Chapter 16: Evolution of Populations - PowerPoint

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					Chapter 16:
1. When Darwin developed his theory of
   evolution, he did not understand:
  • how heredity worked.
   This left him unable to explain two
          a. source of variation
          b. how inheritable traits pass
               from one generation to the
In the 1940’s, Mendel’s work on genetics
  was “rediscovered” and scientists began
  to combine the ideas of many branches of
  biology to develop a modern theory of
  evolution. When studying evolution
  today, biologists often focus on a
  particular population. This evolution of
  populations is called microevolution.
2. Vocabulary:
  population: group of individuals of the
  same species living in the same area that
  breed with each other.
2. gene pool: combined genetic info.
  for all members of a population
2. allele: one form of a gene
2. relative frequency of an allele: #
  times an allele occurs in the gene pool
  compared to other alleles (percent)

                        Relative Frequency:
                        70% Allele B
                        30% Allele b
3. Sources of Variation:
  a. mutations: any change in DNA sequence

  ♦ Can occur because of:
       ♦mistakes in replication
       ♦ environmental chemicals

  ♦ May or may not affect an organism’s
3. Sources of Variation
  b. Gene Shuffling: recombination of genes
  that occurs during production of gametes
  ♦ Cause most inheritable differences between
  ♦ Occurs during meiosis
  ♦ As a result, sexual reproduction is a major
    source of variation in organisms.
  ♦ Despite gene shuffling, the frequency of alleles
    does not change in a population. Explain why
    this is true.
Similar to a deck of cards – no matter how
  many times you shuffle, same cards (alleles)
  are always there.
4. Gene Traits:
A) Single gene trait: controlled by single
  gene with two alleles
     ♦ Examples: widow’s peak, hitchhiker’s thumb,
       tongue rolling
(4. Gene Traits:)
  B) Polygenic trait: controlled by 2 or more
  genes, each with 2 or more alleles
     ♦ Examples: height, hair color, skin color, eye color
Most human traits are polygenic.
 Do the following graphs show the distribution of
   phenotypes for single-gene or polygenic traits? Explain.

type: single gene            type: polygenic
why? Only two                why? Multiple (many)
phenotypes possible          phenotypes possible
Example: tongue roller       Example: height range 4feet
or non-tongue roller         to 9 feet all
5. Natural selection acts on phenotypes, not
Example: in a forest covered in brown leaves,
  dirt and rocks which mouse will survive better
  brown or white?
            Brown, more hidden.
5. If brown is dominant can the a predator tell
  the difference between:
         BB                            Bb

Mouse with highest fitness will have the most
 alleles passed on to the next generation.
White mouse will have low fitness
5. Which mouse will have the lowest fitness?
          White, bb (recessive)
         BB                          Bb

Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness
 advantage of being brown
6. Three ways in which
natural selection affects
    polygenic traits.
 a. Directional Selection: individuals at
 one end of the curve have higher fitness
 so evolution causes increase in
 individuals with that trait
                Food becomes scarce.     , high
                                         , low

♦ Individuals with highest fitness: those at
  one end of the curve
♦ Example: Galapagos finches – beak size
Directional Selection (page 398)
     Directional Selection          Low mortality,
                                    high fitness
                                    High mortality,
     Food becomes scarce.           low fitness
b. Stabilizing Selection: individuals at the
center of the curve have highest fitness;
evolution keeps center in the same position
but narrows the curve          Stabilizing Selection

    Individuals with                   mortality,
                                       High fitness
                                       mortality, low
                                                        against both
    highest fitness: near              fitness            extremes
                                                         keep curve
    the center of the                                   narrow and
                                                           in same

    curve (average                                          place.

    phenotype)                  Birth Weight

    Example: human
    birth weight
       Stabilizing Selection
                 Stabilizing Selection

         Low mortality, high
         High mortality, low
         fitness                      Selection
                                    against both
                                   extremes keep
                                    curve narrow
                                    and in same

Birth Weight
 c. Disruptive Selection: individuals at
 both ends of the curve survive better
 than the middle of the curve.
                                             Disruptive Selection

                                          Largest and smallest seeds become more common.

♦ Individuals with highest fitness: both
                     Number of Birds

                                                                             Number of Birds
                      in Population

                                                                              in Population
   Low mortality,                                       Population splits
   high fitness                                         into two subgroups
  ends of curve
   High mortality,
   low fitness
                                                        specializing in
                                                        different seeds.

