Download Reebop Populations

Reebop Populations
Do Reebops evolve?
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Based on what we already know
about Reebop genetics and
reproduction, do you think that
Reebops could evolve? Why?
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Under what conditions might
Reebops evolve? Why?
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How would we know if they did
evolve?
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?
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Soon, we are going to find out if
Reebops evolve under these new
conditions.
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In a scientific inquiry, we must know
what data we should gather to help
us answer our question.
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So, what data can we gather to
provide evidence of evolution?
Measuring Evolution

To help us decide what data to
gather, let’s look at a definition for
biological evolution:
A process that results in heritable
changes in a population spread over
many generations.
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Problem:
This definition does not suggest a way to
measure these “heritable changes.”
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.
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…
Measuring Evolution

Let’s focus on the frequency of the
tail trail alleles—T and t.

The parent Reebops were
heterozygous (Tt) for this gene.

What percentage of alleles in the parent
generation gene pool were “t”? What
percentage were “T”?
Measuring Evolution

If all parent Reebops were Tt,
50% of the gene pool will be T and
50% 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%.
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!
Measuring Evolution
Imagine 100 F1 offspring…
T
t
T
TT
Tt
t
Tt
tt
25 are TT = 50 copies of T
50 are Tt = 50 copies of T
25 are tt = 0 copies of T
Population has 100 copies of T
Measuring Evolution
Imagine 100 F1 offspring…
T
t
T
TT
Tt
t
Tt
tt
25 are TT = 0 copies of t
50 are Tt = 50 copies of t
25 are tt = 50 copies of t
Population has 100 copies of t
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%
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.
Measuring Evolution
Let’s apply some selection
pressure to the tail trait and
see what happens to our allele
frequencies…
Stop for
Reebop
Population
activity.
Did Our Reebop Population Evolve?
What happened to the frequency of T?
 What happened to the frequency of t?

Did Our Reebop Population Evolve?
Biological Evolution: Any change in the frequency
of alleles within a gene pool from one generation
to the next.
Did Our Reebop Population Evolve?
T (.50)
T (.50)
t (.50)
TT (.50)2
Tt (.50)(.50)
t (.50) Tt (.50)(.50)
tt (.50)2
T (.73)
T (.73)
t (.27)
TT (.73)2
Tt (.73)(.27)
t (.27) Tt (.73)(.27)
tt (.27)2
TT = (.5)2 = .25
TT = (.73)2 = .53
Tt = 2(.5)(.5) = .50
Tt = 2(.73)(.27) = .40
tt = (.5)2 = .25
tt = (.27)2 = .07
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.
A New Population
John Endler’s Guppies

Why do these guppies look so different?
John Endler’s Guppies
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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.
When Do Populations Evolve?

Remember how the gene pool frequencies of
the T and t alleles stayed 50:50 from the
parent generation to the F1 (with no
selection)?

It turns out that gene pool frequencies don’t
change (evolution does not occur) unless
certain factors cause them to change.

Think of a similar idea in physics—an object
at rest stays at rest until a force acts on it.
When Do Populations Evolve?
These factors cause allele
frequencies in populations to change:
 Mutations
 Non-random mating
 Natural selection (you knew that!)
 Migration
 Isolation

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 HardyWeinberg equilibrium.

The Hardy-Weinberg Law Says

If these conditions are met:
– No mutation
– No natural selection—all 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.
Hardy-Weinberg Equilibrium

We say that a population
meeting these conditions is in
Hardy-Weinberg Equilibrium.
Hardy-Weinberg Equation
If a population is in equilibrium, you can
calculate allele frequencies and
genotype frequencies using the Hardy
Weinberg equation.
Hardy-Weinberg Equation
If we call the
frequency of the
dominant allele “p”
and the frequency
of the recessive
allele “q”, then
T (p)
T (p)
t (q)
TT (p)2
Tt (p x q)
Tt (p x q)
tt (q)2
TT = (p)2
Tt = 2(p x q)
t (q)
tt = (q)2
And…(p)2 + 2(p x q) + (q)2 = 1
Hardy-Weinberg Equation
Ta da…The 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.
Using Hardy-Weinberg
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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”
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.
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Since we know that the 1 in 20,000
people with albinism are aa, the
following must be true:

q2 = 1/20,000 = .00005
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.
Using Hardy-Weinberg
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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%
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 1
in 72), are far more common.
 The majority of humans (98.6%)
probably are homozygous dominant
and do not have the albinism allele.
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Using Hardy-Weinberg
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You can find an interactive example of using
the Hardy-Weinberg equation at
http://www.phschool.com/science/biology_place/labbench/lab8/
allfreq.html.
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Lab Bench Activity: Estimating Allelic Frequency
Not In Equilibrium?
Many populations are not in HardyWeinberg 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
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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
tt’s), 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
didn’t even exist in the original population.
New Species

Finally, they didn’t 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.
Can Diseases Be Good?
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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?
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.

Let’s look at some examples:
Can Diseases Be Good?
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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.
Can Diseases Be Good?
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Mutation Story
Can Diseases Be Good?
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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 lowerthan-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.
Can Diseases Be Good?
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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.
Defining Biological Evolution
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“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 substantial;
it 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.”
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Douglas J. Futuyma in Evolutionary Biology,
Sinauer Associates 1986