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

2. In nature, what evolves? Individuals Populations Gene pool
Individuals
Populations
Gene pool
Genetic drift 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

3. In nature, what evolves? The strongest The smartest The “fittest”
The strongest
The smartest
The “fittest”
The biggest 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

4. What is considered being biologically fit?
Being adaptable
Eating the most
Strongest
Most intelligent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

5. 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

6. Genetic variations in populations
Contribute to evolution
Figure 23.1

7. The change in the genetic makeup of a population from generation to generation is
Mutation
Hardy-Weinberg
Macroevolution
Microevolution 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

8. In nature, what evolves? The strongest The smartest The “fittest”5
The strongest
The smartest
The “fittest”
The biggest 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

9. 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

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

11. 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

12. 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

13. In Hardy-Weinberg, what does p2 represent?
Homozygous dom.
Homozygous rec.
Heterozygous
None of the above 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

14. In Hardy-Weinberg, what does q2 represent?
Homozygous dom.
Homozygous rec.
Heterozygous
None of the above 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

15. In Hardy-Weinberg, what does 2pq represent?
Homozygous dom.
Homozygous rec.
Heterozygous
None of the above 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

16. 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

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

18. 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

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

20. 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

21. 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

22. 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

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. 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

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

26. 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

27. 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.

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

29. 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

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

31. 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

32. 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

33. 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

34. 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

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

36. 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

37. 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