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Darwin’s Voyage
Darwin’s Voyage
Darwin’s Observations
• As Darwin traveled around the world on a
British naval ship, he was amazed by the
incredible diversity of the organisms or
species that he saw.
• Species
– A group of similar organisms that can mate
with each other and produce fertile offspring.
Galapagos Island & South
America
• Darwin was surprised that many of the plants
and animals were similar to organisms on
mainland South America, yet there were also
important differences.
• Darwin inferred that a small number of different
species had come to the island from the
mainland and that eventually their offspring
became different from the mainland relatives.
• Adaptation
– A trait that helps an organism survive and reproduce.
– Ex. Beak shape and size differences in the finches
Darwin’s Finches
Evolution
• Evolution
– The gradual change in species over many
generations in order to become better adapted to the
environmental conditions.
– Darwin proposed that evolution occurs by natural
selection.
• Natural Selection
– 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.
Factors Affecting Natural
Selection
• Overpopulation
– Most species produce far more individuals than can
possibly survive.
• Competition
– Offspring must compete for food and other resources
to survive.
• Variations
– Difference between individuals of the same species.
• Selection
– Some variations make certain individuals better
adapted to their environment.
– These will survive and reproduce, thus possibly
passing this allele onto their offspring.
Types of Selection
• Stabilizing Selection
– Natural selection favors average individuals.
– Reduces variation in a population.
• Directional Selection
– Natural selection favors one of the extreme variations
of a trait.
– Leads to rapid evolution of a population.
• Disruptive Selection
– Favors both extreme variations of trait
– Results eventually in no intermediate forms of the trait
and leads to the evolution of two new species.
Relating Natural Selection
to Evolution
• Over a long period of time, natural
selection can lead to evolution.
• Helpful variations gradually accumulate in
a species, while unfavorable ones disappear.
• Ex. Faster turtles will be able to escape
predators and thus might pass this trait onto
their offspring. This species might then be
marked by the “fast-swimmer” trait.
Role of Genes in Evolution
• NOTE: Only traits that are inherited, or
controlled by genes, can be acted upon by
natural selection.
• Ex. Color in moths during the Industrial
Revolution
How Do New Species Form?
• Geographic Isolation
– New species can form when a group of
individuals remains separated from the rest of
its species long enough to evolve different
traits.
– Ex. Kaibab vs. Abert’s Squirrel
• Continental Drift Theory
– Emphasizes that Pangea breaking apart
resulted in geographic isolation which led to
evolution of various species
Development of New Species
Patterns of Evolution
• Divergent Evolution
– Pattern of evolution in which species that once were
similar to an ancestral species diverge, or become
increasingly different
– Caused by populations adapting to different
environmental conditions and eventually resulting in
new species
• Convergent Evolution
– Pattern of evolution in which unrelated species evolve
similar traits because they occupy similar
environments in different parts of the world.
The Flip Side of the Coin
• Mutations or changes in the DNA are
rarely helpful but are usually neutral,
harmful, or fatal.
Summary Questions
• What is evolution? What did Darwin observe on
the Galapagos Islands that he thought was the
result of evolution?
• Explain why variations are needed for natural
selection to occur.
• Describe how geographic isolation can result in
the formation of a new species.
• Some insects look just like sticks. How could
this be an advantage to the insects? How could
this trait have evolved through natural selection?