1. Genes Within Populations
Chapter 20

2. Genetic Variation and Evolution
Darwin: Evolution is descent with modification
Evolution: changes through time
Species accumulate difference
Descendants differ from their ancestors
New species arise from existing ones

3. Natural selection: mechanism of evolutionary change
Natural selection: proposed by Darwin as the mechanism of evolution
individuals have specific inherited characteristics
they produce more surviving offspring
the population includes more individuals with these specific characteristics
the population evolves and is better adapted to its present environment

4. Darwin’s theory for how long necks evolved in giraffes

5. Natural selection: mechanism of evolutionary change
Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck
Individuals passed on physical and behavioral changes to their offspring
Variation by experience…not genetic
Darwin’s natural selection: variation a result of preexisting genetic differences

6. Lamarck’s theory of how giraffes’ long necks evolved

7. Gene Variation in Nature
Measuring levels of genetic variation
blood groups
enzymes
Enzyme polymorphism
A locus with more variation than can be explained by mutation is termed polymorphic.
Natural populations tend to have more polymorphic loci than can be accounted for by mutation.
DNA sequence polymorphism

8. Hardy-Weinberg Principle
Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met

9. Hardy-Weinberg Principle
Five assumptions :
No mutation takes place
No genes are transferred to or from other sources
Random mating is occurring
The population size is very large
No selection occurs

10. Hardy-Weinberg Principle
Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2
p = individuals homozygous for first allele
2pq = individuals heterozygous for both alleles
q = individuals homozygous for second allele
because there are only two alleles: p plus q must always equal 1

11. Hardy-Weinberg Principle

12. Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict frequencies in subsequent generations

13. Five agents of evolutionary change
A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population
Five agents of evolutionary change

14. Agents of Evolutionary Change
Mutation: A change in a cell’s DNA
Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
Ultimate source of genetic variation
Gene flow: A movement of alleles from one population to another
Powerful agent of change
Tends to homogenize allele frequencies

16. Agents of Evolutionary Change
Nonrandom Mating: mating with specific genotypes
Shifts genotype frequencies
Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals
Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes

17. Genetic Drift
Genetic drift: Random fluctuation in allele frequencies over time by chance
important in small populations
founder effect - few individuals found new population (small allelic pool)
bottleneck effect - drastic reduction in population, and gene pool size

19. Genetic Drift: A bottleneck effect

20. Bottleneck effect: case study

21. Selection
Artificial selection: a breeder selects for desired characteristics

22. Selection
Natural selection: environmental conditions determine which individuals in a population produce the most offspring
3 conditions for natural selection to occur
Variation must exist among individuals in a population
Variation among individuals must result in differences in the number of offspring surviving
Variation must be genetically inherited

23. Selection

24. Pocket mice from the Tularosa Basin
Selection
Pocket mice from the Tularosa Basin

25. Selection to match climatic conditions
Enzyme allele frequencies vary with latitude
Lactate dehydrogenase in Fundulus heteroclitus (mummichog fish) varies with latitude
Enzymes formed function differently at different temperatures
North latitudes: Lactate dehydrogenase is a better catalyst at low temperatures

26. Selection for pesticide resistance

27. Fitness and Its Measurement
Fitness: A phenotype with greater fitness usually increases in frequency
Most fit is given a value of 1
Fitness is a combination of:
Survival: how long does an organism live
Mating success: how often it mates
Number of offspring per mating that survive

28. Fitness and its Measurement
Body size and egg-laying in water striders

29. Interactions Among Evolutionary Forces
Mutation and genetic drift may counter selection
The magnitude of drift is inversely related to population size

30. Interactions Among Evolutionary Forces
Gene flow may promote or constrain evolutionary change
Spread a beneficial mutation
Impede adaptation by continual flow of inferior alleles from other populations
Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow
High in birds & wind-pollinated plants
Low in sedentary species

31. Interactions Among Evolutionary Forces
Degree of copper tolerance

32. Maintenance of Variation
Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population
Negative frequency-dependent selection: rare phenotypes are favored by selection
Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population
Strength of selection changes through time

33. Maintenance of Variation
Negative frequency - dependent selection

34. Maintenance of Variation
Positive frequency-dependent selection

35. Maintenance of Variation
Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time
Galápagos Islands ground finches
Wet conditions favor big bills (abundant seeds)
Dry conditions favor small bills

36. Maintenance of Variation
Fitness of a phenotype does not depend on its frequency
Environmental changes lead to oscillation in selection

37. Maintenance of Variation
Heterozygotes may exhibit greater fitness than homozygotes
Heterozygote advantage: keep deleterious alleles in a population
Example: Sickle cell anemia
Homozygous recessive phenotype: exhibit severe anemia

38. Maintenance of Variation
Homozygous dominant phenotype: no anemia; susceptible to malaria
Heterozygous phenotype: no anemia; less susceptible to malaria

39. Maintenance of Variation
Frequency of sickle cell allele

40. Maintenance of Variation
Disruptive selection acts to eliminate intermediate types

41. Maintenance of Variation
Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa

42. Maintenance of Variation
Directional selection: acts to eliminate one extreme from an array of phenotypes

43. Maintenance of Variation
Directional selection for negative phototropism in Drosophila

44. Maintenance of Variation
Stabilizing selection: acts to eliminate both extremes

45. Maintenance of Variation
Stabilizing selection for birth weight in humans

46. Experimental Studies of Natural Selection
In some cases, evolutionary change can occur rapidly
Evolutionary studies can be devised to test evolutionary hypotheses
Guppy studies (Poecilia reticulata) in the lab and field
Populations above the waterfalls: low predation
Populations below the waterfalls: high predation

47. Experimental Studies
High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.

48. The evolution of protective coloration in guppies
Experimental Studies
The evolution of protective coloration in guppies

49. Experimental Studies The laboratory experiment 10 large pools
2000 guppies
4 pools with pike cichlids (predator)
4 pools with killifish (nonpredator)
2 pools as control (no other fish added)
10 generations

50. Experimental Studies The field experiment
Removed guppies from below the waterfalls (high predation)
Placed guppies in pools above the falls
10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies

51. Evolutionary change in spot number
Experimental Studies
Evolutionary change in spot number

52. The Limits of Selection
Genes have multiple effects
Pleiotropy: sets limits on how much a phenotype can be altered
Evolution requires genetic variation
Thoroughbred horse speed
Compound eyes of insects: same genes affect both eyes
Control of ommatidia number in left and right eye

53. Selection for increased speed in racehorses is no longer effective
Experimental Studies
Selection for increased speed in racehorses is no longer effective

54. Phenotypic variation in insect ommatidia
Experimental Studies
Phenotypic variation in insect ommatidia