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Ch16 Evolution of Populations Notes

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									       Ch#16 Evolution of
          Populations
Genetic Variation- differences in DNA that is
   inherited and causes different traits in
                 organisms
  Evolution is any change in the relative
     frequency of alleles in a population
 Three Main sources of Variation
1. Mutations- any change in a
    sequence of DNA
• Causes—replication mistakes
    or radiation or chemicals in the
    environment
2. Gene shuffling- various
    combinations of genes
    inherited via sexual
    reproduction (in humans 23
    chromosome pairs can create
    8.4 million different
    combinations of genes)
Three Main sources of Variation
3. Lateral gene transfer: in single celled
  organisms like bacterial transfer of
  plasmids to each other
         Single-Gene Traits
• # of phenotypes produced for
  a trait depends on how many
  genes control the trait
• Widow’s Peak is controlled
  by 1 gene with 2 alleles, so
  only 2 phenotypes are
  produced
• The frequency of person’s
  with a widow’s peak is fairly
  low (38%) so it’s inheritance is
  low, even though it is
  dominant
             Polygenic Traits
• Traits controlled by 2 or
  more genes
• Most polygenic traits usually
  are exhibited in a bell
  shaped curve or normal
  distribution
• An example is height
• Keep in mind the height can
  also be influenced by
  environmental factors such
  as nutrition and exercise
2
               Natural Selection
    • Single-gene traits that arise in a population
      may lead to evolutionary changes
    • Ie: if a trait will allow better survival, that
      organism will pass on the trait to future
      generations



                                            Resulting in an
                                            increase in the
                                            brown gene
                                            frequency
    Natural Selection & polygenic traits
•  Can affect phenotypes in 3
   ways:
1. Directional Selection- one
   end of a phenotype causes
   better survival & thus more
   #’s
2. Stabilizing Selection- creates
   a narrow bell curve (better
   survival near the middle)
3. Disruptive Selection -high &
   low ends survive better than
   middle
         Directional Selection
• Great example is the
  peppered moth
• The industrial
  revolution has
  darkened the tree bark
  from pollution killing
                           •This has almost
  the lichens that used to
                           wiped out the light
  cover the bark
                           colored form
         Stabilizing Selection
• Sparrow population after
  harsh winter
• Small & large birds tend
  to die, causing a
  medium weight
  population
• Human infant survival
  tends to be around a
  medium birth rate of 7.5-
  8.5 lbs (98.5% survival)
        Disruptive Selection
• Darwin’s finches
• Tend to be either small
  or large beaked due to
  size of seeds being
  either small or large
• Medium beaked
  finches did not survive
                Genetic Drift
    • In small populations, changes in the gene
      pool due to chance




Brown gene                              Brown gene
occurs in 25%                           occurs in 29%
of parents                              of offspring
       Hardy-Weinberg principle
  • Allele frequencies in a population will
    remain constant unless one or more
    factor cause those frequencies to
    change
  • Genetic Equilibrium is when the
    population does not change
  • p2 + 2pq + q2= 1, p+q=1 where
    p=dominant,
Random mating q=recessive
in beatles causes
different
phenotypes
         Genetic Equilibrium
5 conditions are required to maintain the
    same basic phenotypes
1. Random mating
2. Large population
3. No migration
4. No mutations
5. No natural selection
3

              Founder Effect
• When a small group
  colonizes a new habitat
  and genetic drift occurs
• The new colony begins
  to be phenotypically
  different from the
  original colonists
• It’s mostly due to
  inbreeding, therefore       Each jellyfish lives in a
  limiting the variation in   unique colony
  genes
             Bottleneck Effect
• Major change in allele
  frequency due to a large
  change in population size
• Caused by major
  disasters such as
  disease, fire, earthquake,
  etc that wipes out a
  majority of the population
       Process of Speciation
• As new species evolve, populations
  become reproductively isolated from each
  other
• Reproductive isolation- when 2 different
  population members cannot create
  offspring (ie: they have separate gene
  pools)
         Behavioral Isolation
• 2 populations
  have different
  courtship rituals
  (ie: birds with
  different mating
  songs, etc.) and
  therefore will not
  interact enough
  to reproduce
        Geographic Isolation
• Populations separated by
  barriers like rivers,
  mountains, etc.
• Classic example:
  Colorado river separating
  the Abert squirrel from the
  Kaibab squirrel
           Temporal Isolation
• Species reproduce at
  different times
• Several orchids in rain
  forest release pollen
  on different days &
  cannot pollinate each
  other
        Adaptive Radiation
• Single species evolving
  into several diverse forms
  over millions of years
• Darwin’s ancestor finch
  radiated into over a dozen
  different species
• Scientists believe that when
  dinosaurs disappeared,
  mammals underwent adaptive     These
                                 butterflies
  radiation                      evolved from
                                 a common
                                 ancestor.
          Convergent Evolution
• Groups of different organisms that are
  unrelated genetically look similar
• When in similar environments, species often
  need similar adaptations to survive (moving
  through air—need wings)
  (moving through water---need flippers or gills)
              Coevolution
• 2 species evolve
  in response to
  changes in each
  other over time
• This happens with
  plants & insects
  (insects help to
  pollinate the
  plants)
            Molecular Evolution
  • Molecular clock is the idea of marking the
    passage of evolutionary time
• Uses mutation
  rates of DNA by
  looking at DNA
  changes in species
  to estimate the
  time that 2 species
  have been
  evolving
  independently
         Classifying Organisms
• Originally organisms were classified by
  Carolus Linnaus using a binomial
  nomenclature system
  – Each organism had a genus species name
  – Humans: homo sapiens
  Classifying Organisms using
     Genomic Information
• Modern system of Phylogeny:
  evolutionary history of lineages reflects
  evolutionary descent of organisms
• Cladograms: diagrams that link groups by
  showing how evolutionary lines diverged
  from common ancestors
  – Clade: group of species that includes a single
    common ancestor & all their offspring
      3 basic assumptions in
            Cladistics
1. Any group of organisms are related by
   descent from a common ancestor.
2. There is a bifurcating pattern of
   cladogenesis.
3. Change in characteristics occurs in
   lineages over time.
                  Cladogram
• Node- represents last
  point 2 species had a
  close common
  ancestor
• Node C represents a
  common ancestor
  went extinct & 2
  species Human &
                        Therefore they are sister taxa
  Gorilla were left     (more closely related to each
                           other than to either the
                           chimpanzee or baboon)
      Derived Characteristic
• Trait arose in most recent common
  ancestor of a particular lineage
  – Ex: All groups above Lancelet share vertebral
    column
     Interpreting Cladograms
• According to the
  cladogram, which
  groups– X & Y or  X
  X,Y, & Z---- have     Y
  the most recent
                               Z
  common ancestor?
Answer: X & Y
have the most
recent common
ancestor
     Interpreting Cladograms
• According to the
  cladogram, which
  species– X & Y or   X
  X & Z---- share         Y
  more derived
                               Z
  characters?
Answer: X & Y
share more
derived characters
                 Tree of Life
• Linnaeus originally labeled 2 kingdoms:
  Animals & Plants
• By the 1990’s, researchers had created
  the 6 Kingdom system of classification
  1.   Eubacteria
  2.   Archaebacteria
  3.   Protista
  4.   Fungi
  5.   Plantae
  6.   Animalia
            Genomic Analysis
• Tree of Life shows current hypotheses
  regarding evolutionary relationships among
  taxa
• Domain: a larger, more inclusive category than
  a kingdom
• New System has 3 Domains
Tree of Life

								
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