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Chapter 4
Population Changes
Preview
Section 1 Change over Time
Section 2 How Do Population Changes Happen?
Section 3 Natural Selection in Action
Concept Mapping
Chapter 4
Section 1 Change over Time
Bellringer
The cockroach originated on Earth over 250
million years ago and is thriving today all over the
world. A giant deer that was 2 m tall first appeared
less than 1 million years ago and became extinct
around 11,000 years ago. Why do you think one
animal thrived and the other one perished?
Record your answer in your science journal.
Chapter 4
Section 1 Change over Time
Objectives
• Identify two kinds of evidence that show that
organisms have changed over time.
• Identify how the fossil record shows that changes
in the kinds of organisms in the environment have
been occurring over time.
• Describe one pathway through which a modern
whale could have arisen from an ancient mammal.
• Explain how comparing organisms can provide
evidence that they have ancestors in common.
Chapter 4
Section 1 Change over Time
Differences Between Organisms
• A characteristic that helps an organism survive and
reproduce in its environment is called an
adaptation.
• Some adaptations are physical, such as fur or
scales.
• Other adaptations are behaviors that help an
organism find food, protect itself, or reproduce.
Chapter 4
Section 1 Change over Time
Differences Between Organisms, continued
• Individual organisms that share the same
characteristics might be members of the same
species.
• A species is a group of organisms that can mate
with one another to produce fertile offspring.
• Groups of individuals of the same species living in
the same place make up a population.
Chapter 4
Section 1 Change over Time
Differences Between Organisms, continued
• Do Species Change over Time? Since life first
appeared on Earth, many species have died out,
and many new species have appeared.
• Scientists observe that the inherited
characteristics in populations change over time.
• Scientists think that as populations change over
time, new species form.
Chapter 4
Section 1 Change over Time
Evidence of Change over Time
• Sediments laid down over time contain the remains
of ancient organisms. Older layers are deeper, and
contain older organisms.
• Fossils The remains or imprints of once-living
organisms found in the layers of rock are called
fossils.
• Examples of fossils vary from footprints to complete
organisms.
Chapter 4
Section 1 Change over Time
Evidence of Change over Time, continued
• The Fossil Record By studying fossils, scientists
have made a timeline of life that is known as the
fossil record.
• The fossil record organizes fossils by their
estimated ages and physical similarities.
• Comparing organisms in the fossil record can reveal
how organisms have changed over time.
Chapter 4
Section 1 Change over Time
Evidence of Ancestry
• Scientists observe common characteristics in all life,
which suggests that all living species descended from
common ancestors.
• Evidence of common ancestors can be found in
fossils and in living organisms.
Chapter 4
Section 1 Change over Time
Evidence of Ancestry, continued
• Drawing Connections Scientists examine the
fossil record to figure out the relationships between
extinct and living organisms.
• Scientists draw models that show proposed
relationships between ancient and modern
organisms.
Chapter 4
Section 1 Change over Time
Examining Organisms
• Case Study: Origins of the Modern Whale
Scientists think that the ancient ancestor of whales
was probably a mammal that lived on land and
could run on four legs.
• Comparisons between modern whales and a large
number of fossils have supported this hypothesis.
Chapter 4
Section 1 Change over Time
Evidence of Whale Origins: A
Chapter 4
Section 1 Change over Time
Examining Organisms, continued
• Walking Whales Each new species in the fossil
record of the whale has shared traits with an earlier
species.
• Some species had new traits that were passed on
to later species. In all cases, the organisms had
traits that allowed them to survive in their unique
environment.
• Modern whales have hip bones, which is a link to
their ancient walking ancestors.
Chapter 4
Section 1 Change over Time
Evidence of Whale Origins: B
Chapter 4
Section 1 Change over Time
Comparing Organisms
• Comparing Skeletal Structures The structure
and order of bones of a human arm are similar to
those of the front limbs of a cat, a dolphin, and a
bat.
• These similarities suggest that cats, dolphins,
bats, and humans had a common ancestor.
Chapter 4
Section 1 Change over Time
Chapter 4
Section 1 Change over Time
Comparing Organisms, continued
• Comparing DNA Molecules Scientists also
compare organisms at the molecular level.
