1. Genes Within Populations
Chapter 19

2. Introduction Natural populations exhibit variation
Species accumulate differences over time
Descendants differ from their ancestors
New species arise from existing ones

3. Genetic Variation and Evolution
Differences in alleles of genes found within individuals in a population
Raw material for natural selection
Evolution
How an entity changes through time
Development of modern concept traced to Darwin
“Descent with modification”

4. “Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones.”
Charles Darwin

5. Darwin was not the first to propose a theory of evolution
Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution
Rival theory of Jean-Baptiste Lamarck was evolution by inheritance of acquired characteristics

6. giraffes has characteristics of modern-day okapi.
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Stretching
Stretching
Proposed ancestor of
giraffes has characteristics
of modern-day okapi.
The giraffe ancestor lengthened its
neck by stretching to reach tree
leaves, then passed the change to
offspring.
Reproduction
a. Lamarck’s theory: acquired variation is passed on to descendants.

7. Some individuals born happen to have longer necks due to
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Some individuals born happen to have longer necks due to
genetic differences.
Reproduction
Individuals pass on their traits to next generation.
Natural
selection
Reproduction
Over many generations, longer-necked individuals are more
successful, perhaps because they can feed on taller trees, and
pass the long-neck trait on to their offspring.
b. Darwin’s theory: natural selection or genetically-based
variation leads to evolutionary change.

8. Genetic variation is the raw material for selection
Population genetics
Study of properties of genes in a population
Evolution results in a change in the genetic composition of a population
Genetic variation is the raw material for selection
In nature, genetic variation is the rule

9. Human populations contain substantial amounts of genetic variation
Genes that influence blood groups
Genes that influence enzymes
Enzyme polymorphisms

10. Polymorphic variation
More than one allele at frequencies greater than mutation alone
SNPs
DNA polymorphism
Used to assess patterns in human and natural populations

11. Hardy–Weinberg principle
Changes in allele frequency
Hardy–Weinberg equilibrium
Proportions of genotypes do not change in a population if
No mutation takes place
No genes are transferred to or from other sources (no immigration or emigration)
Random mating is occurring
The population size is very large
No selection occurs

12. Principle can be written as an equation
Used to calculate allele frequencies
For 2 alleles, p and q
p = B for black coat color
Black cat is BB or Bb
q = b for white coat color
White cats are bb
p2 + 2pq + q2 = 1
BB + Bb + bb = 1

13. genotype in population 0.36 0.48 0.16
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Generation One
Phenotypes
84%
16%
Genotypes BB Bb bb
Frequency of
genotype in population
0.36
0.48
0.16
Frequency of gametes
= 0.60B
= 0.40b

14. Generation Two B p = 0.60 b q = 0.40 Eggs B p = 0.60 BB p2 = 0.36 Bb
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Generation Two B
p = 0.60 b
q = 0.40
Eggs B
p = 0.60 BB
p2 = 0.36 Bb
pq = 0.24
Sperm b
q = 0.40 Bb
pq = 0.24 bb
q2 = 0.16
p2 + 2 pq + q2 = 1

15. If all 5 assumptions for Hardy-Weinberg equilibrium are true, allele and genotype frequencies do not change from one generation to the next
In reality, most populations will not meet all 5 assumptions
To determine this, look for changes in frequency
Suggest hypotheses about what process or processes are at work to cause changes to the frequencies

16. 5 agents of evolutionary change
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Mutation
Rates generally low
Other evolutionary processes usually more important in changing allele frequency
Ultimate source of genetic variation
Makes evolution possible
Mutation
Mutagen
DNA C G T G C G A G
a. The ultimate source of
variation. Individual
mutations occur so
rarely that mutation
alone usually does not change allele
frequency much.

17. Gene flow Movement of alleles from one population to another
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Gene flow
Movement of alleles from one population to another
Animal physically moves into new population
Drifting of gametes or immature stages into an area
Mating of individuals from adjacent populations
Gene Flow
b. A very potent agent of
change. Individuals or
gametes move from one
population to another.

