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Natural Selection
Biological Evolution and Classification
Part I: Everybody in the Pool!
Each of us has a variety of physical traits which our DNA helps to determine. Our genes are
discrete DNA sequences which affect those traits. Alternative versions of genes are called alleles.
Some alleles affect the physical traits that are inherited by children from their parents. A gene pool
is the total collection of alleles within a population. Within a gene pool, some alleles are more
frequent than others. Every species has its own gene pool which contains all the possible alleles
which can impact on the various traits possessed by the species.
For example, the color of your eyes is determined by the alleles you inherited from your mother
and father. Each individual has two alleles for eye color. If your mother has one allele for blue eyes
and one for brown, and your father has one allele for green eyes and one for brown, you may have
brown eyes but carry a second allele for either blue, green, or brown eye color. Whichever
combination of alleles you have, each of your offspring will inherit one of your alleles for eye color.
Procedure:
1.  In Part I of your Student Journal, write down your eye color.
2.  When your teacher announces “GO,” find five different people in your class to survey. Ask their
name and eye color as quickly as you can, then record that information in the data table found
in your Student Journal.
3.  Once you have recorded all of your data, complete the questions found below the data table in
your Student Journal.
Complete Part I of your Student Journal.
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Natural Selection
Biological Evolution and Classification
Part II: Changes Over Time
Biological evolution is based on changes in the frequencies of alleles from generation to
generation. In some organisms, a single generation spans many years. Evolutionary time scales
are generally measured in hundreds or even thousands of generations. Remember, an organism's
genes generally do not change during the lifetime of the organism. Natural selection cannot
change an individual. It can only select which individuals within a population have more or less
reproductive success.
A Case Study
There are two variations of the European peppered moth (Biston betularia) in England: typica
(“typical”), which are light colored, and carbonaria (“melanic”), which are dark colored.
In the early 1800s, almost 99 percent of peppered moths
near English cities were of the typica variety. Observations
at the time noted that the light-colored moths were almost
invisible when resting on trees that were covered by
lichen. By the 1850s, however, coal-fueled factories were
polluting the air with large amounts of black soot. The soot
killed the lichen and blackened the tree trunks in industrial
areas.
European Peppered Moth
Biston betularia
By 1900, naturalist J.W. Tutt estimated that in the
industrial city of Manchester, the percentage of carbonaria
moths had grown from less than 1 percent of the total
peppered moth population to over 98 percent. He
hypothesized that this was due to higher predation by
birds on the typica moths. Tutt's hypothesis was based on
the theory of natural selection.
According to the theory of natural selection, the change in
frequencies of the different moth varieties could be
caused by differential reproductive success driven by
predation. Was this a case of natural selection at work?
Variety: carbonaria
Photo credit: Olaf Leillinger
Please continue to the next page.
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Natural Selection
Biological Evolution and Classification
Part II: Changes Over Time, continued
In the late 1950s, British ecologist Bernard Kettlewell conducted an experiment to test Tutt's
hypothesis. He raised both moth varieties in a laboratory, then marked the moths with a drop of
paint so he could recapture the same moths after release. He then released several hundred of the
moths in a wooded area near an industrial city where the tree trunks were darkened by pollution.
During the day, he observed birds preying on the moths, taking more of the typica than the
carbonaria moths. That night, he recaptured 27.5 percent of the carbonaria moths that he had
released and 13.5 percent of the typica moths. A second trial was conducted that was identical to
the first, except that it was performed in an unpolluted woodland area where the tree trunks were
light colored. The results of this second experiment were the opposite of those of the first trial.
Kettlewell observed higher predation by birds on the carbonaria moths than on the typica moths,
and he recaptured more of the typica moths than the carbonaria moths at the end of the day.
Kettlewell concluded that the moths that were more visible to the birds were more likely to be
eaten, making the less-visible moths more likely to pass on their genes to the next generation.
After decades of differential predation resulting in differential reproductive success, the initally rare,
dark- colored carbonaria moths eventually became more and more common in industrialized
areas.
Kettlewell concluded that Tutt's hypothesis was valid, and that the changes in the moth populations
were caused by natural selection.
Flawed Science Exposed, Better Experimental Design Implemented
Although his conclusions seemed sound, Kettlewell's experimental design was flawed. For
example, he released and recaptured the moths on exposed tree trunks rather than on their
natural resting places on trunk/branch joints and on branches. Furthermore, Kettlewell did not
measure the abundance of lichen coverage in the different areas. This flaw became apparent
years later when, after the rate of pollution was reduced, the proportion of typica moths
overcame that of carbonaria moths before the lichens had returned.
Years later, other scientists conducted their own experiments to try to explain the changes in
moth varieties with changes in pollution. One such scientist was Michael Majerus, who
conducted research from the mid-1960s to the 1990s. Using improved experimental methods
and technologies, he (and others) obtained results supporting Kettlewell's conclusions, even
though Kettlewell's own experiments are now considered to be too deeply flawed to do so
themselves.
Complete Part II of your Student Journal.
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Natural Selection
Biological Evolution and Classification
Part III: Differential Reproductive Success
A + B + C = Differential Reproductive Success
A.  Genetic variation
Allele frequency changes in a population due to variation, or differences in
alleles that are passed on. Different alleles are sometimes better suited
to different environments than others. Allele frequencies change from
generation to generation, as some are passed on more easily than others.
