Reebop Populations

1
Reebop Populations
2
Do Reebops evolve?

• Based on what we already know about Reebop
  genetics and reproduction, do you think that
  Reebops could evolve? Why?
• Under what conditions might Reebops evolve? Why?
• How would we know if they did evolve?

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Do Reebops Evolve?

• A drought has forced our Reebop population to a
  new location.
• The vegetation in this new location is not as
  tall as before.
• The tails of the straight-tailed Reebops stick
  above the vegetation, and they are more visible
  to predators.
• All of the straight-tailed Reebops are eaten
  before they can reproduce.

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So, DO Reebops Evolve?

• Soon, we are going to find out if Reebops evolve
  under these new conditions.
• In a scientific inquiry, we must know what data
  we should gather to help us answer our question.
• So, what data can we gather to provide evidence
  of evolution?

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Measuring Evolution

• To help us decide what data to gather, lets look
  at a definition for biological evolutionA
  process that results in heritable changes in a
  population spread over many generations.
• ProblemThis definition does not suggest a way
  to measure these heritable changes.

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Measuring Evolution

• Does this definition for biological evolution
  help us decide what to measure?Any change in the
  frequency (proportions) of alleles within a gene
  pool from one generation to the next.
• Gene Pool all of the genes in all of the
  individuals in a breeding population.

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Measuring Evolution

• To decide if a population is evolving, we can
  measure change in the frequency of alleles within
  a gene pool from one generation to the next.
• But, to measure a change, we must first know
  where we started

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Measuring Evolution

• Lets focus on the frequency of the tail trail
  allelesT and t.
• The parent Reebops wereheterozygous (Tt) for
  this gene.
• What percentage of alleles in the parent
  generation gene pool were t? What percentage
  were T?

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Measuring Evolution

• If all parent Reebops were Tt,50 of the gene
  pool will be T and50 of the gene pool will be
  t.
• In other words, in the parent generation, the
  frequency of T is 50 and the frequency of t is
  50.

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Measuring Evolution

• Remember breeding the Reebops?
• Were the frequencies in the gene pool of the F1
  generation still 50 T and 50 t ?
• Our Punnett square can provide the answer!

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Measuring Evolution
Imagine 100 F1 offspring
T
t
25 are TT 50 copies of T
Tt
TT
T
50 are Tt 50 copies of T
tt
Tt
t
25 are tt 0 copies of T
Population has 100 copies of T
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Measuring Evolution
Imagine 100 F1 offspring
T
t
25 are TT 0 copies of t
Tt
TT
T
50 are Tt 50 copies of t
tt
Tt
t
25 are tt 50 copies of t
Population has 100 copies of t
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Measuring Evolution
F1 Population has 100 copies of T
F1 Population has 100 copies of t
Frequency of T is still 50
Frequency of t is still 50
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Measuring Evolution

• Now we know that the starting frequency of T is
  50 and the starting frequency of t is 50.
• Without selection pressure, these frequencies did
  not change from the parent generation to the F1
  generation.
• If we see changes in these frequencies over
  generations, we are seeing evidence of evolution.

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Measuring Evolution
Lets apply some selection pressure to the tail
trait and see what happens to our allele
frequencies
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Stop for Reebop Population activity.
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Did Our Reebop Population Evolve?

• What happened to the frequency of T?
• What happened to the frequency of t?

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Did Our Reebop Population Evolve?
Biological Evolution Any change in the frequency
of alleles within a gene pool from one generation
to the next.
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Did Our Reebop Population Evolve?
T (.73)
t (.27)
T (.50)
t (.50)
T (.73)
TT (.73)2
Tt (.73)(.27)
T (.50)
TT (.50)2
Tt (.50)(.50)
t (.27)
tt (.27)2
Tt (.73)(.27)
t (.50)
tt (.50)2
Tt (.50)(.50)
TT (.73)2 .53 Tt 2(.73)(.27) .40 tt
(.27)2 .07
TT (.5)2 .25 Tt 2(.5)(.5) .50 tt (.5)2
.25
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Quick Think Time

• If selection pressure against tt continues, will
  the t allele ever disappear from the population?
• If no selection pressure against Tt individuals
  exists, the t allele will persist in the
  population.

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A New Population
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John Endlers Guppies

• Why do these guppies look so different?

