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PEPPERED MOTH AND NATURAL SELECTION
adapted from http://www.mothscount.org/text/63/peppered_moth_and_natural_selection.html
The Peppered Moth is widespread in Britain
and Ireland and frequently found in ordinary
back gardens, yet its amazing story has
made it famous all over the world. It is one
of the best known examples of evolution by
natural selection, Darwin's great discovery,
and is often referred to as 'Darwin's moth'.
lichen on a tree
The Peppered Moth is normally
white with black speckles across
the wings. It looks as if it has
been “peppered,” giving it its
name. Its ideal environment has
many trees covered by lichens.
(lichens are the moss-like dry
fungi that grows on rocks and
trees.) The moth’s patterning
makes it well camouflaged against lichencovered tree trunks when it rests on them
during the day. Predators cannot see them
easily (as you can see in the picture to the
left).
In the moth species, however there is also
a naturally-occurring genetic mutation
which causes some moths to have almost
moth on lichen (on tree)
black wings. These black forms (called
'melanic') are not as well camouflaged on
the lichen as their relatives (the 'peppered' forms). They are
more likely to be eaten by birds and other predators. Fewer
black moths survive to breed and so the black moths are less
common in the population than the paler peppered forms.
There was a time, however, that
this pattern was different. In the
nineteenth century towns and cities
were starting to find more and more
black moths and fewer peppered
moths. In the 1800’s industries and
homes were using coal in large
quantities for power. This created a
large amount of soot in the air and
polluted the air with black dust. This
soot killed the lichens on trees and
blackened the tree trunks and
building walls. This made the pale
peppered form of the moth more
visible as the lichen were gone and
the surfaces were dark. The pale form
of the moth was more obvious to predators. The darker moth
(the melanic one) was actually better camouflaged now. It was
the one that was more likely to survive and produce offspring.
As a result, during the industrial revolution time frame and
over successive generations, the black moths came to
outnumber the pale forms in the towns and cities where there
was a lot of soot in the air. Since moths are short-lived, this
evolution by natural selection happened quite quickly. For
example, the first black Peppered Moth was recorded in
Manchester in 1848 and by 1895 98% of Peppered Moths in the
city were black.
After the cities were beginning to become polluted with soot,
there was an endeavor to clean up the towns. In the midtwentieth century laws and controls were introduced to reduce
air pollution. As a result, the air quality improved, tree trunks
became cleaner, and lichen growth increased again. The town
was no longer black and sooty. The black moths were losing
their camouflage advantage. They were being seen and picked
off by predators once again. The pale Peppered Moths were
once again camouflaged in the lighter lichen-covered trees and
the black forms were more noticeable. Normal pale Peppered
Moths became again far more common than the black forms.
Natural selection worked in both directions, always favouring
the moth that is best suited to the environmental conditions.
Sadly though, this species is now declining overall. Between
1968 and 2002 numbers of the Peppered Moth in Britain fell by
almost two thirds, although the causes are as yet unknown.
Something else is affecting them. Will they survive? Will they
adapt? Will they become extinct? Nature will decide.
Assignment:
Requirements:
 10 points: at least 5 panels long (2 points per panel)
 3 points creativity (be unique and make it worth reading)
 3 points neatness (needs to be legible and easy to see pictures)
 5 points accuracy with information
o 2 points for including peppered moth
o 2 points for including melanic moth
o 1 point for including industrial pollution
 10 points: on the back of your comic strip, draw a graph like the one
below. Draw a line to represent the amount of pollution over time
based on the reading. Then on the same graph, draw a line to
represent the amount of the black (melanic) moths corresponding to
your pollution line.
Amounts increasing -->
DON’T SKIP THIS
STEP!!!!
 20 points: Make a comic strip about the story of the peppered moths
(including the ones that were melanic). Be creative! It doesn’t have to
be colored.
Legend:
 Amount of black moths
 Amount of pollution
Time increasing -->