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AP Biology Chapter 23 The Evolution of Populations

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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


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

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

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