CH 16 Evolution of Populations

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• What is Evolution?
• What did Darwin use as evidence of evolution?

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CH 16 Evolution of Populations
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Darwin's disadvantage

• Darwin did not know how heredity worked
• Darwin developed his theory in 1836
• Mendel's work on inheritance was published in
  the1860s
• 1910 biologists realized genes carry information
  that determine traits
• Darwin did not understand the mechanism of
  inheritance

Darwin 1836 (Theory of Evolution)
Mendel 1860s (Genes and Inheritance)
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Gene Pools

• Gene Pool- The combined genetic information of
  all the members of a particular population

Relative Frequency- number of times an allele
occurs in a gene pool compared with the number of
times other alleles occur
Total of 50 alleles, 20 alleles are (B) black and
30 are (b) brown
Relative frequency of brown alleles 30/50 or 60
of alleles for brown fur
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Sources of Genetic Variation

• Variations- can be physical, behavioral, or
  biochemical
• 2 main sources of variations
• Mutations- Any chance in a sequence of DNA.
  Caused by mistakes of replication of mutagens
• Gene Shuffling- Most inheritable differences are
  due to gene shuffling during the production of
  gametes (meiosis).
• Chromosomes move independently and crossing over
  takes place

1 cause of variations
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Gene Shuffling

• Gene shuffling occurs due to sexual reproduction
• This does not change the relative frequencies of
  alleles in a population

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Single-Gene and Polygenic Traits

• The number of phenotypes produced for a given
  trait depend on how many genes control the trait
• Single-Gene Trait- Controlled by a single gene
  that has 2 alleles
• Example Widows Peak

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Single-Gene and Polygenic Traits

• Polygenic traits- traits controlled by two or
  more genes. Each gene of a polygenic trait has 2
  or more alleles.
• Results in many possible genotypes and even more
  phenotypes
• Height in humans is a polygenic trait

Polygenic traits give a bell curve- many
different phenotypes possible.
Short
Tall
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Evolution as Genetic Change 16-2
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Evolution as Genetic Change 16-2

• Natural selection acts on phenotypes not
  genotypes
• Acting on phenotypes can change allele
  frequencies in a population over time.
• When an individual dies without reproducing its
  genes are removed from a population

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Natural selection on single-gene traits lead to
changes in allele frequencies and thus to
evolution
Soil Color
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Natural selection on Polygenic Traits

• Polygenic Traits- (many different phenotypes
  caused by multiple alleles)
• Natural selection can affect phenotypes in 3 ways
• Directional Selection
• Stabilizing Selection
• Disruptive Selection

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Directional Selection

• When individuals at one end of the curve have a
  higher fitness.

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Stabilizing Selection

• When individuals near the center of the curve
  have higher fitness

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Disruptive Selection

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle

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Genetic Drift
Genetic Drift- random change in allele
frequencies that occurs in small populations

• Genetics are controlled by the laws of
  probability.
• In small populations, individuals that carry a
  particular gene may leave more descendants than
  other individuals just by chance. Over time this
  can lead to an allele becoming more popular.

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Stop and think whygtGenetic drift more likely in
small populations

• Flip a coin twice
• Chance of 100 heads is high
• Flip a coin 25 times
• Greater chance of being close to 50 heads and
  50 tails

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Genetic Drift and the Founder Effect

• Founder effect- allele frequencies change as a
  result of the migration of a small subgroup of a
  population.
• Example (evolution of finches separated on
  different islands)

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Evolution Versus Genetic Equilibrium

• When evolution does not occur
• Hardy-Weinberg Principle- allele frequencies in a
  population will remain constant unless one or
  more factors cause those frequencies to change
• Genetic Equilibrium- The situation in which
  allele frequencies do not change, and a
  population does not evolve/

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5 conditions needed to maintain genetic
equilibrium (Hardy-Weinberg Principle)
all members have equal opportunity to produce
offspring.

• 1. Random Mating
• 2. Large Population
• 3. No Movement into or out of a population
• 4. No Mutations
• 5. No Natural Selection
• If any of these are not true then genetic
  equilibrium is disrupted and populations evolve

less genetic drift
No new alleles added
All alleles have equal fitness
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Hardy-Weinberg Principle

• Hardy-Weinberg Equation
• p²  2pq  q²  1
• p is the frequency of the dominant allele
• q is the frequency of the recessive allele
• By knowing p you should be able to find q

p² frequency of homozygous dominant 2pq
frequency of heterozygous q² frequency of
homozygous recessive
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163 The Process of Speciation 
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• Speciation- the formation of a new species
• 1. Mutations causes variation
• 2. Some variations that increases fitness is are
  adaptations
• 3. Adaptation alleles are seen more and more in
  the gene pools because they increase fitness
• 4. Evolution- A new species is created by many
  adaptations over time

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Speciation

• As new species evolve, populations become
  reproductively isolated from each other.
• Reproductive isolation- members of 2 populations
  cannot interbreed and produce fertile offspring
• Isolating Mechanisms
• Behavioral Isolation
• Geographic Isolation
• Temporal Isolation

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Behavioral Isolation

• Occurs when two populations are capable of
  breeding but have different courtship rituals

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Geographic Isolation

• Two populations are separated by geographical
  barriers such as rivers, mountains, or bodies of
  water

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Temporal Isolation

• Two or more species reproduce at different times

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Darwin hypothesized that finches on the Galapagos
Islands had descended from a common ancestor.
Over time, he proposed, natural selection shaped
the beaks of different bird populations as they
adapted to eat different foods.
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Speciation in Darwin's Finches (pg 410 book)

• Founders arrived from mainland
• Separation of populations
• Changes in the gene pool
• Reproductive isolation
• Ecological Competition
• Continued Evolution

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Speciation in Darwins Finches

• Founders arrived from mainland

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Speciation in Darwins Finches

• 2. Separation of populations

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Speciation in Darwins Finches

• 3. Changes in the Gene Pool

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Speciation in Darwins Finches

• 4. Reproductive Isolation

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Speciation in Darwins Finches

• 5. Ecological competition

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Speciation in Darwins Finches

• 6. Continued Evolution