Population Genetics and Natural Selection by li3490

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									Population Genetics and
   Natural Selection
        Chapter 8




                          1
                     Outline
 Darwin
 Gregor  Mendel
 Variation Within Populations
     Plant Populations
     Animal Populations
 Hardy Weinberg
 Natural Selection
     Evolution
 Change    Due to Chance
                                 2
                    Darwin
   1835 Charles Darwin visited the Galapagos
    Islands and became convinced various
    populations evolved from ancestral forms.




                                                3
                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.




                                              4
                    Darwin
 Theconcept of evolution (change through
 time) had been around long before
 Darwin.
     A mechanism for evolution was lacking.
 Darwinprovided that mechanism: Natural
 selection



                                               5
      Darwin’s 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.
                                               6
       Darwin’s 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.



                                                      7
       Darwin’s Theory of Natural
               Selection
 Partof Darwin’s theory involved the
 heritability of traits.
     Mechanism for this heritability was lacking in
      Darwin’s time.




                                                       8
                        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
                                        9
                       Gregor Mendel
   Mendel worked with
    several traits
    including flower color.
       Started with true
        breeding parents.
       All offspring were
        purple.
       Next generation –
        white reappeared.
         • 3:1 ratio


                                       10
                    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


                                                         11
  Modern Evolutionary Theory
 Thesynthesis of the theory of natural
 selection and genetics gave rise to
 modern evolutionary ecology.




                                          12
   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.




                                                13
     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.




                                           14
        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.

                                        15
    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.

                                     16
    Variation in Animal Populations
   Chuckwalla
    (Sauromalus obesus)
       Herbivorous lizard in
        desert SW.




                                      17
Variation in Animal Populations
   Variation in rainfall
    translates into
    variation in food
    availability.




                                  18
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.




                                  19
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.
                                  20
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.




                                                               21
Variation in Animal Populations
 Douglas  and Brunner used microsatellite
 DNA to conclude Coregonus populations
 are highly diverse and exhibit a high level
 of differentiation.




                                               22
        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.



                                                23
           Population Genetics
 Evolutioninvolves a change in a
 population over time.
     Changes in the frequencies of alleles within a
      popultion.




                                                       24
                 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
                       p2+2pq+q2 = 1.0

p = frequency of the most common allele
q = frequency of the less common allele

                                                     25
 Calculating Gene Frequencies
 Harmonia, Asian lady beetles, show a
 great deal of variation in color and pattern.




                                                 26
 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.




                                                    27
 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



                                                 28
    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




                                           29
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.

                                                    30
    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.




                                                          31
     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.

                                               32
            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.



                                                 33
        Stabilizing Selection
            selection acts to impede
 Stabilizing
 changes in a population by acting against
 extreme phenotypes and favoring average
 phenotypes.




                                             34
       Directional Selection
 Directionalselection leads to changes in
 phenotypes by favoring an extreme
 phenotype over other phenotypes in the
 population.




                                             35
        Disruptive Selection
 Disruptive  selection creates bimodal
 distributions by favoring two or more extreme
 phenotypes over the average phenotype in a
 population.




                                            36
 Evolution by Natural Selection
 Naturalselection, which changes
 genotypic and phenotypic frequencies in
 populations, can result in adaptation to the
 environment.




                                                37
 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.
                                                38
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.




                                                  39
  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).

                                                        40
   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.

                                               41
     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.

                                     42
     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.



                                     43
Rapid Adaptation by Soapberry
            Bugs
 Carrolland 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.




                                                                       44
    Rapid Adaptation by Soapberry
                Bugs
   Distance from outside
    fruit wall to seeds
    varies widely - beak
    length should be
    under selection.




                                    45
    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.
                                    46
     Change Due to Chance
 Random  processes, such as genetic drift,
 can change gene frequencies in
 populations, especially in small
 populations.




                                              47
      Evidence of Genetic Drift in
          Chihuahua Spruce
      chihuahuana now restricted to
 Picea
 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.




                                                  48
       Evidence of Genetic Drift in
           Chihuahua Spruce
   Found significant
    positive correlation
    between population
    size and genetic
    diversity of study
    populations.




                                      49
        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.


                                      50
      Genetic Variation In Island
             Populations
   Endemic populations
    show much less
    variation than
    mainland populations.




                                    51
        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.


                                          52
      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.




                                              53

								
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