The advantage or disadvantage of a given allele often depends on
environmental conditions..
B.  Potential of a population to produce more offspring than can
survive (overpopulation)
Each individual within a population will produce as many offspring as they
can. This increases the chances that at least some of the offspring will
survive to adulthood and reproduce. Hence, the population as a whole will
often produce more offspring than the environment can support.
C. Finite supply of environmental resources
The environment provides a limited supply of food, water, shelter, and
space. Moreover, the amount of each resource provided by the
environment fluctuates or changes over time. Those that are best suited
to obtain resources in their environment (best adapted) are more likely to
survive and reproduce.
= Differential Reproductive Success
Some individuals/variants produce more offspring that survive to
adulthood and reproduce than others. Those that are the most fit, or able
to obtain resources and reproduce, tend to produce more offspring and
thus pass on their alleles to the next generation.
Please continue to the next page.
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Natural Selection
Biological Evolution and Classification
Part III: Differential Reproductive Success, continued
You are going to analyze how the ABC's of differential reproductive success work to produce
changes in a population of organisms over time.
Scenario:
There are two islands consisting of true breeding tortoises. True breeding organisms are
homozygous for a specific trait. Island A consists of true breeding long neck tortoises and Island B
contains true breeding short neck tortoises. You are going to relocate tortoises from Island B to
Island A. They will mate and help us model trait frequency.
Procedure:
Reproduction Rules
1. Obtain two sets of True Breeding Tortoises.
Females are represented by one color of paper and
the males by another color of paper.
2. Place six long neck and four short neck females
in a plastic cup. Then place six long neck and
four short neck males in another cup.
3. Without looking, select a male and a female
tortoise from each cup to mate.
4. In Part III of your Student Journal, record the
neck type of each parent and the resulting offspring.
5. Return the female piece to the cup containing the
female breeding tortoises, and the male piece to
the cup containing the male breeding tortoises.
Each mated pair will produce
10 offspring from each trial.
Dominant Phenotype:
Long Neck
Parents
Long
Neck
Short
Neck
Long
Neck
NN
Nn
Short
Neck
Nn
nn
6. Gently mix the pieces in each cup before repeating
the breeding process for a total of eight trials.
Then, answer questions 1-5 to evaluate the effects of
the ABC's of natural selection on this population of
tortoises.
Complete Part III of your Student Journal.
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Natural Selection
Biological Evolution and Classification
Part IV: From One Species to Another
As you have learned, natural selection is a process that drives
certain alleles to higher or lower frequencies within a
population of organisms. If a population is divided, and the
subpopulations remain isolated for a long period of time, then
natural selection may increase and decrease the frequencies
of different alleles in each subpopulation, causing the
subpopulations to diverge from one another. If the genetic
differences between the subpopulations become large enough, then the subpopulations become
genetically incompatible and can no longer produce viable offspring by interbreeding.
Speciation is the formation of new species via the divergence of subpopulations. Natural selection
can contribute to speciation by causing the gene pools of subpopulations to become increasingly
different from one another.
In the 1830's when Charles Darwin visited the Galápagos Islands, the vice-governor accurately
informed him that, just by looking at the shell of a tortoise, one could identify which island it came
from. Although, they are identified as a “vulnerable species,” seven different subspecies of
tortoises (Chelonoidis nigra) live in the wild on the Galápagos Islands today.
In order to be considered part of the same species, two organisms must be able to breed and
produce fertile offspring. Subspecies are populations of organisms that can interbreed, but they
are unlikely to do so in nature as they are separated by geography or other factors. Over many
generations, natural selection can cause one subspecies or the other to lose some alleles from
their gene pool while the frequencies of other alleles increase. Eventually, each subspecies
develops different traits that are unique to its gene pool, just like the shapes of the Galápagos
tortoises' shells!
To understand how natural selection could lead to the evolution of the many diverse subspecies of
tortoise in the Galápagos Islands, it is necessary to know about the geography of the Galápagos
Islands.
Work on the questions in Part IV of your Student Journal and continue to the next page.
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Natural Selection
Biological Evolution and Classification
Part IV: From One Species to Another, continued
The Galápagos Islands are located about 600 miles west of
Ecuador and sit right along the equator. They are a geologically
young island chain, with the oldest (and largest) islands having
formed around 5 million years ago.
Scientists hypothesize that the original tortoises were carried to the Galápagos Islands by a strong
current from the South American mainland. Tortoises do not swim, but they can float and are
capable of going a long time without eating or having fresh water.
Since Darwin's visit nearly 200 years ago, scientists have used DNA evidence to trace the genetic
history of the tortoises. They have concluded that the larger islands were colonized by the original
drifters. Over thousands of years, new islands formed that were later colonized by plants and other
organisms. The tortoises gradually dispersed to the newer islands using the local currents over
many generations. As moving between the islands was a rare event, each island colony had an
extremely limited gene pool containing only a subset of the alleles from the gene pools on the
larger islands. The current phenotypic diversity observed among the seven tortoise subspecies is
due to the different frequencies of alleles among the isolated subpopulations on the different
islands.
The closest relative to the Galápagos tortoise is the Argentine tortoise (Chelonoidis chilensis),
which is a smaller species of tortoise that lives on the mainland of South America.
Complete Part IV and the Reflections and Conclusions of your Student Journal.
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