1. The Evolution of Populations
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
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

3. Fig. 23-2
Nonheritable variation. These caterpillars of the moth Nemoria arizonaria owe their different appearances to chemicals in their diets, not to their genotypes. Caterpillars raised on a diet of oak flowers resembled the flowers (a), whereas their siblings raised on oak leaves resembled oak twigs (b).
(a)
(b)
Figure 23.2 Nonheritable variation

4. Variation Within a Population
Both discrete and quantitative characters contribute to variation within a population
Discrete characters can be classified on an either-or basis (purple pea flowers or white pea flowers)
Quantitative characters vary along a continuum within a population (human skin color)
Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
Average heterozygosity measures the average percent of loci that are heterozygous in a population
Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

5. Fig. 23-3
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups. The numbers represent fused chromosomes from ancestral species. 1
2.4
3.14
5.18 6
7.15
8.11
9.12
10.16
13.17 19 XX 1
2.19
3.8
4.16
5.14
6.7
Figure 23.3 Geographic variation in isolated mouse populations on Madeira
9.10
11.12
13.17
15.18 XX

6. Ldh-B b allele frequency
Fig. 23-4
1.0
0.8
0.6
Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
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)

7. Mutation – Point Mutations, Mutations that alter gene number or sequence, Mutation Rates
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
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

8. Mutation – Point Mutations, Mutations that alter gene number or sequence, Mutation Rates
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
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

9. 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
(Remember – In meiosis genetic variation is a result of crossing over, independent assortment of alleles, and random fertilization)

10. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving Gene Pools and Allele Frequencies
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
A gene pool consists of all the alleles for all loci in a population
A locus is fixed if all individuals in a population are homozygous for the same allele
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

11. 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
In Fig there are two caribou populations that are not totally isolated; they sometimes share the same area. Nonetheless, members of either population are more likely to breed with members of their own populations than with members of the other populations.

12. 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

13. 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
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
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

14. Hardy-Weinberg Principle
p + q = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
p2 + 2pq + q2 = 1
p2 = frequency of the homozygous dominant
genotype
2pq = frequency of the heterozygous genotype
q2 = frequency of the recessive genotype

15. Fig. 23-6
If all of these alleles could be placed in a large bin, 80% would be CR and 20% would be Cw.
Assuming random mating, each time two gametes come together, there is an 80% chance the egg carries a CR allele and a 20% chance it carries a Cw allele. (Same %’s for sperm)
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

16. 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

17. 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
Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

18. 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

19. Applying the Hardy-Weinberg Principle
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

20. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Natural selection
Differential success (some traits are selected for and others are selected against) in reproduction results in certain alleles being passed to the next generation in greater proportions
Genetic drift
Gene flow

21. Fig
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
(Highlighted boxes represent, by chance alone, flowers that produce fertile offspring.)
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

22. 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

23. 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

24. 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

25. The Bottleneck Effect Original population Bottlenecking event
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
Figure 23.9 The bottleneck effect
Original
population
Bottlenecking
event
Surviving
population

26. Fig
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%
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
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
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)

27. 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

28. 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
Gene flow can decrease the fitness of a population
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

29. Fig
This image illustrates how gene flow can homogenize the gene pools of populations, thereby reducing geographic variation in appearance.
Figure Gene flow and human evolution

30. Windblown pollen moves these alleles between populations
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 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)

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

32. 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

33. Fig a
Original population
Frequency of individuals
Directional selection favors individuals at one end of the phenotypic range
Phenotypes (fur color)
Figure Modes of selection
Original population
Evolved population
(a) Directional selection

34. Fig b
Original population
Frequency of individuals
Disruptive selection favors individuals at both extremes of the phenotypic range
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(b) Disruptive selection

35. Fig c
Original population
Frequency of individuals
Stabilizing selection favors intermediate variants and acts against extreme phenotypes
Phenotypes (fur color)
Figure Modes of selection
Evolved population
(c) Stabilizing selection

36. 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
As a result, what constitutes a “good match” between an organism and its environment can be a moving target, making adaptive evolution a continuous, dynamic process.

37. Fig a
Natural selection increases the frequencies of alleles that enhance survival and reproduction
Figure Examples of adaptations
(a) Color-changing ability in cuttlefish – allows fish to hide from predators and surprise its prey.

38. snakes – allows them to swallow food much larger than their head.
Fig b
Movable bones
Adaptive evolution occurs as the match between an organism and its environment increases
Figure Examples of adaptations
(b) Movable jaw
bones in
snakes – allows them to swallow food much larger than their head.

39. 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

40. 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
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

41. Fig
EXPERIMENT
Conclusion:
Because offspring fathered by an LC male had higher fitness than their half siblings fathered by an SC male, the duration of a male’s mating call is indicative of the male’s overall genetic quality. This result supports the hypothesis that female mate choice can be based on a trait that indicates whether the male has “good genes.”
Female gray tree frogs prefer to mate with males that give long mating calls.
Question:
Is the genetic makeup of long calling (LC) males superior to that of short calling (SC) males?
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.

42. The Preservation of Genetic Variation – various mechanisms help to preserve genetic variation in a population
Diploidy maintains genetic variation in the form of hidden recessive alleles
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population (includes heterozygote advantage and frequency-dependent selection)
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
In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population

43. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Fig
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
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%

44. “left-mouthed” individuals
Fig
Selection can favor whichever phenotype is less common in a population
“Right-mouthed”
1.0
“Left-mouthed”
Green dot represents adults that reproduced had the opposite phenotype of that which was most common
“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

45. 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

46. Why Natural Selection Cannot Fashion Perfect Organisms (Make sure you read!)
Selection can act only on existing variations - these variations may not be the ideal traits
Evolution is limited by historical constraints – Each species has a legacy of descent with modification. Evolution does not scrap the ancestral anatomy and build each structures from scratch. In birds and bats, an existing pair of limbs took on new functions as these organisms evolved from walking ancestors.
Adaptations are often compromises – Each organism must do many different things. Some adaptations work for one aspect of the organisms exsistence but not for others. (Fig )
Chance, natural selection, and the environment interact – founding populations may not transport individuals that are best suited to new environment and natural disasters change the environment in unpredictable ways

47. This poor little frog’s love calls also attract this bat!
Fig
This poor little frog’s love calls also attract this bat!
Figure Evolutionary compromise

48. You should now be able to:
Explain why the majority of point mutations are harmless
Explain how sexual recombination generates genetic variability
Define the terms population, species, gene pool, relative fitness, and neutral variation
List the five conditions of Hardy-Weinberg equilibrium
Apply the Hardy-Weinberg equation to a population genetics problem
Explain why natural selection is the only mechanism that consistently produces adaptive change
Explain the role of population size in genetic drift
Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection
List four reasons why natural selection cannot produce perfect organisms