1. The Evolution of Populations
Chapter 23
The Evolution of Populations

2. Overview: The Smallest Unit of Evolution
One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but populations evolve

3. Genetic variations in populations
Contribute to evolution
Figure 23.1

4. Concept 23.1: Population genetics provides a foundation for studying evolution
Microevolution
Is change in the genetic makeup of a population from generation to generation
Figure 23.2

5. The Modern Synthesis Population genetics
Is the study of how populations change genetically over time
Reconciled Darwin’s and Mendel’s ideas

6. The modern synthesis
Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection
Focuses on populations as units of evolution

7. Gene Pools and Allele Frequencies
A population
Is a localized group of individuals that are capable of interbreeding and producing fertile offspring
MAP
AREA
ALASKA
CANADA
Beaufort Sea
Porcupine
herd range
Fairbanks
Whitehorse
Fortymile
NORTHWEST
TERRITORIES
YUKON
Figure 23.3

8. The gene pool
Is the total aggregate of genes in a population at any one time
Consists of all gene loci in all individuals of the population

9. The Hardy-Weinberg Theorem
Describes a population that is not evolving
States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work

10. Mendelian inheritance
Preserves genetic variation in a population
Figure 23.4
Generation 1
CRCR
genotype
CWCW
Plants mate
All CRCW
(all pink flowers)
50% CR
gametes
50% CW
Come together at random 2 3 4
25% CRCR
50% CRCW
25% CWCW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions

11. Preservation of Allele Frequencies
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

12. Hardy-Weinberg Equilibrium
Describes a population in which random mating occurs
Describes a population where allele frequencies do not change

13. A population in Hardy-Weinberg equilibrium
Figure 23.5
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) p2
64%
CRCR
16%
CRCW 4%
CWCW qp
Eggs pq
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium: q2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
64% CR from
CRCR homozygotes
16% CR from
CRCW homozygotes + =
80% CR = 0.8 = p
16% CW from
CRCW heterozygotes
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
64% CRCR, 32% CRCW and 4% CWCW plants
4% CW from
CWCW homozygotes

14. 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
And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

15. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem
Describes a hypothetical population
In real populations
Allele and genotype frequencies do change over time

16. The five conditions for non-evolving populations are rarely met in nature
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection

17. Population Genetics and Human Health
We can use the Hardy-Weinberg equation
To estimate the percentage of the human population carrying the allele for an inherited disease

18. Two processes, mutation and sexual recombination
Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible
Two processes, mutation and sexual recombination
Produce the variation in gene pools that contributes to differences among individuals

19. Mutation Mutations Cause new genes and alleles to arise
Are changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Figure 23.6

20. Point Mutations A point mutation Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact

21. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial

22. Gene duplication
Duplicates chromosome segments

23. Mutation Rates Mutation rates Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation
Are more rapid in microorganisms

24. In sexually reproducing populations, sexual recombination
Is far more important than mutation in producing the genetic differences that make adaptation possible

25. Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow

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

27. Statistically, the smaller a sample
Genetic Drift
Statistically, the smaller a sample
The greater the chance of deviation from a predicted result

28. Genetic drift
Describes how allele frequencies can fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation
Figure 23.7
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

29. In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a population
The gene pool may no longer be reflective of the original population’s gene pool
Figure 23.8 A
(a)
Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.
Original
population
Bottlenecking
event
Surviving
population

30. Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
Figure 23.8 B
(b)
Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

31. The Founder Effect The founder effect
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population

32. Gene Flow Gene flow Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes
Tends to reduce differences between populations over time

33. Concept 23.4: Natural selection is the primary mechanism of adaptive evolution
Accumulates and maintains favorable genotypes in a population

34. Genetic Variation Genetic variation
Occurs in individuals in populations of all species
Is not always heritable
Figure 23.9 A, B
(a) Map butterflies that emerge in spring: orange and brown
(b) Map butterflies that emerge in late summer: black and white

35. Variation Within a Population
Both discrete and quantitative characters
Contribute to variation within a population

36. Quantitative characters
Discrete characters
Can be classified on an either-or basis
Quantitative characters
Vary along a continuum within a population

37. Phenotypic polymorphism
Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable
Genetic polymorphisms
Are the heritable components of characters that occur along a continuum in a population

38. Measuring Genetic Variation Population geneticists
Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level
Average heterozygosity
Measures the average percent of loci that are heterozygous in a population

39. Variation Between Populations
Most species exhibit geographic variation
Differences between gene pools of separate populations or population subgroups 1
2.4
3.14
5.18 6
7.15 XX 19
13.17
10.16
9.12
8.11
2.19
3.8
4.16
5.14
6.7
15.18
11.12
9.10
Figure 23.10

40. Heights of yarrow plants grown in common garden
Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
Figure 23.11
EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the resulting plants.
RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences.
Mean height (cm)
Atitude (m)
Heights of yarrow plants grown in common garden
Seed collection sites
Sierra Nevada Range
Great Basin Plateau

41. A Closer Look at Natural Selection
From the range of variations available in a population
Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations

42. The phrases “struggle for existence” and “survival of the fittest”
Evolutionary Fitness
The phrases “struggle for existence” and “survival of the fittest”
Are commonly used to describe natural selection
Can be misleading
Reproductive success
Is generally more subtle and depends on many factors

43. Fitness Relative fitness
Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Relative fitness
Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

44. Directional, Disruptive, and Stabilizing Selection
Favors certain genotypes by acting on the phenotypes of certain organisms
Three modes of selection are
Directional
Disruptive
Stabilizing

45. 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

46. The three modes of selection
Fig A–C
(a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators.
(b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.
(c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.
Phenotypes (fur color)
Original population
Original
population
Evolved
Frequency of individuals

47. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population

48. Diploidy
Diploidy
Maintains genetic variation in the form of hidden recessive alleles

49. Balancing Selection Balancing selection
Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
Leads to a state called balanced polymorphism

50. Heterozygote Advantage
Some individuals who are heterozygous at a particular locus
Have greater fitness than homozygotes
Natural selection
Will tend to maintain two or more alleles at that locus

51. The sickle-cell allele
Causes mutations in hemoglobin but also confers malaria resistance
Exemplifies the heterozygote advantage
Figure 23.13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)

52. Frequency-Dependent Selection In frequency-dependent selection
The fitness of any morph declines if it becomes too common in the population

53. Parental population sample Experimental group sample
An example of frequency-dependent selection
Figure 23.14
Parental population sample
Experimental group sample
Plain background
Patterned background
On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity).
0.06
0.05
0.04
0.03
0.02 20 40 60 80
100
Generation number
Frequency- independent control
Phenotypic diversity

54. Neutral Variation Neutral variation
Is genetic variation that appears to confer no selective advantage

55. Sexual Selection Sexual selection
Is natural selection for mating success
Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

56. Intrasexual selection
Is a direct competition among individuals of one sex for mates of the opposite sex

57. Intersexual selection
Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex
May depend on the showiness of the male’s appearance
Figure 23.15

58. The Evolutionary Enigma of Sexual Reproduction
Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap
Figure 23.16
Asexual reproduction
Female
Sexual reproduction
Male
Generation 1
Generation 2
Generation 3
Generation 4

59. If sexual reproduction is a handicap, why has it persisted?
It produces genetic variation that may aid in disease resistance

60. Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Adaptations are often compromises

61. Chance and natural selection interact
Selection can only edit existing variations