Thursday April 17
• Essential Question: What is a selective
pressure? What are some examples?
• Today Peppered Moth Case Study. For
Full points Answer Questions in complete
sentences.
• Finish Study Guides
The Peppered Moth
The Peppered Moth has 2 variations: light gray
and dark gray.
The "typica" form of the
moth (light-colored)
The "carbonaria"
form (dark-colored)
The peppered moth lives in a forest
in England.
The trees in the forest are covered
with lichen.
The black moths
were easy to see
against the
lichen, but the
light moths
blended in and
were hard to see.
Birds ate more
of the dark
moths, until
there were a lot
of light moths
and very few
dark ones.
Ha ha! You can’t
see me…
If you were a
hungry bird,
which of these
peppered
moths would
you grab for
lunch?
One day factories
moved into the area,
and released
pollution into the air.
The trees were
blackened with
soot.
What do you think happened to the moths?
You’re right!
The birds ate more of the light moths because
they were now easier to see than the dark moths.
Eventually there were a lot of dark moths
and very few light ones.
Peppered
moths on a
normal tree
background
Peppered
moths on a
tree
darkened by
soot
When the
monster came,
Lola, like the
peppered moth
and the arctic
hare, remained
motionless and
undetected.
Harold, of
course, was
immediately
devoured.
Peppered moths have lived in the forests of
England for thousands of years. They rest on
the trunks of trees during the day, and are a
source of food for many birds.
Peppered moths vary in color, from lightcolored to dark-colored. These color variations
are genetic.
Before the Industrial Revolution, the tree trunks
were light-colored. The trunks and branches
were also covered with silvery-white lichens. As
industry grew, pollution killed the lichens and
blackened the tree bark.
In the 1950s, Oxford University professor H.B.D.
Kettlewell and his students performed an
experiment. They released the same number of
light and dark-colored moths in two areas. One
was an unpolluted area that had lichen-covered
oak trees; the other was a wooded area that had
experienced pollution for many years.
After a certain amount of time, they recaptured as
many moths as they could, and counted them.
What do you think the results of the experiment
were?
You’re right!
In the unpolluted area (light-colored tree
trunks), more light-colored moths
survived.
In the polluted area (dark-colored tree
trunks), more dark-colored moths
survived.
How does this experiment show the
process of natural selection?
Note that this example illustrates Darwin's four
points:
 moth eggs are over-produced; many moths
will not survive to reproduce.
 there is variation among individuals -- some
are black in color, others are light gray
 the variation is genetic –black moths have
different alleles for color than do gray moths
 the different colors differ in advantage-- in
industrial environments, black moths survive
to reproduce better than gray moths do. In
unpolluted environments, the opposite is true.