Evolution of Populations

1
Evolution of Populations

• Chapter 16

2
Genetic Variation

• Heterozygotes make up between 4-8 in mammals and
  15 in insects.
• The gene pool is total of all the alleles in a
  population. All the genes.
• Relative Frequency is the number of times a
  particular allele appears in a gene pool.
• In genetic terms evolution is any change in
  relative frequency in a gene pool.

3
 Relative Frequencies of Alleles
Sample Population
Frequency of Alleles
allele for brown fur
allele for black fur
48 heterozygous black
16 homozygous black
36 homozygous brown
Brown is the recessive trait but more common in
frequency.
4
Sources of Variation

• The two main sources for genetic variation are
  Mutation and Gene Shuffling.
• Mutations occur as DNA is changed by mistakes in
  replication, radiation or chemicals in the
  environment.
• Gene Shuffling occurs as meiosis creates gametes.
  Crossing Over and Independent Assortment creates
  new combinations.

5
Single Gene Polygenic Traits

• Single-gene traits are controlled by one gene
  with two phenotypes.
• The variation leads to only two phenotypes.
• Polygenic traits are controlled by two or more
  genes and have variable phenotypes.
• The phenotypes usually fall in a bell curve.

6
Generic Bell Curve for Polygenic Trait
Frequency of Phenotype
Phenotype (height)
7
Natural Selection on Single-gene Traits

• Evolutionary fitness can be measured by the
  ability to reproduce and pass genes.
• Natural selection works only on the organism and
  its phenotype to survive.
• Natural selection on a single-gene trait can
  change gene frequencies and cause evolution. Fig
  16-5
• Red, Brown, Black Lizards

8
Natural Selection on Polygenic Traits

• Natural Selection on Polygenic traits is more
  complex.
• Directional Selection occurs when members on one
  end of the curve have greater fitness and create
  a shift in that direction
• Stabilizing Selection the middle of the curve is
  most fit and the ends shrink.
• Disruptive Selection the lower and upper ends of
  the curve have greater fitness and the middle
  declines.

9
 Graph of Directional Selection
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
10
 Graph of Stabilizing Selection
Stabilizing Selection
Selection against both extremes keep curve narrow
and in same place.
Key
Low mortality, high fitness
High mortality, low fitness
Percentage of Population
Birth Weight
11
 Graph of Disruptive Selection
Disruptive Selection
Largest and smallest seeds become more common.
Key
Population splits into two subgroups specializing
in different seeds.
Low mortality, high fitness
Number of Birdsin Population
Number of Birdsin Population
High mortality, low fitness
Beak Size
Beak Size
12
Genetic Drift

• In small populations random mating leads to a
  change in gene frequency called Genetic Drift.
• By chance in a small population some organisms
  might leave more offspring thus increasing their
  gene frequency.
• The founder effect occurs when a new population
  is created from an existing one and the founding
  gene frequency is different than the original
  population due to chance.

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Genetic Drift
Sample of Original Population
Descendants
Founding Population A
Founding Population B
14
Evolution vs. Genetic Equilibrium

• Hardy-Weinberg Principle states that allele
  frequencies will remain in genetic equilibrium
  and unchanged unless acted upon by evolutionary
  forces.
• There are five conditions for Hardy-Weinberg to
  work
• Random mating
• Very large population
• No movement in or out of a population
• No mutations
• No natural selection

15
Speciation

• Speciation is the process of a species evolving
  into a new species.
• Separation of the gene pool is necessary for
  speciation to occur.
• Behavioral Isolation differences in mating
  rituals lead to isolation
• Geographic Isolation a population is divided by
  a geographic boundary. River, mountains
• Temporal Isolation different breeding times
  separate a population.

16
Concept Map
Reproductive Isolation
results from
Isolating mechanisms
which include
produced by
produced by
produced by
which result in
Independentlyevolving populations
which result in
Formation ofnew species
17
Testing Natural Selection

• Peter Rosemary Grant tested Natural Selection
  on the Galapagos Islands.
• Variation in the Finches followed the bell curve
  and was genetic.
• During drought bigger beaked birds were naturally
  selected, also a behavior for big beaked birds
  mating with like birds led to directional
  selection.
• The finches beaks became bigger in only a few
  decades.

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Speciation in Darwins

• The species of Galapagos finches evolved by
• Founding population from South America
• Separation by geographic boundaries
• Changes in gene pool by directional selection
• Reproductive Isolation keeps birds separate
• Ecological Competition may change the original
  population
• Evolution continues on the islands
• Discuss Limits and Questions to finch evolution