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Lesson 6-2
The Theory of Evolution by Natural Selection
Darwin’s Voyage
Charles Darwin served as a naturalist aboard the HMS
Beagle.
A naturalist is someone who studies plants and animals
by observing them.
One of the highlights of Darwin’s voyage were the
Galapagos Islands west of South America.
Darwin noticed different varieties of tortoise, finch, and
mockingbird on each island.
Some were different enough to be separate species.
Darwin’s
Voyage
Galapagos Tortoises
Darwin noticed there was a relationship between each
species and the food sources of the island it lived on.
The species of tortoise living on islands with taller
plants had longer necks.
The species of tortoise living on islands with
shorter plants had shorter necks.
Darwin concluded all of the Galapagos tortoises
shared an ancient common ancestor.
Galapagos Tortoise
Darwin’s Finches
Darwin noticed that finches on the islands had
beaks that matched their primary food source.
Finches that ate mostly insects had shorter,
pointier beaks.
Finches that ate mostly large seeds had bigger,
more powerful beaks.
Darwin’s Finches
Darwin’s Theory
Darwin spent many years studying notes and specimens
from the Galapagos Islands.
He realized that variations in populations could help
explain how different species on the Galapagos evolved.
Variations are slight differences within members of
a species.
Populations are groups of individuals from the
same species living in the same area.
Natural Selection
Darwin’s experiences with the Galapagos led him to
develop the theory of evolution by natural selection.
Natural selection is the process by which the best
adapted individuals in a population become most
likely to survive and reproduce.
It predicts that species will develop adaptations
that match their environment over time.
Natural selection is driven by four main factors.
Natural Selection
1. Limited resources mean parents produce more
offspring than can survive.
2. Members of a species must compete for resources.
3. Helpful variations give some individuals an
advantage in the competition for survival.
4. Survivors reproduce and pass on their traits to
offspring.
Adaptation
An adaptation is a characteristic of a species that
helps the species survive in its environment.
There are three main categories of adaptation:
Structural
Behavioral
Functional
Adaptation
Structural adaptations involve shape, size, color, and other
physical features.
Example: A bird’s wing.
Behavioral adaptations involve actions.
Example: A tiger hunting.
Functional adaptations involve internal body systems.
Example: Expanding blood vessel’s in a rabbits ear to
cool off.
Specialty Adaptations
Camouflage is a structural adaptation that aids
members of a species with blending in with their
environment.
Example: An owl that looks like tree bark.
Mimicry is an adaptation in which one species
resembles another species.
Example: Some flies are striped like bees.
Camouflage
Mimicry
Artificial Selection
Selective breeding is the breeding of organisms by
humans for desired traits.
Example: Dog breeds are bred to have a certain
size, color, or behavior.
Since humans are choosing traits instead of nature,
selective breeding is also called artificial selection.
Artificial Selection