18. Nonrandom mating Assortative mating Disassortative mating
Phenotypically similar individuals mate
Increases proportion of homozygous individuals
Disassortative mating
Phenotypically different individuals mate
Produces excess of heterozygotes
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Nonrandom Mating
Self-fertilization
c. Inbreeding is the most
common form. It does
not alter allele
frequency but reduces
the proportion of
heterozygotes.

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Genetic Drift
Genetic drift
In small populations, allele frequency may change by chance alone
Magnitude of genetic drift is negatively related to population size
Founder effect
Bottleneck effect
d. Statistical accidents.
The random fluctuation
in allele frequencies
increases as population
size decreases.

20. Genetic drift Alters allele frequencies in small populations
Population must be large
Small number of individuals drift from population
Can lead to the loss of alleles in isolated populations

21. Genetic drift can lead to the loss of alleles in isolated populations
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Parent
population
Bottleneck
(drastic reduction
in population)
Surviving
individuals
Next
generation
Genetic drift can lead to the loss of alleles in isolated populations
Alleles that initially are uncommon are particularly vulnerable

22. Bottleneck effect
If organisms do not move from place to place their populations may be drastically reduced
Survivors may constitute a random genetic sample of the original population
Results in loss of genetic variability

23. Northern Elephant Seal
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population in 1890,
reduced to inhabiting
Guadalupe only
UNITED
STATES
current population
Guadalupe
MEXICO
Northern Elephant Seal
Bottleneck case study
Nearly hunted to extinction in 19th century
As a result, species has lost almost all of its genetic variation
Population now numbers in tens of thousands

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25. Selection
Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior
Artificial selection
Natural selection

26. 3 conditions for natural selection to occur and to result in evolutionary change
Variation must exist among individuals in a population
Variation among individuals must result in differences in the number of offspring surviving in the next generation
Variation must be genetically inherited

27. Natural selection and evolution are not the same
Natural selection is a process
Only one of several processes that can result in evolution
Evolution is the historical record, or outcome, of change through time
Result of evolution driven by natural selection is that populations become better adapted to their environment

28. Pocket mice come in different colors
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Light coat color pocket mouse is vulnerable on lava rock
Light coat color favored by natural selection because it matches sand color
Dark coat color favored by natural selection because it matches black lava color
Pocket mice come in different colors
Population living on rocks favor dark color
Populations living on sand favor light color

29. Fitness and its measurement
Individuals with one phenotype leave more surviving offspring in the next generation than individuals with an alternative phenotype
Relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number of offspring

30. Fitness has many components
Survival
Sexual selection – some individuals more successful at attracting mates
Number of offspring per mating
Traits favored for one component may be a disadvantage for others
Selection favors phenotypes with the greatest fitness
Phenotype with greater fitness usually increases in frequency

31. Selection for increased speed in racehorses is no longer effective

32. Interactions Mutations and genetic drift may counter selection
In nature, mutation rates are rarely high enough to counter selection
Selection is nonrandom but genetic drift is random
Drift may decrease an allele favored by selection
Selection usually overwhelms drift except in small populations

33. Gene flow promotes or constrains evolutionary change
Degree of copper tolerance in grass plants on and near ancient mine sites

34. Gene flow can be Constructive Constraining
Spread beneficial mutation to other populations
Constraining
Can impede adaptation by continual flow of inferior alleles from other populations

35. Maintenance of variation
Frequency-dependent selection
Fitness of a phenotype depends on its frequency within the population
Negative frequency-dependent selection
Rare phenotypes favored by selection
Rare forms may not be in “search image”
Positive frequency-dependent selection
Favors common form
Tends to eliminate variation
“Oddballs” stand out

36. Taken by Fish Predators Color Type Frequency in Population
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Negative frequency-dependent selection
In water boatman, fish eat the most common color type more than they would by chance alone
100 80 60
Percent of Color Type
Taken by Fish Predators 40
Color type of
water boatman 20
dark brown
medium brown
light brown 20 40 60 80
100
Color Type Frequency in Population

37. Oscillating selection
Selection favors one phenotype at one time and another phenotype at another time
Effect will be to maintain genetic variation in the population
Medium ground finch of Galápagos Islands
Birds with big bills favored during drought
Birds with smaller bills favored in wet conditions

