1.
Natural selection, genetic drift, and gene flow can alter a population’s genetic composition

2. 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

3. Genetic Drift
Unpredictable fluctuation in alleles frequency from one generation to the next because of a population finite size.
Statistically, the smaller a sample
The greater the chance of deviation from a predicted result

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

5. 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

6. 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
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

7. Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
Figure 23.8 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.

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

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

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

11. Genetic Variation Seasonal differences in hormones
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
Seasonal differences in hormones

12. 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
The different forms can be called morphs.
Quantitative characters
Vary along a continuum within a population

13. Polymorphism Phenotypic polymorphism Genetic polymorphisms
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
Alleles of several loci affect the character
height

14. Measuring Genetic Variation
Population geneticists measure the number of polymorphisms in a population by determining the amount of heterozygosity
At the gene level
Average heterozygosity: measures the average percent of loci that are heterozygous in a population
At the molecular level: nucleotide variability
Average heterozygosity tends to be greater than nucleotide variability

15. 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
In the island of Madeira, house mice separated by mountains have evolve in isolation of each other.
Differences in Karyotypes. The patterns of fused chromosomes differ from one population to another.
These mutations leave the genes intact, the effects on the mice appear neutral.

16. Heights of yarrow plants grown in common garden
Some examples of geographic variation occur as a cline: a graded change in a trait along a geographic axis (parallels to the gradient in the environment)
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

17. 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

18. Evolutionary Fitness Reproductive success
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

19. In a more quantitative way: Relative fitness
Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
In a more quantitative way: Relative fitness
the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

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

21. 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

22. The three modes of selection
Directional selection
Disruptive selection
Stabilizing selection

23. 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.

24. 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.

25. 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.

26. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population
Diploidy (Eukaryotes are diploid)
Maintains genetic variation in the form of hidden recessive alleles

27. 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

28. 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

29. 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)

30. Frequency-Dependent Selection
The fitness of any morph declines if it becomes too common in the population

31. 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

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

33. Sexual Selection Is natural selection for mating success
Sexual dimorphism:
marked differences between the sexes in secondary sexual characteristics

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

35. 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

36. 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

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

38. Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Each species has a legacy of descent with modification. (birds 4 legs & wins?)
Adaptations are often compromises
Seals swim well, walk not as well
Chance and natural selection interact
Selection can only edit existing variations