                                       Beak Size                                               Beak Size

♦ Example: birds where seeds are either
  large or small
      Disruptive Selection (pg 399)

                                            Disruptive Selection

                                         Largest and smallest seeds become more common.

                                                                            Number of Birds
                    Number of Birds

                     in Population

                                                                             in Population
  Low mortality,                                       Population splits
  high fitness                                         into two subgroups
  High mortality,                                      specializing in
  low fitness                                          different seeds.

                                      Beak Size                                               Beak Size
              Quiz Monday!!
• Evolution review ½ sheet. (yes some
  questions are missing)
• Thursday and Friday’s concepts will be
  on the quiz:
  – Directional, Stabilizing and Disruptive
  – Geographic, Behavioral, Temporal
  – Small populations caused by bottleneck
    and founder effect
The Process of Speciation
• The formation of new biological
  species, usually by the division of a
  single species into two or more
  genetically distinct ones.
Three Isolating Mechanisms:
Isolate species forming subspecies
 and perhaps causing speciation.

  1. Geographic Isolation
  2. Behavioral Isolation
  3. Temporal Isolation
 1. Geographic Isolation
• Two populations separated
  by a geographic barrier;
  river, lake, canyon, mountain
  range, or ocean.
Example: 10,000 years ago the Colorado River separated two squirrel
They COULD still mate, but do not, because they are geographically

    • Kaibab Squirrel                  Abert Squirrel
     2. Behavioral Isolation
• Two populations are capable of
  interbreeding but do not interbreed
  because they have different
  ‘courtship rituals’ or other lifestyle
  habits that differ.
  Example: Eastern and Western
• Eastern and Western Meadowlark
 populations overlap      in the
 middle of the US
    Example: Eastern and Western
• Male birds sing a mating song
  that females like, East and West
  have different songs. Females only
  respond to their subspecies song.
   3. Temporal Isolation
Populations reproduce at
 different times

          1   2    3   4 5 6
        7 8 9 10 11 12 13
 Example: Northern Leopard Frog
   & North American Bullfrog
• Mates in:         Mates in:

 April                July
• Geographic, Behavioral and
  Temporal Isolation are all
  believed to lead to speciation.
–No examples ever observed in
–A couple examples that may
 demonstrate speciation exist
 in plants and some insects.
• Diseases: Some diseases are caused by a gene
  mutation, or a change at the molecular level in
  the DNA strand.
• Tay-Sachs: individuals with Tay-Sachs die in
  early childhood so they do not reproduce.
• WHY? How is it possible for the gene to still
  exist in the gene pool
• ANSWER: It is recessive and carried in
  heterozygous individuals.
              Gene Mutations:
Cystic fibrosis:
WHY? Individuals who are heterozygous for the allele
  (Cc) do NOT have the trait and can survive and
  reproduce, passing the trait onto their offspring

What is the chance of 2 heterozygous (Cc) individuals
 having a baby with the homozygous recessive trait (cc),
 therefore having a child with the disease?
        Genetic Drift
• random change in allele
 frequency that occurs in small
The results of genetic crosses can
 usually be predicted using the laws of
  probability. In small
  populations, however, these
  predictions are not always accurate.
  a. Founder effect: allele frequencies change due to
  migration of a small subgroup of a population

♦ Example: fruit flies on Hawaiian islands
 Two phenomena that result
small populations and cause
        genetic drift
  1. Founder Effect
  2. Bottleneck Effect
     Founder effect
allele frequencies change due
to migration of a small
subgroup of a population
Founder Effect: : Fruit Flies
   on Hawaiian islands
      Sample of
 Original Population                           Descendants
                       Founding Population A

                       Founding Population B
         2. Bottleneck effect
  major change in allele
  frequencies when population
  decreases dramatically due to
♦ Example: northern elephant seals
     decreased to 20 individuals in 1800’s, now 30,000
     no genetic variation in 24 genes
     Bottleneck Effect: Northern
      Elephant Seal Population
♦ Hunted to near extintion
♦ Population decreased to
  20 individuals in 1800’s,
  those 20 repopulated so
  today’s population is
♦ No genetic variation in 24
        Bottleneck Effect

        Bottleneck Effect


        Bottleneck Effect


 Original                   Surviving
population                  population
Another picture to illustrate
     bottleneck effect

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