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John Endlers Guppies

• Evolutionary biologist John Endler studied guppy
  populations in Trinidad.
• He noticed wide color variation in guppies living
  in different streams.
• Endler also observed differences in the
  distribution of guppy predators and in the color
  of gravel in different locations.
• He found that male guppies are brightly colored
  in streams with few predators, but are drably
  colored in streams with many predators.
• He also found that females prefer brightly
  colored males.

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When Do Populations Evolve?

• Remember how the gene pool frequencies of the T
  and t alleles stayed 5050 from the parent
  generation to the F1 (with no selection)?
• It turns out that gene pool frequencies dont
  change (evolution does not occur) unless certain
  factors cause them to change.
• Think of a similar idea in physicsan object at
  rest stays at rest until a force acts on it.

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When Do Populations Evolve?

• These factors cause allele frequencies in
  populations to change
• Mutations
• Non-random mating
• Natural selection (you knew that!)
• Migration
• Isolation

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No Evolution?

• In the early 1900s, Godfrey Hardy and Wilhelm
  Weinberg used mathematical analysis to developed
  a set of rules now called the Hardy-Weinberg Law.
• These rules describe a population that is not
  evolving.
• Another way to say that a population is not
  evolving is to say it is in Hardy-Weinberg
  equilibrium.

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The Hardy-Weinberg Law Says

• If these conditions are met
• No mutation
• No natural selectionall survive and reproduce
  equally
• Infinitely large population, so no genetic drift
  (deviations due to chance)
• Random mating
• No migration in or out of the population
• Then the frequency of alleles does not change
  over time.

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Hardy-Weinberg Equilibrium

• We say that a population meeting these conditions
  is in Hardy-Weinberg Equilibrium.

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Hardy-Weinberg Equation
If a population is in equilibrium, you can
calculate allele frequencies and genotype
frequencies using the Hardy Weinberg equation.
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Hardy-Weinberg Equation
T (p)
t (q)
If we call the frequency of the dominant allele
p and the frequency of the recessive allele
q, then TT (p)2 Tt 2(p x q) tt (q)2
TT (p)2
Tt (p x q)
T (p)
tt (q)2
Tt (p x q)
t (q)
And(p)2 2(p x q) (q)2 1
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Hardy-Weinberg Equation
Ta daThe Hardy-Weinberg Equation (p)2 2(p x q)
(q)2 1
The Hardy-Weinberg equation tells us If the
frequencies of the alleles in a population remain
the same, the ratio of genotypes will remain the
same from generation to generation.
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Using Hardy-Weinberg

• Albinism is a rare homozygous recessive (aa)
  trait.
• The most characteristic symptom is a deficiency
  in the skin and hair pigment melanin.
• Albinism occurs among humans as well as among
  other animals.
• The average human frequency of albinism in North
  America is about 1 in 20,000.

albino gorilla Snowflake
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Using Hardy-Weinberg

• Referring back to the Hardy-Weinberg equation (p2
  2pq q2 1), the frequency of homozygous
  recessive individuals (aa) in a population is q2.
•  
• Since we know that the 1 in 20,000 people with
  albinism are aa, the following must be true
• q2 1/20,000 .00005

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Using Hardy-Weinberg

• Take the square root of both sides of the
  equation
• q2 .00005 q .007
• So, the frequency of the recessive albinism
  allele (a) is .007 or about 1 in 140.
• Knowing (q), it is easy to solve for (p)
• p 1 - q p 1 - .007 p .993
• So, the frequency of the dominant allele(A) is
  .99293 or about 99 in 100.

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Using Hardy-Weinberg

• Plug the frequencies of p and q into the
  Hardy-Weinberg equation
• p2 2pq q2 1
• (.993)2 2(.993)(.007) (.007)2 1
• .986 .014 .00005 1
• p2 predicted frequency of AA  .986 98.6
• 2pq predicted frequency of Aa  .014 1.4
• q2 predicted frequency of aa  .00005 .005

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Using Hardy-Weinberg

• With a frequency of .005 (about 1 in 20,000),
  persons with albinism are rare. 
• Heterozygous carriers for this trait, with a
  predicted frequency of 1.4 (about 1in 72), are
  far more common. 
• The majority of humans (98.6)probably are
  homozygous dominantand do not have the albinism
  allele.

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Using Hardy-Weinberg

• You can find an interactive example of using the
  Hardy-Weinberg equation at http//www.phschool.com
  /science/biology_place/labbench/lab8/allfreq.html.
  
