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Lesson 2
The Theory of Evolution
By Natural Selection
Charles Darwin
• Scientists had evidence that showed changes
in species over time, but they didn’t know
how.
• Charles Darwin was one scientist who
struggled with this idea.
Charles Darwin
• Darwin was a NATURALIST, a person who studies
plants and animals by observing them.
• He was not the first scientists to develop a theory
about evolution, but his theory is the one best
supported by evidence.
Charles Darwin
• Darwin’s breakthrough came while on a voyage to
the Galapagos Islands (near the equator off the
pacific coast of South America).
• Darwin noticed slight differences in animals living
on each of the islands; he later decided that some
varieties were different enough to be classified as
different species.
Darwin’s Theory
• Darwin noticed a relationship between each species
and the food sources of the island it lived on.
Darwin’s Theory
• The species of tortoise that lived on an island with
tall cacti had long necks, while the species of
tortoise on an island with lots of short grass had
short necks.
Darwin’s Theory
Darwin’s Theory
• Darwin thought all the Galapagos
tortoises shared a common ancestor
that came to one of the islands
millions of years ago.
• He knew that members of the same
species each have slight differences
called VARIATIONS.
• Darwin didn’t know about heredity,
but he realized that variations in
populations could help explain how
the different species of Galapagos
tortoises and finches evolved.
Darwin’s Theory
• Darwin knew that food is a LIMITING RESOURCE, so
members of a species that live in the same area
compete for food.
• If a variation benefited a tortoise, allowing it to
compete better for food than other tortoises (more
“fit” to its environment), the tortoise was more likely
to live longer, reproduce more, and pass on its
variations (traits) to its offspring.
Natural Selection
• NATURAL SELECTION: Individuals within a population
that possess variations which help them survive in
their environment tend to live longer, compete better,
and reproduce more than individuals that do not have
the beneficial trait.
Adaptations
• An ADAPTATION is an inherited trait that
increases an organism’s chance of surviving
and reproducing in its environment.
• Scientists classify adaptations into three
categories.
Adaptation
https://www.youtube.com/watch?v=YX8VQIJVp
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Adaptations
• STRUCTURAL ADAPTATIONS involve color,
shape, and other physical characteristics.
Adaptations
• BEHAVIORAL ADAPTATIONS involve the way
organisms behave or act.
Adaptations
• FUNCTIONAL ADAPTATIONS involve internal
body systems that affect biochemistry.
Environmental Interactions
• A structural adaptation that aids members of a
species in blending in with their environment is
called CAMOUFLAGE.
Environmental Interactions
• An adaptation in which one species resembles
another species is called MIMICRY.
Environmental Interactions
• The living and nonliving parts of the
environment are always changing; species that
cannot adapt to such changes will become
extinct.
Natural Selection vs. Artificial Selection
• Darwin’s theory of natural selection predicts
that species will develop adaptations, which
explains why we see such a diversity of
organisms that are perfectly suited to thrive in
their environment.
Natural Selection vs. Artificial Selection
• SELECTIVE BREEDING is the practice of
breeding organisms for desired characteristics.
Natural Selection vs. Artificial Selection
• Darwin realized that natural selection and
artificial selection are similar processes.
– In natural selection, nature causes the changes in the
species.
– In artificial selection, humans cause the changes in the
species.
Common Misconceptions about
Evolution
Common Misconceptions about
Evolution
Jean-Baptiste Lamarck
Common Misconceptions about
Evolution
Common Misconceptions about
Evolution
Symbiotic Relationships
Symbiosis- two species living together
4 Types of
symbiosis:
1. Commensalism
2. Parasitism
3. Mutualism
4. Predation
Symbiotic Relationships
Commensalismone species benefits
and the other is
neither harmed nor
helped
Ex. orchids on a tree
Symbiotic Relationships
Parasitismone species benefits (parasite) and
the other is harmed (host)
• Parasite-Host relationship
Symbiotic Relationships
Mutualismbeneficial to
both species
Ex. cleaning birds
and cleaner
shrimp
Predation
One benefits, one DIES!
Type of
Species
relationship
harmed
Commensalism
Parasitism
Mutualism
= 1 species
Species
benefits
Species
neutral