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

2. Overview: The Smallest Unit of Evolution
One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only populations evolve
Microevolution is a change in allele frequencies in a population over generations

3. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

4. Fig. 23-2
(a)
(b)
Figure 23.2 Nonheritable variation

5. 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
8.11
9.12
10.16
13.17 19 XX
Figure 23.3 Geographic variation in isolated mouse populations on Madeira 1
2.19
3.8
4.16
5.14
6.7
9.10
11.12
13.17
15.18 XX

6. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

7. Ldh-B b allele frequency
Fig. 23-4
1.0
0.8
0.6
Ldh-B b allele frequency
0.4
0.2
Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30
Latitude (°N)
Maine
Cold (6°C)
Georgia
Warm (21°C)

8. Animation: Genetic Variation from Sexual Recombination
Mutation
Mutations are changes in the nucleotide sequence of DNA
Mutations cause new genes and alleles to arise
Only mutations in cells that produce gametes can be passed to offspring
Animation: Genetic Variation from Sexual Recombination

9. A point mutation is a change in one base in a gene
The effects of point mutations can vary:
Mutations in noncoding regions of DNA are often harmless
Mutations in a gene might not affect protein production because of redundancy in the genetic code

10. The effects of point mutations can vary:
Mutations that result in a change in protein production are often harmful
Can you name an example?
Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

11. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of large chromosome segments is usually harmful
Duplication of small pieces of DNA is sometimes less harmful and increases the genome size

12. Mutation Rates
Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutations rates are often lower in prokaryotes and higher in viruses – as evidenced by the number of flu and cold viruses that affect us

13. Sexual Reproduction
Sexual reproduction can shuffle existing alleles into new combinations through crossing over, independent assortment of chromosomes,

14. Gene Pools and Allele Frequencies
A population is a localized group of individuals capable of interbreeding and producing fertile offspring
Porcupine herd and fortymile herd of caribou – same species, do not interbreed readily
A gene pool consists of all the alleles for all loci in a population

15. Porcupine herd Porcupine herd range Fortymile herd range
Fig. 23-5
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON
Fortymile herd

16. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

17. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time

18. Alleles in the population Frequencies of alleles
Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg:
Each sperm:
CR allele = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele = 0.2
Figure 23.6 Selecting alleles at random from a gene pool

19. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
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
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

20. The five conditions for nonevolving populations are rarely met in nature:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow

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

22. Animation: Causes of Evolutionary Change
Genetic Drift
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next
Genetic drift tends to reduce genetic variation through losses of alleles
Founder Effect
Bottleneck Effect
Animation: Causes of Evolutionary Change

23. The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population
Ex: Ellis-van Creveld Syndrome

24. The Bottleneck Effect
The bottleneck effect is a sudden reduction in population size due to a change in the environment
The resulting gene pool may no longer be reflective of the original population’s gene pool
If the population remains small, it may be further affected by genetic drift

25. Original population Bottlenecking event Surviving population
Fig. 23-9
Figure 23.9 The bottleneck effect
Original
population
Bottlenecking
event
Surviving
population

26. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Understanding the bottleneck effect can increase understanding of how human activity affects other species
Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

27. Fig
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Number
of alleles
per locus
Percentage
of eggs
hatched
Population
size
Location
Illinois
5.2
3.7 93
<50
1930–1960s
1,000–25,000
1993
<50
Figure Bottleneck effect and reduction of genetic variation
Kansas, 1998
    (no bottleneck)
750,000
5.8 99
Nebraska, 1998
    (no bottleneck)
75,000–
200,000
5.8 96
Minnesota, 1998
    (no bottleneck)
4,000
5.3 85
(b)

28. Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck
The results showed a loss of alleles at several loci
Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

29. Gene Flow
Gene flow consists of the movement of alleles among populations
Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce differences between populations over time
Gene flow is more likely than mutation to alter allele frequencies directly

30. Fig
Figure Gene flow and human evolution

31. Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Only natural selection consistently results in adaptive evolution
Natural selection brings about adaptive evolution by acting on an organism’s phenotype

32. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Selection favors certain genotypes by acting on the phenotypes of certain organisms

33. Directional, Disruptive, and Stabilizing Selection
Three modes of selection:
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

34. Frequency of individuals
Fig
Original population
Frequency of individuals
Phenotypes (fur color)
Original
population
Evolved
population
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

35. Stabilizing Selection

36. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

38. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

39. Heterozygote Advantage
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

40. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Fig
Frequencies of the
sickle-cell allele
0–2.5%
Figure Mapping malaria and the sickle-cell allele
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
7.5–10.0%
10.0–12.5%
>12.5%

41. Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact

42. Fig
Figure Evolutionary compromise