Types of Natural Selection

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

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

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Ultimate source of heritable variation is
change in DNA

• Change in DNA caused by1) Mutation2) Genetic
  Recombination

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Mutations

• change in genotype other than by recombination.
  
• Three types
• 1) Point Mutations2) Chromosome Mutations3)
  Change in Chromosome Number

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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
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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
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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)
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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).
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b) Polyploidy

• Evolution of chromosome number which is a
  multiple of some ancestral set.
• Has been a major mechanism of evolution in
  plants.

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Two ways polyploidy can occur
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Polyploid evolution of wheat
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Genetic Recombination(in sexual reproduction)

• Natural, shuffling of existing genes, occurring
  with meiosis and sexual reproduction
• Two types
• Independent Assortment
• Crossing over

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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)

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Independent assortment

• Results in great variation of gametes, and
  therefore progeny.
• E. g., one human223 8,388,608 possible types
  of gametes (each with different combination of
  alleles).

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Crossing over

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

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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.

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Fitness

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

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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.

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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
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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
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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
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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)

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Hardy-Weinberg Equilibrium

• 2) No migration (no gene flow into or out of
  population)But, can occur . . .

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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.)

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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

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Hardy-Weinberg Equilibrium

• 5) All genotypes equally adaptive (i.e., no
  selection)
• But, selection does occur . . .

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If any conditions of Hardy-Weinberg not met

• Genotype frequencies change
• Evolution occurs!
• Evolution change in gene frequency of a
  population over time.

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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

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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

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Population Bottleneck
Fig. 23-9
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Summary Evolution can occur by two major
mechanisms

• Natural Selection (non-random)
• Genetic Drift (random)

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Pepper Moth Biston betulariaSelective
pressurepredation by birds
Single geneAA/Aa darkaa light
Camoflague selected for!
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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

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• Sickle Cell Anemia

• Homozygotes for sickle mutation (HsHs) lethal

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• Sickle Cell Anemia

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

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General Principle

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

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

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

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• 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)