1. Chapter 23
Evolution of Populations

2. Common misconception 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. Population genetics  foundation for studying evolution Microevolution
Chg. in genetic makeup of a population from generation to generation
Figure 23.2

5. The Modern Synthesis
Population genetics  how populations change genetically over time

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

7. Population
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. Gene pool Total aggregate of genes in a population at any one time
 all gene loci in all individuals of the population

9. The Hardy-Weinberg theorem
Describes a population that is not evolving
Frequencies of alleles 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. Where gametes contribute to the next generation randomly, allele frequencies will not change

12. Hardy-Weinberg equilibrium
Random mating
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
p2 and q2  frequencies of homozygous genotypes
2pq  frequency of heterozygous
Also p + q = 1 (allele frequency)

15. The Hardy-Weinberg theorem
Hypothetical population
Real populations
Allele and genotype frequencies do change over time

16. 5 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. Mutation and sexual recombination
 variation in gene pools, contributes to differences among individuals, makes evolution possible

18. Mutations Cause new genes and alleles to arise
Chgs in nucleotide sequence of DNA
Cause new genes and alleles to arise
Figure 23.6

19. Point mutation Chg in 1 base in a gene Significant impact on phenotype
Usually harmless, but may have an adaptive impact

20. Chromosomal mutations that affect many loci
Usually harmful, but may be neutral or even beneficial

21. Mutation rates Low in animals and plants
~ 1 mutation in every 100,000 genes per generation
Rapid in microorganisms

22. In sexually reproducing populations, sexual recombination far more important than mutation f/ genetic differences that make adaptation possible

23. 3 major factors alter allele frequencies  most evolutionary change
Natural selection
Genetic drift
Gene flow

24. Differential success in reproduction
Natural Selection
Differential success in reproduction
Certain alleles pass to the next generation in greater proportions

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

26. Genetic drift
Allele freq can fluctuate unpredictably from one generation to the next
Reduces 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

27. Bottleneck effect
Sudden change in the environment may drastically reduce the size of a population
Gene pool no longer reflective of the original population
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

28. Bottleneck effect
Can increase understanding of how human activity affects other species, e.g. hunting
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.

29. Founder effect
A few individuals become isolated f/ a larger population
Can affect allele frequencies in a population

30. Gene flow Causes a population to gain or lose alleles
Results f/ movement of fertile individuals

31. Natural selection (primary mechanism of adaptive evolution)
Accumulates and maintains favorable genotypes in a population
Reduces differences between populations

34. Genetic variation Occurs in individuals in populations of all species
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. Quantitative characters
Variation
Discrete characters
Can be classified on an either-or basis
Quantitative characters
Vary along a continuum within a population

36. Phenotypic polymorphism
Two or more distinct morphs in high enough freq.to be noticeable
Genetic polymorphisms
Heritable components of characters that occur along a continuum

37. 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

38. Heights of yarrow plants grown in common garden
e.g. a cline, 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

39. Phrases “struggle for existence” and “survival of the fittest”
Used to describe natural selection, but can be misleading
Reproductive success
Is generally more subtle

40. Fitness Relative fitness
Contribution to the gene pool of the next generation
Relative fitness
Contribution of a genotype to the next generation

41. Selection
Favors certain genotypes by acting on the phenotypes of certain organisms

42. Directional selection
Favors individuals at one end of the phenotypic range
Disruptive selection
Favors extremes
Stabilizing selection
Favors intermediates

43. Phenotypes (fur color) Frequency of individuals
3 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

44. Diploidy
Maintains genetic variation hidden recessive alleles

45. Balancing selection
Natural selection maintains stable frequencies of diff. forms
 balanced polymorphism

46. Heterozygote Advantage
Sometimes heterozygous at a particular locus
 greater fitness
Natural selection
Will maintain two or more alleles

47. The sickle-cell allele
Causes mutations in hemoglobin but also confers malaria resistance
 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)

48. Frequency-Dependent Selection
Fitness of any morph declines if it becomes too common in the population

49. 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

50. Sexual selection Natural selection for mating success
sexual dimorphism, differences between the sexes

51. Intrasexual selection
Direct competition f/ mates

52. Intersexual selection
One sex (usually females) are choosy in selecting mates
 showiness of the male’s appearance
Figure 23.15

53. The Evolutionary Enigma of Sexual Reproduction
Produces fewer reproductive offspring than asexual reproduction,  handicap
Figure 23.16
Asexual reproduction
Female
Sexual reproduction
Male
Generation 1
Generation 2
Generation 3
Generation 4

54. Why has it persisted?
 genetic variation

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

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