causes 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

42. 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 differences between populations over time
Gene flow is more likely than mutation to alter allele frequencies directly

43. Fig
Figure Gene flow and human evolution

44. Gene flow can decrease the fitness of a population
In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils
Windblown pollen moves these alleles between populations
The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

45. Bent Grass on toxic mine site
Fig
Bent Grass on toxic mine site 70
NON-
MINE
SOIL
MINE
SOIL
NON-
MINE
SOIL 60 50
Prevailing wind direction
Index of copper tolerance 40 30
Figure Gene flow and selection 20 10 20 20 20 40 60 80
100
120
140
160
Distance from mine edge (meters)

46. Gene flow can increase the fitness of a population
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

47. Only natural selection consistently results in adaptive evolution
Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Only natural selection consistently results in adaptive evolution
Natural selection brings about adaptive evolution by acting on an organism’s phenotype

48. Relative Fitness
The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals
Reproductive success is generally more subtle and depends on many factors

49. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Selection favors certain genotypes by acting on the phenotypes of certain organisms

50. Directional, Disruptive, and Stabilizing Selection
Three modes of selection:
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes

51. Frequency of individuals
Fig
Original population
Frequency of individuals
Phenotypes (fur color)
Original
population
Evolved
population
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

52. The Key Role of Natural Selection in Adaptive Evolution
Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between an organism and its environment increases

53. Fig
(a) Color-changing ability in cuttlefish
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes

54. Because the environment can change, adaptive evolution is a continuous process
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

55. 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

56. Fig
Figure Sexual dimorphism and sexual selection

57. 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

58. How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for

59. Larval growth NSD LC better Larval survival LC better NSD
Fig
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
Figure Do females select mates based on traits indicative of “good genes”?
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.

60. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population
1.) Diploidy maintains genetic variation in the form of hidden recessive alleles
2.) Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

61. 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
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

62. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Fig
Frequencies of the
sickle-cell allele
0–2.5%
Figure Mapping malaria and the sickle-cell allele
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
7.5–10.0%
10.0–12.5%
>12.5%

63. Frequency-Dependent Selection
In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population
Selection can favor whichever phenotype is less common in a population

64. “left-mouthed” individuals
Fig
“Right-mouthed”
1.0
“Left-mouthed”
“left-mouthed” individuals
Frequency of
0.5
Figure Frequency-dependent selection in scale-eating fish (Perissodus microlepis)
1981
’82
’83
’84
’85
’86
’87
’88
’89
’90
Sample year

65. Neutral Variation
Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage
For example,
Variation in noncoding regions of DNA
Variation in proteins that have little effect on protein function or reproductive fitness

66. 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