1. AP Biology Chapter 23 The Evolution of Populations
Campbell and Reece 10th Edition

2. Individuals do not evolve, populations do over time
Medium Ground Finch from island of Daphne Major in Galápagos Islands
Long period of drought altered their food supply to mostly larger nuts & over the years those individuals with larger beaks were more successful

3. Overall size of bird b/4 & after drought years

4. Average beak size & size of individual birds larger after the drought so….
The medium ground finch population had evolved by natural selection

5. Genetic Variation as Cause of Evolution
Darwin reasoned that natural selection acted on genetic variation of populations
He knew nothing about genes
Few yrs later: Mendel’s paper on inheritance in pea plants: stage set for understanding variation

6. Genetic Variation Genotype inheritable, phenotypes are not
Example: moth, Nemoria arizonaria, appears very different eating oak flowers vs. oak leaves

7. Phenotype is result of : genotype + environmental influence

8. In general, only the genetically determined part of a phenotype can affect evolution
Discrete characters = “either/or” ( Mendel’s pea) = single gene
Most heritable variations involve quantitative characters: vary along a continuum ≥ 2 genes

9. Average Heterozygosity
way to quantify gene variability
average % of loci that are heterozygous
can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act  evolutionary change

10. Gel Electrophoresis used to calculate heterozygosity

11. Gel Electrophoresis
does not show silent mutations (DNA changes but still codes for same a.a.)

12. Gene Variation Between Populations
Geographic Variation
differences in genetic composition of separate populations

13. Cline: a graded change in a character along a geographic axis

14. Sources of Genetic Variation
Mutation
Gene Duplication
Sexual Reproduction
Other process that results in new alleles or new genes

15. Sources of genetic variation
Organisms with short life spans new genetic variants arise fairly rapidly

16. Formation of New Alleles
Mutations
can’t predict where in genome or what type mutation
for multicellular organisms only mutations in gametes cell line passed to new generations (most are in somatic cell line)
most point mutations silent or only slightly harmful, rarely are they beneficial

17. Altering Gene # or Position
chromosomal changes that delete, disrupt, or rearrange usually lethal or harmful
if genes left intact they may be neutral changes
Translocation:
Part of 1 chromosome breaks off & attaches to another chromosome

18. Translocations

19. Duplications of Chromosomes
if large segments duplicated usually harmful
duplications of small pieces may be beneficial
mutations accumulate over time
eventually that duplication takes on new role
end result: expanded genome

20. Rapid Reproduction
average mutation rate in plants & animals is considered low
~ 1 mutation in every
100,000 genes / generation

21. Prokaryotic Mutation Rates
shorter generation spans allows for generation of genetic variation in a population
virus populations, especially retroviruses process is fastest

22. HIV single stranded RNA: less complicated to duplicate
fewer RNA repair mechanisms in host cells

23. HIV
most effective treatment for a quickly mutating retrovirus has been combination protocols

24. Sexual Reproduction
most of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization

25. Hardy-Weinberg Equation can tell you if a Population is Evolving
1 of 3 factors that cause evolution must be at work in a population
Population: group of same species in same area that interbreed, with fertile offspring
presence of genetic variation does not guarantee that population is evolving

26. Populations examples of isolated populations:
Islands
Lakes
even populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups

27. Gene Pools
consists of all copies of every allele at every locus in all members of a population

28. Fixed Genes
if there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene
if there are 2 or more alleles for a locus then individuals may be homozygous or heterozygous

29. Each allele has a frequency in the population

30. The Hardy-Weinberg Principle
to test whether natural selection is acting on a particular locus:
Determine what the frequency would be if it were not evolving
Then compare that calculation with what you measure in the population
No difference: not evolving
difference: evolving

31. Hardy-Weinberg Principle 1908

32. Hardy-Weinberg Principle
states that the frequencies of alleles & genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work
If that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUM

33. Hardy-Weinberg Equilibrium

34. Hardy-Weinberg Equilibrium

35. assumes random mating

36. Hardy-Weinberg Problems
ench/lab8/intro.html
Problem 2:
If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

37. Answer to problem 2
2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)

38. Conditions for Hardy-Weinberg Equilibrium
No Mutations
Random Mating
No Natural Selection
Extremely Large Populations
No Gene Flow

39. Departure from any of the 5 conditions usually results in evolutionary changes
A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci

40. Applying the Hardy-Weinberg Principle
Can be used to estimate the frequency of a gene causing inherited disease in a population
Must assume:
No new mutations
Random mating
Ignore any effects of differential survival & reproductive success
No genetic drift

41. How Allele Frequencies are Altered in a Population

42. Conditions Necessary for H-W Equilibrium
No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) way
Random Mating: not usually significant
No Natural Selection cause most
Extremely Large Populations evolutionary
No Gene Flow change

