Evolution & Microevolution Tutorial

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Evolution Microevolution Tutorial

• Introduction
• Microevolution
• Hardy Weinberg Equilibrium
• Practice!

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In this tutorial, you will learn

• The difference between macroevolution
  microevolution.
• How Hardy-Weinberg equilibrium works as well as
  factors that can upset this equilibrium.
• How to use the equation, p2 2pq q2 1, to
  calculate allele frequencies in a population.

Credits Figures and images by N. Wheat unless
otherwise noted. Lesser ball python image used
with permission from Tim Bailey, Bailey Bailey
Reptiles. Funded by Title V-STEM grant
P031S090007.
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Introduction

• Evolution includes all of the changes in the
  characteristics and diversity of life that occur
  throughout time.
• Evolution can occur on both large and small
  scales.

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Macroevolution Evolution on a Large Scale

• Macroevolution evolutionary change on a grand
  scale.
• Origin of novel designs
• Evolutionary trends
• Adaptive radiation

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Microevolution Evolution on a Small Scale

• Microevolution - a change in the genetic
  composition of a population over time.
• A change in the frequency of certain alleles in a
  population over several generations.

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Polymorphism

• Polymorphism occurs when there are different
  allelic forms of a gene in a population.
• Mojave (left) and Lesser (middle) are different
  alleles of the same gene. Wild type ball python
  is shown on the right.

Photo courtesy of Bailey Bailey Reptiles
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Gene Pool

• All of the alleles of all of the genes possessed
  by all of the members of the population are
  contained in the gene pool of the population.
• We can measure the relative frequency of a
  particular allele in a population.
• Allelic frequency

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Population Genetics

• Population Genetics the study of how
  populations change over time.
• Dependent on both Darwins theory of natural
  selection and Mendels laws of inheritance.
• All heritable traits have a genetic basis, some
  are controlled by multiple genes not as simple
  as in Mendels studies.

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Genetic Equilibrium

• According to Hardy-Weinberg equilibrium, the
  hereditary process alone does not produce
  evolutionary change.
• Allelic frequency will remain constant generation
  to generation unless disturbed by mutation,
  natural selection, migration, nonrandom mating,
  or genetic drift.
• These are sources of microevolutionary change.

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Frequency of Alleles

• Each allele has a frequency (proportion) in the
  population.
• Example population of 500 wildflowers.
• CRCR red CRCW pink CWCW white
• 250 red, 100 pink, 200 white
• Frequency of CR
• (250 x 2) 100 / 1000 600/1000 .6 60

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Frequency of Alleles

• p is the frequency of the most common allele (CR
  in this case).
• p 0.6 or 60
• q is the frequency of the less common allele (CW
  in this case).
• p q 1
• q 1- p 1 0.6 0.4 or 40

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Hardy-Weinberg Theorem

• Populations that are not evolving are said to be
  in Hardy-Weinberg equilibrium.

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Hardy-Weinberg Theorem

• As long as Mendels laws are at work, the
  frequency of alleles will remain unchanged.

Review Punnett squares in the genetics tutorial.
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Hardy-Weinberg Theorem

• The Hardy-Weinberg theorem assumes random mating.
• Generation after generation allele frequencies
  are the same.

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Hardy-Weinberg Theorem

• Conditions required for Hardy-Weinberg
  equilibrium to hold true
• Very large population
• No gene flow into or out of the population
• No mutations
• Random mating
• No natural selection

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Hardy-Weinberg Theorem

• Departure from these conditions results in a
  change in allele frequencies in the population.
• Evolution has occurred!

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Practice with Hardy Weinberg

• Frequency the proportion of individuals in a
  category in relation to the total number of
  individuals.
• 100 cats, 75 black, 25 white frequency of black
  75/100 0.75, white 0.25.
• Two alleles p is common, q is less common.
• pq 1

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The frequency of black cats is
Question 1

• 0.75
• 75
• 0.25
• 25
• 100

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Sorry!
Question 1

• That is incorrect.
• Try again!

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Congratulations!
Question 1

• You are correct!

