Population Genetics and Natural Selection

1
Population Genetics and Natural Selection

• Chapter 8

2
Outline

• Darwin
• Gregor Mendel
• Variation Within Populations
• Plant Populations
• Animal Populations
• Hardy Weinberg
• Natural Selection
• Evolution
• Change Due to Chance

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Darwin

• 1835 Charles Darwin visited the Galapagos Islands
  and became convinced various populations evolved
  from ancestral forms.

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Darwin

• 1838 After reading an essay by Thomas Malthus, he
  theorized some individuals would have a
  competitive advantage conferred by favorable
  characteristics and that these characteristics
  could be passed on to offspring.

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Darwin

• The concept of evolution (change through time)
  had been around long before Darwin.
• A mechanism for evolution was lacking.
• Darwin provided that mechanism Natural selection

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Darwins Theory of Natural Selection

• Organisms beget 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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Darwins Theory of Natural Selection

• Darwin proposed that differential survival and
  reproduction of individuals would produce changes
  in species populations over time.
• The environment acting on variation among
  individuals would result in adaptation to the
  environment.

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Darwins Theory of Natural Selection

• Part of Darwins theory involved the heritability
  of traits.
• Mechanism for this heritability was lacking in
  Darwins time.

9
Gregor Mendel

• Gregor Mendel was a monk who studied the garden
  pea (Pisum sativum).
• Background in math statistics
• Emphasized experimentation
• Replicates
• Looked for patterns in his data

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Gregor Mendel

• Mendel worked with several traits including
  flower color.
• Started with true breeding parents.
• All offspring were purple.
• Next generation white reappeared.
• 31 ratio

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Gregor Mendel

• Mendel discovered that characteristics pass from
  parent to offspring in form of discrete packets
  called genes.
• Exist in alternate forms - alleles.
• Some prevent expression of others.
• Dominant vs. recessive
• Phenotype what the plant looks like.
• Purple or white
• Genotype which alleles are actually present.
• PP, Pp or pp

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Modern Evolutionary Theory

• The synthesis of the theory of natural selection
  and genetics gave rise to modern evolutionary
  ecology.

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Variation Within Populations

• Phenotypic variation among individuals in a
  population results from the combined effects of
  genes and the environment.
• Variation exists in all populations.
• Much of this variation is heritable.

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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.

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Variation Within Populations

• Sticky cinquefoil was grown at 30 m, 1400 m, and
  at 3050 m.
• They used clones found at low, mid, and high
  elevations.
• They found differences in growth indicating
  genetic variation among populations.
• Each population is adapted to its native
  environment.

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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.

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Variation in Animal Populations

• Variation in rainfall translates into variation
  in food availability.

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Variation in Animal Populations

• 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

• Juvenile Chuckwallas from a variety of elevations
  were grown together with unlimited food.
• Those from high elevation populations grew to a
  larger size.
• Genetic component to size differences observed in
  the field.

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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.

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Variation in Animal 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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Population Genetics

• Populations have genetic variation.
• This variation is required for evolutionary
  change.
• Population genetics allows us to study how common
  a trait is in a population, and how that may
  change over time.

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Population Genetics

• Evolution involves a change in a population over
  time.
• Changes in the frequencies of alleles within a
  popultion.

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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 from generation to generation.
• p q 1
• p22pqq2 1.0
• p frequency of the most common allele
• q frequency of the less common allele

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Calculating Gene Frequencies

• Harmonia, Asian lady beetles, show a great deal
  of variation in color and pattern.

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Calculating Gene Frequencies

• There are more than a dozen possible alleles we
  will consider only two.
• 10-signata (SS) yellow with several black
  spots.
• Aulica (AA) black borders with large oval
  yellow or orange area.

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Calculating Gene Frequencies

• SS (81) SA (18) AA (1)
• Frequency of S allele?
• SS 1/2SA 0.81 ½(0.18) 0.90
• Frequency of A allele?
• AA 1/2SA 0.01 ½(0.18) 0.1
• p q 1
• 0.90 0.10 1.00

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Calculating Gene Frequencies

• p2 2pq q2 1.0
• (0.90)2 2(0.9 x 0.1) (0.10)2 1.0
• 0.81 0.18 .01 1.0

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Conditions Necessary for Hardy Weinberg

• Random Mating
• No Mutations
• Large Population Size
• Avoid a change in allele frequencies due to
  chance (genetic drift)
• No Immigration
• Equitable Fitness Between All Genotypes
• No natural selection is occurring.

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Conditions Necessary for Hardy Weinberg

• It is likely that 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.

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Evolution by Natural Selection

• Depends on heritability of trait.
• h2 VG / (VG VE)
• VG Genetic variance
• VE Environmental variance
• VG VE VP
• VP Phenotypic variance
• Natural selection only works on heritable traits.

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Adaptive Change in Colonizing Lizards

• Losos et al. worked with the genus Anolis
• Great diversity of species includes large amount
  of variation in size and body proportions.

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Adaptive Change in Colonizing Lizards

• Length of hind limbs appears to reflect selection
  for effective use of vegetation.
• Diameter of perching surfaces seems to be the
  most significant feature of the vegetation.
• Hind limb length appears to be the result of a
  trade-off between selection for maximum speed
  (longer limbs run faster) and selection for
  moving efficiently on narrow branches (shorter
  limbs are more efficient).

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Adaptive Change in Colonizing Lizards

• Vegetation on the source area (Staniel Cay) was
  up to 10 m tall.
• Experimental island populations had vegetation
  1-3 m tall and no native lizard populations.
• Lizards from the source area were released on
  each small island.
• After 10-14 years they returned to measure lizard
  morphology.

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Adaptive Change in Colonizing Lizards

• Positive correlation between the difference in
  vegetative height on experimental islands
  compared to Staniel Cay and the degree to which
  introduced lizards diverged from the ancestral
  population.

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Adaptive Change in Colonizing Lizards

• The hind limb length in the lizard populations
  was positively correlated with the average perch
  diameter the lizards used on each island.

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Rapid Adaptation by Soapberry Bugs

• Carroll and Boyd studied the soapberry bug
  (Jadera haematoloma) which feeds on seeds from
  family Sapindaceae.
• Slender beaks to pierce fruit walls.
• Found close relationship between fruit radius and
  beak length.

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Rapid Adaptation by Soapberry Bugs

• Distance from outside fruit wall to seeds varies
  widely - beak length should be under selection.

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Rapid Adaptation by Soapberry Bugs

• The results show a close correlation between beak
  length in the bugs and the radius of fruits.
• Juveniles from various populations were reared on
  alternative host plants.
• Shows genetic basis for the change.

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Change Due to Chance

• Random processes, such as genetic drift, can
  change gene frequencies in populations,
  especially in small populations.

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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.

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Evidence of Genetic Drift in Chihuahua Spruce

• 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.
• Increased probability of extinction.

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Genetic Variation In Island Populations

• Endemic populations show much less variation than
  mainland populations.

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Genetic Diversity and Butterfly Extinctions

• 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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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.