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Darwin and the
Theory of Evolution
Darwin’s Voyage
In 1831, Charles Darwin set sail on a five
year long voyage on a British Naval ship
called the Beagle. Everywhere he went he
observed the plants and animals.
H.M.S. Beagle
Darwin’s Voyage
Galapagos Islands
When Darwin reached the Galapagos
Islands, he found animals very similar to
the ones he saw on mainland South
America. However, they were a little
different.
South America Iguana Galapagos Iguana
South America Finch Galapagos Finch
South America
Cormorant
Galapagos
Cormorant
Animal Differences
On the Galapagos, the Cormorants
couldn’t fly, the iguanas had long
claws, and the beaks of the finches
were shaped differently.
Adaptations
Darwin realized these were examples of
adaptations. Adaptations are traits
that help an organism survive and
reproduce.
But, how did the animals
get these traits???
Darwin’s Theory
Darwin thought that the ancestors of these
animals lived on the mainland many, many
years before. Maybe they had floated on
logs or were blown to the Galapagos
Islands by a storm.
Theory of Evolution
To survive on the islands, the animals had to
change to become better adapted to the
new conditions. Evolution is the gradual
change in a species over time.
How does
evolution occur?
Darwin wrote a book entitled The Origin of
Species that explained how evolution
happens.
Natural Selection
He said evolution occurs by means of natural
selection. Natural selection is where
individuals that are better adapted to their
environment are more likely to survive and
reproduce. It’s like “survival of the fittest.”
What affects
natural selection?
Darwin wrote in his book that three things
affect natural selection: overproduction,
competition, and variations.
Overproduction
Overproduction means that most species
produce far more offspring than could ever
survive. Sea turtles, for example, lay over
100 eggs. Why do you think they do this?
Competition
Competition means that the offspring will
have to compete with one another for such
things as food and mates.
Variations
Variations are the differences between
individuals of the same species. For
example, some turtles can swim faster
than other turtles after they hatch.
What does it all mean?
Those offspring that are born with the traits
that are better adapted for their
environment will survive more often and
will be the ones to have offspring some
day.
They will pass those traits to their offspring
through their genes and then their
offspring will be more likely to have those
same traits.
The new offspring will have a better chance
of surviving and passing those traits to
their own offspring some day. If this goes
on and on, generation after generation
then…
…this leads to evolution. Helpful variations
accumulate in a species and unfavorable
ones disappear.
Since traits are controlled by genes which
are passed from parents to offspring, only
traits that are inherited can be acted upon
by natural selection.
Here’s an example.
Think about it!
Why do you think the shape of the beak of
the finches varied from one island to
another?
Hint: Think about how they use their beaks.
Think about it!
Why do you think the cormorants on the
Galapagos Islands could not fly?
Hint: The cormorants had to get their food
from the ocean.
Think about it!
Why do you think the iguanas had long claws
on the Galapagos?
Hint: They climbed on slippery rocks to eat
seaweed.
More Evidence
Before the industrial revolution in England,
peppered moths were white. After
factories were built they became black.
Why?
Peppered Moths before the Industrial
Revolution
Peppered Moths after the Industrial Revolution