Population Genetics and Natural Selection

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Population Genetics and Natural Selection

• Chapter 8

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Outline

• Darwin
• Gregor Mendel
• Variation Within Populations
• Plant Populations
• Animal Populations
• Hardy Weinberg
• Natural Selection
• Evolution

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Darwin

• 1835 Charles Darwin visited the Galapagos Islands
  and became convinced various populations evolved
  from ancestral form.
• 1838 After reading an essay by Thomas Malthus, he
  theorized some individuals would have a
  competitive advantage conferred by favorable
  characteristics.

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Darwins Theory of Natural Selection

• Organisms begat like organisms.
• Chance variation between individuals.
• Some are heritable.
• More offspring are produced each generation than
  can survive.
• Some individuals, because of physical or
  behavioral traits, have a higher chance of
  surviving than others in the same population.

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Gregor Mendel

• Augustinian Monk
• Studied garden pea (Pisum sativum).
• Discovered characteristics pass from parent to
  offspring in form of discrete packets called
  genes.
• Exist in alternate forms - alleles.
• Some prevent expression of others.

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Variation Within Populations

• Variation in Plant Populations
• Many plant species differ dramatically in form
  from one elevation to another.
• Clausen et.al. found evidence of adaptation by
  ecotypes to local environmental conditions in
  Potentilla glandulosa.
• Distinctive ecotypes.

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Variation in Plant Populations

• Molecular and Morphological Information
• Hansen et. al. used randomly amplified
  polymorphic DNA (RAPD) along with morphological
  data to support separation of three species of
  Potentilla.

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Variation in Animal Populations

• Chuckwalla (Sauromalus obesus)
• Herbivorous lizard in desert SW.
• Variation in rainfall translates into variation
  in food availability.
• Case found lizards from food-rich higher
  elevations were approx 25 longer and 2x body
  weight of those from lower elevations.

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Variation in Animal Populations

• Genetic Variation in Alpine Fish
• Movement of cold adapted aquatic species into the
  headwaters of glacial valleys that lace the Alps
  created clusters of geographically isolated
  populations.
• Douglas and Brunner used microsatellite DNA to
  conclude Coregonus populations are highly diverse
  and exhibit a high level of differentiation.

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Hardy Weinberg

• Hardy Weinberg principle states that in a
  population mating at random in the absence of
  evolutionary forces, allele frequencies will
  remain constant.
• (pq)2 p22pqq2

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Calculating Gene Frequencies

• SS (81) SA (18) AA (1)
• Frequency of S allele ?
• SS 1/2SA .81 ½(.18) .90
• (.90)2 2(.9x.1) (.10)2 1.0

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Conditions Necessary for Hardy Weinberg

• Random Mating
• No Mutations
• Large Population Size
• No Immigration
• Equitable Fitness Between All Genotypes
• Likely, at least one of these will not be met and
  allele frequencies will change.
• Potential for evolutionary change in natural
  populations is very great.

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Change Due To Chance

• Random processes such as genetic drift can change
  gene frequencies in populations, especially in
  small populations.
• Major concern of habitat fragmentation is
  reducing habitat availability to the point where
  genetic drift will reduce genetic diversity
  within natural populations.

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Natural Selection

• Some individuals in a population, because of
  their phenotypic characteristics, produce more
  offspring that themselves live to reproduce.
• Natural selection can favor, disfavor, or
  conserve the genetic make-up of a population.

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Stabilizing Selection

• Stabilizing selection acts to impede changes in a
  population by acting against extreme phenotypes
  and favoring average phenotypes.

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Directional Selection

• Directional selection leads to changes in
  phenotypes by favoring an extreme phenotype over
  other phenotypes in the population.

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Disruptive Selection

• Disruptive selection creates bimodal
  distributions by favoring two or more extreme
  phenotypes over the average phenotype in a
  population.

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Evolution by Natural Selection

• Natural selection, which changes genotypic and
  phenotypic frequencies in populations, can result
  in adaptation to the environment.
• Depends on heritability of trait.
• h2 VG / VP
• VG Genetic variance
• VP Phenotypic variance

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Adaptive Change in Colonizing Lizards

• Losos et.al.
• Genus Anolis
• Great diversity includes large amount of
  variation in size and body proportions.
• Length of hind limbs appears to reflect selection
  for effective use of vegetation.
• Diameter of perching surfaces.

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Rapid Adaptation by Soapberry Bugs

• Carroll and Boyd
• Soapberry Bug (Jadera haematoloma) feeds on seeds
  from family Sapindaceae.
• Slender beaks to pierce fruit walls.
• Distance from outside fruit wall to seeds varies
  widely - beak length should be under selection.
• Found close relationship between fruit radius and
  beak length.

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Evidence of Genetic Drift in Chihuahua Spruce

• Picea chihuahuana now restricted to peaks of
  Sierra Madre Occidental in N. Mexico.
• Ledig et.al. examined populations to determine if
  the species has lost genetic diversity as a
  consequence of reduced population size.
• Found significant positive correlation between
  population size and genetic diversity of study
  populations.

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Genetic Variation In Island Populations

• In general, genetic variation is lower in
  isolated and generally smaller, island
  populations.
• Reduced genetic variation indicates a lower
  potential for a population to evolve.

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Genetic Diversity and Butterfly Extinctions

• Frankham and Ralls point out inbreeding may be a
  contributor to higher extinction rates in small
  populations.
• Reduced fecundity, depressed juvenile survival,
  shortened life-span.
• Saccheri conducted genetic studies on populations
  of Glanville fritillary butterflies (Melitacea
  cinxia).
• Populations with highest levels of inbreeding had
  highest probabilities of extinction.

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Review

• Darwin
• Gregor Mendel
• Variation Within Populations
• Plant Populations
• Animal Populations
• Hardy Weinberg
• Natural Selection
• Evolution

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