43. NATURAL SELECTION
is based on differential success in survival & reproduction
if NS consistently favoring some alleles over others, NS can cause adaptive evolution (dfn: evolution that results in a better match between organisms & their environment)

44. GENETIC DRIFT
process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next
the smaller the population the more pronounced the effect

45. GENETIC DRIFT

46. 2 Examples of Genetic Drift
Founder Effect
genetic drift that occurs when a few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of original population

47. Founder Effect

48. Founder Effect Tristan da Cunha 15 British colonist in 1814

49. Tristan da Cunha
1 colonist carried recessive allele for retinitis pigmentosa

50. Tristan da Cunha
By late 1960’s, there were 240 descendants of the original founders
4 had retinitis pigmentosa
This frequency is 10x higher than frequency of retinitis pigmentosa in England

51. 2nd Example of Genetic Drift
2. Bottleneck Effect: occurs when the size of a population is reduced, as by a natural disaster or human actions. The resulting population is genetically different than original population.

52. Bottleneck Effect

54. Summarizing Effects of Genetic Drift
is significant in small populations
can cause allele frequencies to change at random
can lead to a loss of genetic variation w/in populations
can cause harmful alleles to become fixed

55. GENE FLOW
the transfer of alleles from one population to another as result of movement of fertile individuals or their gametes

56. Gene Flow

57. Gene Flow
transferred alleles may increase a population’s ability to adapt to local conditions
Culex pipiens spread of insecticide- resistant alleles used to treat mosquitoes to
prevent spread of West Nile

58. Natural Selection is only mechanism that consistently causes adaptive evolution
outcome of NS is not random
NS increases frequencies of alleles that provide reproductive advantage so, leads to adaptive evolution
NS acts directly on the phenotype & indirectly on the genotype

60. Relative Fitness
the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the population

61. Types of Selection DIRECTIONAL SELECTION
conditions favor individuals favoring one extreme of a phenotype
shifts curve in one direction or other

62. Types of Selection 2. DISRUPTIVE SELECTION
conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes

63. Types of Selection 3. STABILIZING SELECTION
conditions favor the intermediate phenotype and act against both extremes
reduces variation

64. ADAPTIVE EVOLUTION
Natural selection will increase the frequencies of alleles that enhances survival & reproduction so… over time adaptations arise
genetic drift & gene flow may cause changes that are either advantageous or disastrous

65. Sexual Selection
individuals with certain inherited characteristics are more likely to obtain mates
can result in sexual dimorphism: marked differences between the 2• sex characteristics

66. Sexual Dimorphism: House Finches

67. Mechanisms of Sexual Selection
Intrasexual Selection
selection w/in same sex
Alpha male
Intersexual Selection
aka mate choice
females choosy about their mate (often depends on male showiness)

68. Intrasexual Selection

69. Intersexual Selection: Sandpiper male

70. How do female preferences devlop?
1 hypothesis:
females have linked “good genes” with trait
study: gray tree frog
2nd hypothesis
females have linked “good health” with these traits
study: birds

71. The Preservation of Genetic Variation
neutral variation: differences in DNA that do not confer an advantage or disadvantage
Why don’t all genes move toward neutrality?
tendency for directional or stabilizing selection countered by mechanisms that preserve or restore variation

72. Diploidy recessive alleles hidden and carried forward in heterozygotes
heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but could bring benefits in environment changes

73. Advantages determined by Alleles

74. Balancing Selection
occurs when natural selection maintains 2 or more forms in a population
2 types:
heterozygote advantage
frequency-dependent selection

75. Heterozygote Advantage
Heterozygotes have survival advantage
If phenotype of a heterozygote is intermediate between the 2 homozygotes then this advantage is: stabilizing selection
If phenotype of heterozygote same as dominant homozygote this advantage is directional selection

76. Heterozygote Advantage
Example: Sickle Cell

77. Sickle Cell Allele SS Ss ss homozygous dominant
no protection against malaria Ss
heterozygous
protection against malaria
few sickle cells but not harmful ss
homozygous recessive
die young of sickle cell

78. Frequency-Dependent Selection
the fitness of a phenotype depends on how common it is in the population

79. Scale-Eating Fish in Lake Tanganyika
eat scales off flank of prey
some left- mouthed some right-mouthed
right-mouthed dominant to left- mouthed

80. Scale-eating Fish
selection favors whichever mouth phenotype is least common (prey fish learn to avoid attacks from more common

81. Why Natural Selection does not Result in a “Perfect” Organism
1. Selection can only act on existing variations
NS favors only the fittest available phenotypes
2. Evolution is limited by historical constraints
NS has to work with existing structures

82. 3. Adaptations are often compromises
each organisms must do many things: some structures are a compromise (Walrus fins great for swim, not so good for walking on rocks)
4. Chance, natural selection, & the environment interact
founding population may not carry “best” alleles for new environment; environments can change