Evolution and Population Genetics

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Evolution and Population Genetics

• Evidence for evolutionary change
• Mechanics and
• Hardy-Weinberg Calculations

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Evidence for Evolution

• 1. Fossil record.
• Material imprinted in rock
• Law of superposition older fossils on bottom
• 2. Anatomical evidence
• Homologous structures similar bone structure but
  different function. Ex bat wing, whale flipper,
  human arm
• Vestigial structures no longer functional but
  retained in anatomy Ex appendix, wisdom teeth
• Modified structures adapted for new function Ex
  Pandas thumb wrist bones modified to strip
  leaves from bamboo
• Embryologic similarities
• 3. Molecular evidence
• Shared sequences of bases in DNA

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• Evidence for evolution (continued)
• Adaptive radiation
• Ancestral pioneer arrives at new habitat.
• New species evolve speciation
• By occupying different niches, they reduce
  competition.
• Example Darwins finches 13 different but
  related species on the Galapagos Islands

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Fossils
Archaeopteryx (oldest known fossil bird)
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LAW OF SUPERPOSITION
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Why does a whale have a kneecap?
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The Evolution of Populations

• Remember individual organisms do not evolve.
  Individuals are selected, but it is populations
  that evolve.
• Because evolution occurs when gene pools change
  from one generation to the next, understanding
  evolution require us to understand population
  genetics.

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Some terminology

• Population All the members of one species living
  in single area.
• Gene pool the collection of genes in a
  population. It includes all the alleles of all
  genes in the population.

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Some terminology

• If all individuals in a population all have the
  same allele for a particular gene that allele is
  said to be fixed in the population.
• If there are 2 or more alleles for a given gene
  in the population then individuals may be either
  homozygous or heterozygous (i.e. have two copies
  of one allele or have two different alleles)

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Detecting evolution in nature

• Evolution is defined as changes in the structure
  of gene pools from one generation to the next.
• How can we tell if the gene pool changes from one
  generation to the next?
• We can make use of a simple calculation called
  the Hardy-Weinberg Equilibrium

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Hardy-Weinberg Equilibrium

• In meiosis, individuals alleles are sorted into
  gametes (sperm or egg) which may combine to form
  a zygote.
• A population of 100 organisms has a minimum of
  200 alleles for each trait, one from each parent.
• To determine probability of two independent
  events (sorting of parental alleles), multiply
  the probabilities of individual events.
• If 80 of the alleles in a gene pool are A and
  20 are a, what are the probabilities for each
  genotype (AA, Aa, and aa)?

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Hardy-Weinberg Equilibrium

• If 80 of the alleles in the gene pool are A and
  20 are a, we can predict the genotypes in the
  next generation.
• Basic probability To determine the probability
  of two independent events both occurring, you
  should multiply the probabilities of the
  individual events together.

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Hardy-Weinberg Equilibrium

• Probability of an AA individual is 0.80.8 0.64
• Probability of an aa individual is 0.20.2 0.04
• Probability of an Aa individuals is 0.20.8
  0.16, but there are two ways to produce an Aa
  individual so 0.162 0.32.
• Note the probabilities add up to 1 (100)

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General formulae

• Allele frequencies
• p q 1.
• Genotypic frequencies
• p2 2pq q2 1, where p is frequency
  of one allele and q is frequency of the other
  allele. NOTE p2 and q2 are homozygous and pq is
  heterozygous

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Hardy-Weinberg Equilibrium

• Hardy-Weinberg equilibrium can be used to
  estimate allele frequencies from information
  about phenotypes and genotypes.

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Hardy-Weinberg Conditions

• No gene flow movement of individuals to/from the
  population
• Random mating no significant preference when
  choosing mate. Increases homozygote genotype.
• Large population size reduce effect of small,
  chance events (genetic drift bottleneck or
  founder effect)
• No natural selection which would otherwise select
  for or against a particular genotype.
• No mutations.

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Bottleneck Effect

• The bottleneck effect occurs when some disaster
  causes a dramatic reduction in population size.
• As a result, by chance certain alleles may be
  overrepresented in the survivors, while others
  are underrepresented or eliminated. Genetic
  drift while the population is small may lead to
  further loss or fixation of alleles.
• Humans have been responsible for many bottlenecks
  by driving species close to extinction.

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Bottleneck Effect

• The Northern Elephant seal population for example
  was reduced to about 20 individuals in the
  1890s. Population now gt30,000, but an
  examination of 24 genes found no variation, i.e.
  there was only one allele. Southern Elephant
  Seals in contrast show lots of genetic variation.

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Founder Effect

• Populations founded by only a few individuals
  (ex island communities)
• Gene pool is unlikely to be as diverse as the
  source pool from which it was derived.
• Example polydactylism (having extra fingers) is
  common among the Amish
• Founder effect coupled with inbreeding explains
  the high incidences of certain recessive diseases
  among humans in many isolated island communities.

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Hardy-Weinberg Equilibrium

• If a population is found to depart significantly
  from Hardy-Weinberg equilibrium this is strong
  evidence that evolution is taking place.

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Natural Selection the primary mechanism of
adaptive evolution

• Primary mechanism of adaptive evolution
• survival of the fittest means individuals
  reproduce thus contributing genes to the next
  generation
• May depend on differences in ability to gather
  food, hide from predators, or tolerate extreme
  temperatures, which all may enhance survival and
  ultimately reproduction

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Natural Selection the primary mechanism of
adaptive evolution

• Three major forms of natural selection
• Directional
• Disruptive
• Stabilizing

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Directional Selection

• Favors one extreme in the population
• Average value in population moves in that
  direction
• E.g. Selection for darker fur color in an area
  where the background rocks are dark

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Disruptive selection

• Intermediate forms are selected against.
  Extremes are favored
• E.g. Pipilo dardanus butterflies. Different
  forms of the species mimic the coloration of
  different distasteful butterflies.
• Crosses between forms are poor mimics and so are
  selected against by being eaten by birds.

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Stabilizing Selection

• Most common
• Extreme forms are selected against
• Example Human birth weights. Highest survival
  is at intermediate birth weights.
• Babies that are too large cannot fit through the
  birth canal, babies that are born too small are
  not well developed enough to survive

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