Download Natural Selection And The Peppered Moth

Natural Selection and the Peppered Moth
Natural Selection is one of the ideas suggested by Charles Darwin to explain evolution.
All living things “inherit” traits from their parents. In humans, a trait can be hair color and
how tall we are (height), just to name a couple. In birds it can be the color of its feathers,
the shape of its beak or the strength of its song. In insects it can be body color or wing
shape. If one (or many) of these traits, which they inherit from their parents, helps them
survive longer, so that they can have more offspring of their own, with those same traits
– they are selected. This means that over many generations, there will be more and
more individuals like them in their population.
Charles Darwin accumulated a tremendous collection of facts to support the theory of
evolution by natural selection. One of his difficulties in demonstrating the theory,
however, was the lack of an example of evolution over a short period of time, which
could be observed as it was taking place in nature. Although Darwin was unaware of it,
remarkable examples of evolution, which might have helped to persuade people of his
theory, were in the countryside of his native England. One such example is the
evolution of the peppered moth Biston betularia.
In the early 1900s, collecting butterflies and moths was a popular hobby. Records
indicate that the peppered moth was very common. It had a whitish appearance with
minute dots and speckles from which it got its name. The pigmentation would allow the
moth to blend into the light-coloured lichen on the tree barks on which it rested. It would
be protected by predatory birds that would feed on them. During the industrial
revolution, coal-burning was common in London and the air was thick with pollution.
Coal smoke blackened the trees until their bark was dark brown. The soot discoloured
and generally darkened the surfaces of trees and rocks. In 1848, a dark-coloured moth
(melanic version) was first recorded. The difference between the black (melanic) and
white peppered moth forms is due to a single DNA mutation. The melanic moth was a
speckled brown moth that blended into the dark English tree bark perfectly. Today, in
some areas, 90% or more of the-peppered moths are dark in colour. More than 70
species of moth in England have undergone a change from light to dark. Similar
observations have been made in other industrial nations, including the United States.
Then in 1956, London passed a clean air act and coal was banned in the city.
Smokestacks were made taller to get pollutants further out into the atmosphere. Within
ten years the trees, once brown from coal smoke, began to take on their natural lightcolored bark. As the trees got lighter, the brown peppered moths stood out against the
bark and were easy targets for hungry birds. Lighter moths, however, blended in and
survived to lay eggs. Over many generations, which for insects can be just a couple of
years, all the peppered moths were lighter in color.
The Percentage of Recaptured Melanic (Black) and Non-Melanic (Peppered White)
Moths in Non-Polluted and Polluted Areas after 28 Days
Location
Non-Polluted
Polluted
Number Released
M
N
473
496
447
137
Number Recaptured
M
N
30
62
130
18
% Recaptured
M
N
6.3
12.5
27.5
13.0
This is how natural selection works, though in mammals and other vertebrates it takes
much longer for traits to spread throughout a population. This physical change is also
called adaptation or structure and function.
Analysis:
Answers should be in complete sentences.
1. What caused the first appearance of the melanic form?
2. What was the actual selective agent in this case study?
3. What happened to the moths that were not recaptured?
4. Why were dark-colored peppered moths successful during the Industrial
Revolution in England?
5. What do you think would have happened to the peppered moth population during
the Industrial Revolution if a dark variety of the species had not existed?
6. Why are dark-colored peppered moths rare today?
7. What would happen if there were no predators in the forest? Would the colors of
the moths change over time? Defend your answer.
8. How is the peppered moth story an example of natural selection? Answer the
question including the words variation, adaptation, and natural selection.