Page 114 - Science Course 3 (Book 1)

Mo3-L2a: What is Darwin’s Theory of Evolution?

Charles Darwin Natural selection is the process by which
populations of organisms with variations that help
Darwin found that each island in the Galápagos had a them survive in their environments live longer,
different environment, and tortoises looked different compete better, and reproduce more than those
depending on which island environment they inhabited. that do not have the variations.

Natural selection explains how populations change
as their environments change.

Domed Tortoise Saddle-back Tortoise
• shell come to neck • large space between
• can only reach low shell and neck.
vegetation. • can reach high vegetation

Intermediate Tortoise
• shell shape is between
dome and saddle-back
• can reach low and high
vegetation.

Darwin’s Theory

A variation is a slight difference in an inherited trait of
individual members of a species.

Variations arise naturally in populations, occurring in
offspring as a result of sexual reproduction.
1. Variation in Traits
Genetic changes to phenotype can be passed on to In this population of beetles, some and yellow and
future generations. some are brown. The color does not affect the ability
of the beetles to survive in their environment.

2. Organism Compete
A new predator eats yellow beetles more often
because it sees the yellow beetles more easily that
brown beetle.

3. Traits are inherited
The yellow beetles do not live as long as the brown
beetles, and – since color is inherited – fewer yellow
beetles hatch.

4. Adaption over Time
Nearly all individuals in a population are brown. The
color brown has become an adaption that helps the
beetles avoid predators in that environment.
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