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DARWIN’S THEORY OF NATURAL SELECTION
In 1831, Charles Darwin left England on board the HMS Beagle.
On his five-year voyage, Darwin made many important observations,
included the diversity of living things, the remains of ancient organisms, and
the characteristics of organisms living on the Galapagos Islands, off the
west coast of Ecuador. Many of Darwin’s ideas were used to create his nowfamous theory of evolution. A scientific theory is a well-tested concept
that explains a wide range of observations in a clear and measurable way.
In 1835, the Beagle reached the Galapagos Islands in
the Pacific Ocean. Darwin was surprised that many of the
plants and animals on the Galapagos Islands were similar to the
organisms on mainland South America. However, there were
also important differences. Darwin hypothesized that a small
number of different species had come to the islands from the
mainland. Over millions of years, their offspring became
different from their mainland relatives.
The small finches on the Galapagos Islands were
noticeably different from one another. The most obvious
differences were the varied sizes and shapes of the birds’ beaks.
Beak shape is an example of an adaptation, a trait that helps
an organism to survive and reproduce. Darwin reasoned that
plants or animals that arrived on the Galapagos Islands faced
conditions that were different from those on the mainland.
Darwin deduced that the species may have gradually changed over many
generations and became better adapted to their new habitats. The gradual change in
one species over time is called evolution.
In his book On the Origin of Species, Darwin proposed that evolution occurs by a
process called natural selection. Natural selection is the process by which individuals that are
better adapted to their environment are more likely to survive and reproduce than other
members of the same species. A number of factors affect natural selection: overproduction,
competition, and variations. Any difference between individuals of the same species is called a
variation. Some variations make certain individuals better adapted to their environment
because of helpful traits they possess. Darwin proposed that, over a long period of time,
helpful variations may gradually accumulate in a species, while unfavorable
variations may disappear. Without variations, all members of the same species would have
the same traits. Only traits that can be passed down from parent to offspring, or are inherited,
can be acted upon by natural selection.
DARWIN’S THEORY OF NATURAL SELECTION
1. T or F? Darwin observed the greatest diversity of organisms in the Galapagos Islands?
____________
2. Look at the bird below. Which bird feeds on seeds and which bird feeds on insects?
a) __________
b) __________
3. A trait that helps an organism to survive and reproduce is called a(n) ____________.
4. Circle the letter of each sentence that is true about Darwin’s conclusions.
a) Darwin understood immediately why Galapagos organisms had many adaptations
b) Darwin thought the Galapagos organisms gradually changed over many generations
c) Darwin believed that evolution had occurred in the Galapagos Islands.
d) Darwin noticed that the Galapagos organisms were identical to mainland species.
5. A scientific _______________ is a well-tested concept that explains many observations.
6. In his book, On the Origin of Species, Darwin explained that evolution occurred by means of
_________________________.
7. True of False? Individuals with variations that make them better adapted to their
environment will not survive. __________
Match the factors that affect natural selection with their correct definitions:
8. Coming into conflict over limited food and other resources.
a. overproduction
9. Differences between individuals of the same species.
b. competition
10. Caused by species producing more offspring than can survive.
c. variations
11. True or False? Only traits that are passed from parent to offspring by genes can be acted
upon by natural selection. ____________
12. That some newly hatched turtles can swim faster than others of the same species is
evidence of _________________________ within the species.
13. True or False? Darwin knew about DNA, genes, and chromosomes. ____________