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

2. 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

3. Genetic variations in populations
Contribute to evolution
Figure 23.1

4. 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

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

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

7. 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

8. 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

9. Mendelian inheritance
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
Mendelian inheritance
Preserves genetic variation in a population

10. Two processes, mutation and sexual recombination
Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible
Two processes, mutation and sexual recombination
Produce the variation in gene pools that contributes to differences among individuals

11. 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

12. Point Mutations A point mutation Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact

13. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial

14. 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

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

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

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

18. Genetic Variation Genetic variation
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

19. 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
Quantitative characters
Vary along a continuum within a population

20. Phenotypic polymorphism
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

21. 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

22. 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

23. 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

24. 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

25. 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

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

27. 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

28. 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

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

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

31. 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

32. 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

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

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

35. sexual 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. Sexual selection - a “special case” of natural selection.
Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate.
Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances, and some species deliver persuasive gifts. After all, what female Mormon cricket (bottom right) could resist the gift of a juicy sperm-packet? Going to even more extreme lengths, the male redback spider (bottom left) literally flings itself into the jaws of death in order to mate successfully.
Sexual selection is often powerful enough to produce features that are harmful to the individual’s survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex.

37. Selection is a two-way street.
Sexual selection usually works in two ways, although in some cases we do see sex role reversals:
Male competition Males compete for access to females, the amount of time spent mating with females, and even whose sperm gets to fertilize her eggs. For example, male damselflies scrub rival sperm out of the female reproductive tract when mating.
Female choice Females choose which males to mate with, how long to mate, and even whose sperm will fertilize her eggs. Some females can eject sperm from an undesirable mate.

38. 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

39. Mechanisms: The Processes of Evolution

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

41. Why Natural Selection Cannot Fashion Perfect Organisms?

42. 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
Environment, hence selection forces changes