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What would the frequency of black cats be if the
population size was 80 instead of 100 (still 75
black)?
Question 2

• 0.75
• 75
• 0.94 (75/80)
• 1

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Sorry!
Question 2

• That is incorrect.
• Try again!

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Congratulations!
Question 2

• You are correct!

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Hardy-Weinberg Theorem

• At a locus with two alleles, the three genotypes
  will appear in the following proportions
• (p q) x (p q) p2 2pq q2 1

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Practice with Hardy Weinberg

• (p q)2 p2 2pq q2

Individuals homozygous for allele B
Individuals heterozygous for alleles B b
Individuals homozygous for allele b
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Practice with Hardy Weinberg

• We will use a population of 100 cats as a
  practice example.
• 84 of the 100 cats are black.
• 16 are white.

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Practice with Hardy Weinberg

• We can use the equation and our color
  observations to calculate allele frequencies in
  our population of 100 cats.
• p2 2pq q2 1
• 100 population size

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Practice with Hardy Weinberg

• 84 of our 100 cats are black.
• Black is the dominant phenotype.
• Cats with the genotype Bb or BB will be black.
• The frequency of black cats is 84/100, but we
  cant yet say anything about the B allele.
• See the genetics tutorial to review these terms.

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Practice with Hardy Weinberg

• 16 of our 100 cats are white.
• White is recessive (bb) and is represented by q2
  in our equation p2 2pq q2 1
• So, q2 16/100 0.16
• q square root of 0.16 0.40.

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Practice with Hardy Weinberg

• q square root of 0.16 0.40.
• Since p q 1 p 1 q 0.60.
• p2 0.36
• p2 represents the proportion of individuals in
  the population with the homozygous dominant
  phenotype (BB).
• Remember population size 100

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So, the number of cats in our population that
have the BB genotype would be
Question 3

• 0.36 cats
• 0.36 x 100 36 cats
• 0.16 x 100 16 cats
• 84 cats

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Sorry!
Question 3

• That is incorrect.
• Try again!

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Congratulations!
Question 3

• You are correct!

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Practice with Hardy Weinberg

• Now we know how many of our cats have the BB
  genotype and the bb genotype.
• We can find the number of Bb cats using our
  equation p2 2pq q2 1.
• 2pq represents the proportion of cats with Bb.
• 2 x 0.6(p) x 0.4(q) 0.48
• 0.48 x 100 48 cats with Bb genotype.

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Lets try another! In our population of 100 cats,
75 are black 25 are white. Where do we start?
Question 4

• 75 black cats p2.
• 75/100 0.75 black cats p2.
• 25 white cats q2.
• 25/100 0.25 white cats q2.
• Need more information.

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Sorry!
Question 4

• That is incorrect.
• Try again!

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Congratulations!
Question 4

• You are correct!

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If q2 0.25, q
Question 5

• 0.05
• 5
• 0.5
• 50

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Sorry!
Question 5

• That is incorrect.
• Try again!

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Congratulations!
Question 5

• You are correct!

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If q0.5, p
Question 6

• 0.5
• 5
• 0.6
• 0.1

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Sorry!
Question 6

• That is incorrect.
• Try again!

43
Congratulations!
Question 6

• You are correct!

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So, if p0.5, and p20.25, how many of our cats
have the BB genotype?
Question 7

• 0.25
• 25
• 50
• 75

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Sorry!
Question 7

• That is incorrect.
• Try again!

46
Congratulations!
Question 7

• You are correct!

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Now, how many of the cats are heterozygous (Bb)?
Question 8

• 48
• 100
• .5
• 50

48
Sorry!
Question 8

• That is incorrect.
• Try again!

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Congratulations!
Question 8

• You are correct!

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If we measure allele frequency one year at p0.8
q0.2 and then go back 5 generations later to
find p0.5 q0.5, what has happened?
Question 9

• The population has remained in Hardy-Weinberg
  equilibrium.
• The population has doubled in size.
• There has been a change in allele frequencies
  evolution has occurred.
• Nothing has changed.

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Sorry!
Question 9

• That is incorrect.
• Try again!

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Congratulations!
Question 9

• You are correct!