BIOL 1107 - Chapter 20 - Dr. Yoga Sundram - PowerPoint PPT Presentation

About This Presentation
Title:

BIOL 1107 - Chapter 20 - Dr. Yoga Sundram

Description:

Populations above the waterfalls: low predation. Populations below the waterfalls: high predation ... Removed guppies from below the waterfalls (high predation) ... – PowerPoint PPT presentation

Number of Views:70
Avg rating:3.0/5.0
Slides: 55
Provided by: arle65
Learn more at: http://faculty.sgc.edu
Category:

less

Transcript and Presenter's Notes

Title: BIOL 1107 - Chapter 20 - Dr. Yoga Sundram


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
Darwins 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 experiencenot genetic
  • Darwins natural selection variation a result of
    preexisting genetic differences

6
Lamarcks 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
Godfrey H. Hardy English mathematicianWilhelm
Weinberg German physicianConcluded thatThe
original proportions of the genotypes in a
population will remain constant from generation
to generation as long as five assumptions are met
Hardy-Weinberg Principle
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 (pq)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
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 cells 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

15
(No Transcript)
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

18
(No Transcript)
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
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
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
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
Experimental Studies
Selection for increased speed in racehorses is no
longer effective
54
Experimental Studies
Phenotypic variation in insect ommatidia
Write a Comment
User Comments (0)
About PowerShow.com