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

2. 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

3. Genetic variations in populations
Contribute to evolution
Figure 23.1

4. Where does variation come from?
Mutation and sexual recombination produce the variation that makes evolution possible
Produce the variation in gene pools that contributes to differences among individuals

5. 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

6. In sexually reproducing populations, sexual recombination
Is far more important than mutation in producing the genetic differences that make adaptation possible

7. Altering allele frequencies in a population
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow

8. Genetic drift
Chance events cause allele frequencies to fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation
More likely in small populations
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

9. The Bottleneck Effect (one example of genetic drift)
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

10. 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.

11. The Founder Effect (a second example of genetic drift)
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population

12. 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

13. Natural Selection
Natural selection is the primary mechanism of adaptive evolution
From the range of variations available in a population natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations.

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

15. 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

16. 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

17. Sexual Selection Sexual selection
Is natural selection for mating success
Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

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

19. 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

20. 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

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

23. The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population

24. Diploidy
Diploidy
Maintains genetic variation in the form of hidden recessive alleles

25. Balancing Selection 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

26. 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

27. 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)

28. Frequency-Dependent Selection In frequency-dependent selection
The fitness of any morph declines if it becomes too common in the population

29. 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

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

31. Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Adaptations are often compromises
Chance and natural selection interact
Selection can only edit existing variations