• Lab Bench Activity Estimating Allelic Frequency

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Not In Equilibrium?

• Many populations are not in Hardy-Weinberg
  equilibrium.
• So how is the Hardy-Weinberg equation useful
  then?
• The model of a population in equilibrium allows
  us to see if data from other populations conforms
  or deviates.
• Deviations from the model equilibrium population
  help us identify evolutionary processes.

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New Species

• Imagine a flood washed some Reebops across a
  river where they became permanently isolated from
  the original population.
• Not only was the grass shorter (too bad for the
  tts), but the environment was different in a lot
  of other ways.
• Over many, generations, the gene frequencies for
  tail shape changed. In a similar way, the gene
  frequencies for lots and lots of other genes also
  changed.
• Mutations of some genes added new alleles that
  didnt even exist in the original population.

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New Species

• Finally, they didnt look like the original
  Reebops, nor were they able to mate with them on
  the rare occasions when they did come into
  contact.
• They had evolved into a new species.
• The accumulation of genetic differences between
  populations in different habitats over many
  generations is what gives rise to new species.

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Can Diseases Be Good?

• Some human genetic diseases result from
  inheriting two recessive alleles.
• Without modern medical treatment, most of these
  diseases are fatal in childhood. So why do the
  alleles for these diseases persist?
• We know that some recessive alleles will remain
  in the population as long as heterozygotes are
  not selected against.
• But in some situations, the percentage of the
  recessive allele actually rises in the
  population, even though the homozygous recessive
  is often fatal.
• How can that happen?

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Can Diseases Be Good?

• Some of these diseases actually provide an
  advantage to heterozygotes over the homozygous
  dominant individuals.
• When carriers of an allele have advantages that
  allow a detrimental allele to persist in a
  population, balanced polymorphism is at work.
• This form of polymorphism often entails
  heterozygosity for an inherited illness that
  protects against an infectious illness.
• Lets look at some examples

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Can Diseases Be Good?

• Sickle Cell Disease causes anemia, joint pain, a
  swollen spleen, and frequent, severe infections.
  Carriers (heterozygotes) are resistant to
  malaria, an infection of the blood cells by the
  parasite Plasmodium falciparum.
• People who inherit one copy of the sickle cell
  allele have red blood cell membranes that do not
  admit the parasite.
• In East Africa, during a period when land being
  cleared for cultivation produced an ideal
  mosquito habitat, the frequency of the sickle
  cell allele rose from 0.1 percent to 45 percent
  in 35 generations.

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Can Diseases Be Good?

• Mutation Story

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Can Diseases Be Good?

• Phenylketnonuria (PKU) is an error of metabolism
  in which a missing enzyme causes the amino acid
  phenylalanine to build up, with devastating
  effects on the nervous system unless the
  individual follows a restrictive diet.
• Carriers (heterozygotes) have slightly elevated
  phenylalanine levels. Physicians have observed
  that women who are PKU carriers have a much
  lower-than-average incidence of miscarriage.
• One theory is that excess phenylalanine somehow
  inactivates a poison, called ochratoxin A, that
  certain fungi produce and that is known to cause
  spontaneous abortion.

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Can Diseases Be Good?

• Tay-Sachs is a fatal disease of the central
  nervous system. Babies lack an enzyme called
  hexosaminidase A (hex A) necessary for breaking
  down certain fatty substances. These substances
  build up and gradually destroy brain and nerve
  cells. Death occurs by age 5.
• In Eastern European Jewish populations, up to 11
  percent of the people are Tay-Sachs carriers.
• During World War II, Tuberculosis was rampant in
  Eastern European Jewish settlements. Often,
  healthy relatives of children with Tay-Sachs
  disease (probably heterozygotes) did not contact
  Tuberculosis, even when repeatedly exposed.

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Defining Biological Evolution

• The changes in populations that are considered
  evolutionary are those that are inheritable via
  the genetic material from one generation to the
  next.
• Biological evolution may be slight or
  substantialit embraces everything from slight
  changes in the proportion of different alleles
  within a population (such as those determining
  blood types) to the successive alterations that
  led from the earliest proto-organism to snails,
  bees, giraffes, and dandelions.
• Douglas J. Futuyma in Evolutionary Biology,
  Sinauer Associates 1986