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Types of Natural Selection

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Microevolution evolution at the population level = change in allele frequencies over generations Homozygotes for sickle mutation (HsHs): lethal Sickle Cell Anemia ... – PowerPoint PPT presentation

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Title: Types of Natural Selection


1
  • Types of Natural Selection
  • Microevolution evolution at the population
    level change in allele frequencies over
    generations

2
  • Genetics
  • science dealing with inheritance or heredity,
    the transmission of acquired traits

3
Ultimate source of heritable variation is
change in DNA
  • Change in DNA caused by 1) Mutation 2) Genetic
    Recombination

4
Mutations
  • change in genotype other than by recombination.
  • Three types
  • 1) Point Mutations 2) Chromosome Mutations 3)
    Change in Chromosome Number

5
1) Point Mutation
  • Change in a single DNA Nucleotide.

Point mutation rate per gene 1 in 100,000
gametes. In humans 1 mutation/gene x
(25,000 genes) 100,000 gametes 0.25 point
mutations/gamete
6
E.g., human hemoglobin
  • 2 alpha chains (141 amino acids)
  • 2 beta chains (146 amino acids)
  • 1973 sampling of population (thousands) 169
    mutation types recorded
  • 62 substitutions in alpha
  • 99 substitutions in beta
  • 1 deletion in alpha
  • 7 deletions in beta
  • 1 in 2,000 people have mutant hemoglobin gene.

hemoglobin
7
2) Chromosome Mutations
  • Rearrangements (including losses and gains) of
    large pieces of DNA. E.g., inversion

A B C D E F G A B F
E D C G
Re-attaches here and here
3 of pop. of Edinburgh, Scotland have inversion
in Chromosome 1 Humans differ from chimps by
6 inversions, from gorillas by 8 (also difference
in chromosome number)
8
3) Change in Chromosome No.
  • a) Aneuploidy - change in chromosome number of
    less than an entire genome.

Horse (2n 64) versus donkey (2n 62) Humans
(2n 46) versus chimp or gorilla (2n 48) Some
Genetic Diseases Trisomy (addition of a
chromosome to the original diploid pair) of
chromosome 21 in humans Down's syndrome.
Extra or one sex chromosomes ( e. g., XYY, XXY,
X).
9
b) Polyploidy
  • Evolution of chromosome number which is a
    multiple of some ancestral set.
  • Has been a major mechanism of evolution in
    plants.

10
Two ways polyploidy can occur
11
Polyploid evolution of wheat
12
Genetic Recombination (in sexual reproduction)
  • Natural, shuffling of existing genes, occurring
    with meiosis and sexual reproduction
  • Two types
  • Independent Assortment
  • Crossing over

13
Independent assortment
  • Sorting of homologous chromosomes independently
    of one another during meiosis
  • E. g., (where A,B,C genes are unlinked)
  • AaBBcc X AabbCC ---gt AaBbCc (one of
    many possibilities)

14
Independent assortment
  • Results in great variation of gametes, and
    therefore progeny.
  • E. g., one human 223 8,388,608 possible types
    of gametes (each with different combination of
    alleles).

15
Crossing over
  • Exchange of chromatid segments of two adjacent
    homologous chromosomes during meiosis (prophase).
  • Greatly increases variability of gametes and,
    therefore, of progeny.

16
Genetic Variation
  • Genetic recombination - source of most variation
    (in sexual organisms), via new allele
    combinations.
  • Mutation - ultimate source of variation, source
    of new alleles and genes.

17
Fitness
  • measure of the relative contribution of a given
    genotype to the next generation
  • Can measure for individual or population.

18
Fitness
  • allele/genotype freq. in future generation
    allele/genotype freq. in prev. generation
  • E. g., 1st gen. 25AA 50Aa 25aa freq. A
    25 .5(50) 50
  • 2nd gen. 36AA 48Aa 16aa freq. A
    36 .5(48) 60
  • Fitness of A allele is 60/50 1.2 a is 40/50
    0.8
  • Fitness of AA genotype is 36/25 1.44 , etc.