38. Heterozygote advantage
Heterozygotes are favored over homozygotes
Works to maintain both alleles in the population
Sickle cell anemia
Hereditary disease affecting hemoglobin
Causes severe anemia
Homozygotes for sickle cell allele usually die before reproducing (without medical treatment)

39. Why is the sickle cell allele not eliminated?
Leading cause of death in central Africa is malaria
Heterozygotes for sickle cell allele do not suffer anemia and are much less susceptible to malaria
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Normal red blood cells
Sickled red blood cells
Sickle cell allele in Africa
1–5%
5–10%
10–20%
Geographic distribution of P. falciparum

40. Selection Many traits affected by more than one gene
Selection operates on all the genes for the trait
Changes the population depending on which genotypes are favored
Types of selection
Disruptive
Directional
Stabilizing

41. Acts to eliminate intermediate types
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Disruptive selection
Acts to eliminate intermediate types
Different beak sizes of African black-bellied seedcracker finch
Available seeds fall into 2 categories
Favors bill sizes for one or the other
Number of Individuals 25 50 75
100
125
Body Size (g)
Selection for small and large individuals
Two peaks
form
Number of Individuals 25 50 75
100
125
Body Size (g)
a. Disruptive selection

42. Birds with intermediate-sized beaks are at a disadvantage with both seed types – they are unable to open large seeds and too clumsy to efficiently process small seeds
Fix runover style in em dash entry?

43. Directional selection Acts to eliminate one extreme
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Directional selection
Acts to eliminate one extreme
Often occurs in nature when the environment changes
In Drosophila, artificially selected flies that moved toward the light
Now fewer have that behavior
Number of Individuals 25 50 75
100
125
Body Size (g)
Selection for larger individuals
Peak shifts
Number of Individuals 25 50 75
100
125
Body Size (g)
b. Directional selection

44. Directional selection for negative phototropism in Drosophila
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Light 11 10 9 8 7
Average Tendency to Fly Toward Light 6 5 4 3 2
Dark 1 2 4 6 8 10 12 14 16 18 20
Number of Generations
Directional selection for negative phototropism in Drosophila

45. Stabilizing selection Acts to eliminate both extremes
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Stabilizing selection
Acts to eliminate both extremes
Makes intermediate more common by eliminating extremes
In humans, infants with intermediate weight at birth have the highest survival rate
Number of Individuals 25 50 75
100
125
Body Size (g)
Selection for mid-size individuals
Distribution
gets narrower
Number of Individuals 25 50 75
100
125
Body Size (g)
c. Stabilizing selection

46. Stabilizing selection for birth weight in humans
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births in population
infant mortality 20
100 70 15 50 30 20
Percent of Births in Population
Percent Infant Mortality 10 10 7 5 5 3 2 2 3 4 5 6 7 8 9 10
Birth Weight in Pounds
Stabilizing selection for birth weight in humans

47. Experimental studies
To study evolution, biologists have traditionally investigated what has happened in the past
Fossils or DNA evidence
Laboratory studies on fruit flies common for more than 50 years
Only recently started with lab and field experiments

48. Constraints on evolutionary change
Mutation and genetic drift
May counter or promote selection
Natural in abandoned mine sites in Great Britain
Copper tolerance in plants

49. Slender bent grass at copper mines
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Pollen
Prevailing wind
Non- mine
Mine Site
Nonmine 60
Index of Copper Tolerance 40
Bent grass (Agrostis tenuis) 20 20 20 40 60 80
100
120
140
160
Distance Upwind from Mine (m)
Distance Away from Mine (m)
Slender bent grass at copper mines
Resistance allele occurs at intermediate levels in many areas
Individuals with resistance gene grow slower on uncontaminated sites
Gene flow between sites high enough to counteract selection

50. Limits of selection Multiple phenotypic effects of alleles
Larger clutch size leads to thinner shelled eggs
Lack of genetic variation
Gene pool of thoroughbreds limited and performance times have not improved for more than 50 years
Phenotypic variation may not have genetic basis
Interactions between genes – epistasis
Selective advantage of an allele at one gene may vary from one genotype to another

51. Kentucky Derby Winning Time (seconds)
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130
125
120
Kentucky Derby Winning Time (seconds)
115
110
1900
1920
1940
1960
1980
2000
Year
Selection for increased speed in racehorses is no longer effective