Download Darwin`s Theory of Natural Selection

Darwin’s Theory of Natural Selection
Skim section 15-1 then answer the following questions:
1.
Who was Charles Darwin?
2.
Where are the Galapagos Islands?
3.
What did he notice about the organisms on the Galapagos Islands?
1. There is variation (differences) among offspring, caused mainly by genetic
differences.
2. In nature there is an overproduction of offspring. Or, many more young are
produced than are able to survive.
3. There is struggle for existence as organisms must compete for limited
resources (such as food, space, etc).
4. There is differential survival and reproduction among organisms: individuals
born with favorable variations (traits) that are better suited to their particular
environment are more likely to survive and/or reproduce – passing on the
favorable traits to their offspring (who pass it on to their offspring, who pass
it on to their offspring, etc.) Other organisms die sooner and/or leave fewer
offspring.
5. Over time, the population will be mostly made up of organisms with the
favorable traits (that are better suited to their environment).
Examples: apply the 5 major principles (steps) of natural selection to the following
2 examples:
Jelly bellicus example:
1.
2.
3.
4.
5.
Hummingbirds:
Watch video clip #4 at:
http://www.pbs.org/wgbh/evolution/educators/teachstuds/svideos.html
1.
2.
3.
4.
5.
Peppered Moths: Natural Selection in Action
England’s peppered moth provides an example of natural selection in action. It also offers us a chance
to study the sorts of experiments that can be used to test evolutionary theory. The story is as follows. The
peppered moth spends much of the daytime resting on the bark of oak trees. In the beginning of the
nineteenth century, the trunks of most oak trees in England were light brown speckled with green. Most of
the peppered moths of that time were mottled light brown too. There were always a few dark-colored moths
around, but light-colored moths were always the most common.
Then the Industrial Revolution began in England. Pollution (mostly soot from burning coal) stained
London’s tree trunks dark brown. At about the same time, biologists noticed that more and more moths with
dark coloration were appearing. Why was the population changing color this way?
The evolutionary hypothesis suggested by observation was straightforward. Birds are the major
predators for peppered moths. It is much harder for birds to see, catch, and eat moths that blend in with the
color of the tree bark than it is for them to spot moths whose color makes a strong contrast with the tree
trunks. The moths that blend in with their background are said to be camouflaged.
As the tree trunks darkened, the rarer, dark-colored moths were better camouflaged and harder for
birds to spot. Being harder to spot, the darker individuals were now better able to survive. The darker forms
had greater fitness than the lighter forms. More of the darker moths survived and reproduced, passing on the
genes for dark color to their offspring, and the moth population evolved darker coloration.
But a hypothesis that looks good is not enough. Scientific hypotheses must be tested by experiment
whenever possible. British ecologist H.B.C. Kettlewell devised just such a test for this hypothesis. Kettlewell
learned how to capture and mark both light and dark-colored moths in such a way that birds could not see
the marks.
Kettlewell then released equal numbers of light and dark-colored moths in two types of areas. In one
area, trees were normally-colored. In the other area, they were blackened by soot. Later on, they recaptured,
sorted, and counted all marked moths he could. What type of results do you think Kettlewell needed to either
prove or disprove the hypothesis?
Kettlewell found that in unpolluted areas, more of his light-colored moths had survived. In sootblackened areas, more of his dark-colored moths had survived. Thus, Kettlewell showed that in each
environment, the moths that were better camouflaged had the higher survival rate. It was logical to conclude
that when soot darkened the tree trunks in an area, natural selection caused the dark-colored moths to
become more common. Today Kettlewell’s work is considered to be a classic demonstration of natural
selection in action.
Female peppered moths lay dozens of eggs.
These eggs then go through the typical life cycle of
moths and butterflies ….
egg → larva (caterpillar) → pupae → adult (moth)
Analysis
1. a) What color were most peppered moths prior to the Industrial Revolution? ______________
b) Why (how did this color help them survive longer and/or reproduce more)?
2. a) What color were most peppered moths after the Industrial Revolution? _______________
b) Why (how did this color help them survive longer and/or reproduce more)??
3. Fill in Darwin’s 5 steps of Natural Selection to the peppered moths:
I. Variation among offspring (how do moths differ that is applicable here?)
II. Overproduction of offspring (how many moths does on female lay?)
III. Struggle for existence (what do moths compete for?)
IV. Differential survival and reproduction
(which variation had an advantage for survival – & thus produced MORE offspring?)
 Before the Industrial Revolution......

After the Industrial Revolution.......
V. Population Change: 0ver time, most moths were

___________________ before the Industrial Revolution

___________________ after the Industrial Revolution
4. England has since ‘gone green’ – decreasing its coal burning and thus its soot output. What do you think the
population of moths is like now? Explain your reasoning.
Extra Credit 3 pts: On a piece of small poster paper, create a series of drawings that illustrate the change in
proportion of light vs. dark coloration in moth population over time (at least 1 drawing for pre-Industrial, postIndustrial, current day). Remember that some moths of the less advantageous color still persist at all times!