19
Hardy-Weinberg Equilibrium (1908)
  • The frequency of a gene / allele does not change
    over time (given certain conditions). A,a
    alleles of one gene, combine as AA, Aa, or aa
  • Generation 1 p freq. A q freq. a p
    q 1 (100)
  • pA qa pA p2AA pqAa qa pqAa q2aa


gene frequencies in generation 1
p2AA 2pqAa q2aa 1
20
Hardy-Weinberg Equilibrium (1908)
  • Example
  • Generation 1 p 0.4 q 0.6 p q
    1 (100)
  • 0.4A 0.6a 0.4A 0.16AA 0.24Aa 0.6a 0.24Aa 0.
    36aa


gene frequencies in generation 1
p2AA 2pqAa q2aa 0.16 0.48 0.36
1
21
Hardy-Weinberg Equilibrium (1908)
  • The frequency of a gene / allele does not change
    over time (given certain conditions). What
    will be the frequency of alleles in the second
    generation?
  • p2AA 2pqAa q2aa 1
  • freq. A (generation 2) (p2 pq) / (p2 2pq
    q2)
  • p(p q) / (p q)2 p
    / (p q) p
  • Therefore, freq. A p freq. a q, same as in
    generation 1.


gene frequencies in generation 1
22
Hardy-Weinberg Equilibrium
  • Maintained only if
  • 1) No mutation Mutations rare, but do occur
    (1 new mutation in 10,000 - 1,000,000 genes per
    individual per generation)

23
Hardy-Weinberg Equilibrium
  • 2) No migration (no gene flow into or out of
    population) But, can occur . . .

24
Hardy-Weinberg Equilibrium
  • 3) Population size large
  • Two things can disrupt
  • a) Population bottleneck (large pop. gets very
    small)
  • b) Founder effect (one or a few individuals
    dispersed from a large pop.)

25
Hardy-Weinberg Equilibrium
  • 4) Mating is random
  • But, most animals mate selectively, e.g.,
  • 1) harem breeding (e. g., elephant seals)
  • 2) assortative mating (like mates with like)
  • 3) sexual selection

26
Hardy-Weinberg Equilibrium
  • 5) All genotypes equally adaptive (i.e., no
    selection)
  • But, selection does occur . . .

27
If any conditions of Hardy-Weinberg not met
  • Genotype frequencies change
  • Evolution occurs!
  • Evolution change in gene frequency of a
    population over time.

28
Selective Pressure
  • agent or causative force that results in
    selection.
  • E. g., for dark skin, selective pressure UV
    radiation (UV increases sunburn and skin cancer
    in lighter skinned individuals)
  • E. g., for light skin, selective pressure
    Vitamin D synthesis

29
Genetic Drift
  • change in genotype solely by chance effects
  • random!
  • promoted by
  • Population Bottleneck -drastic reduction in
    population size
  • Founder Effect - isolated colonies founded by
    small no. individuals

30
Population Bottleneck
Fig. 23-9
31
Summary Evolution can occur by two major
mechanisms
  • Natural Selection (non-random)
  • Genetic Drift (random)

32
Pepper Moth Biston betularia Selective
pressurepredation by birds
Single gene AA/Aa dark aa light
Camoflague selected for!
33
Result Balanced polymorphism
  • E.g., Sickle Cell Anemia Mutation single
    amino acid subst. in beta chain of hemoglobin --gt
    single a.a. difference.
  • Sickle blood cells
  • Normal blood cells

34
  • Sickle Cell Anemia
  • Homozygotes for sickle mutation (HsHs) lethal

35
  • Sickle Cell Anemia
  • Heterozygotes (HsHn) resistant to malaria,
  • selected for in malaria-infested regions,
  • selected against where malaria not present.

36
General Principle
  • Selection dependent on the environment!
  • If environmental conditions change, selective
    pressure can change!!

37
  • Stabilizing selection - selection against the
    two extremes in a population (e.g., birth weight
    in humans, clutch size in birds)

38
  • Directional selection - selection for one
    extreme in a population, against the other
    extreme (e.g., pesticide resistance in
    insects antibiotic resistance in bacteria)

39
  • Disruptive selection - selection for the two
    extremes in a population, against the average
    forms (e.g., limpets w/ 2 color forms light
    dark in mosaic environment flies on two hosts
    apple hawthorn)
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