• Species that have more DNA in common are more
closely related and have shared a common
ancestor more recently.
• The fact that all existing species have DNA supports
the theory that all species share a common
ancestor.
Chapter 4
Section 2 How Do Population
Changes Happen?
Bellringer
The following are traits that almost all humans have in
common: upright walking, hair, fingerprints, binocular
vision, and speech. List the advantages and
disadvantages of each trait. Do you think the
advantages are greater than the disadvantages? Why or
why not?
Record your responses in your science journal.
Chapter 4
Section 2 How Do Population
Changes Happen?
Objectives
• Describe how some finch species of the Galapagos
Islands developed adaptations in response to their
environment.
• Describe the four parts of Charles Darwin’s
process of natural selection.
• Explain how variation in each species can occur.
Chapter 4
Section 2 How Do Population
Changes Happen?
Charles Darwin
• In the 1800s, scientists were beginning to see
evidence of change in the fossil record, but no one
was able to explain how change happens until
Charles Darwin.
• Darwin signed-on for a five-year voyage around the
world and worked as a naturalist.
• His observations helped him form a theory about
how change happens.
Chapter 4
Section 2 How Do Population
Changes Happen?
Charles Darwin, continued
• Darwin’s Excellent Adventure Darwin collected
thousands of plant and animal samples.
Observations made on the Galápagos Islands west
of Ecuador were especially important to his theory.
Chapter 4
Section 2 How Do Population
Changes Happen?
Charles Darwin, continued
• Darwin’s Finches Darwin noticed that the finches
of the Galápagos Islands were a lot like those in
Ecuador. And the finches on each island differed
from the finches on the other islands.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Thinking
• Darwin hypothesized that the island finches
descended from South American finches.
• And Darwin suggested that over many generations,
the finches developed adaptations for the various
island environments.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Thinking, continued
• Ideas About Breeding In Darwin’s time, farmers
and breeders had produced many kinds of farm
animals and plants. These plants and animals had
traits that were desired by the farmers and breeders.
• A trait is a form of a genetically determined
characteristic.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Thinking, continued
• The practice by which humans select plants or
animals for breeding based on desired traits is
selective breeding.
• Most pets, such as dogs, have been bred for their
desired traits.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Thinking, continued
• Ideas About Population Darwin was influenced by
an essay that described the reasons that human
populations do not grow uncontrollably.
• Darwin knew that the populations of all species are
limited by starvation, disease, competition, and
predation.
• Darwin reasoned that the offspring of the survivors
inherit traits that help the offspring survive in their
environment.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Thinking, continued
• Ideas About Earth’s History New ideas abut the
age of the Earth provided enough time for
adaptations to occur. This helped to support
Darwin’s belief that species change over time.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Theory of Natural Selection
• In 1859, Darwin published a famous book called On
the Origin of Species by Means of Natural Selection.
• In his book, Darwin proposed the theory that change
in populations happens through natural selection.
• Natural selection is the process by which organisms
that are better adapted to their environment survive
and reproduce more successfully than less well
adapted organisms do.
Chapter 4
Section 2 How Do Population
Changes Happen?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 4
Section 2 How Do Population
Changes Happen?
Darwin’s Theory of Natural Selection,
continued
• Genetics and Inherited Traits Today, scientists know
that variation happens as a result of of the exchange of
genetic information as it is passed from parent to
offspring.
• Some genes make an organism more likely to survive
to reproduce.
• The process called selection happens when only
organisms that carry these genes can survive to
reproduce.
Chapter 4
Section 3 Natural Selection in Action
Bellringer
A mnemonic device is a trick that can help you
remember words. Write down the four steps of
natural selection. Then create a mnemonic device
to help you remember each step by using the first
letter of the main word from each step as the first
letter of a word in a phrase that is easy for you to
remember.
Write your answer in your science journal.
Chapter 4
Section 3 Natural Selection in Action
Objectives
• Give two examples of how natural selection can
result in an adaptation that helps an organism
survive.
• Outline the process of speciation.
Chapter 4
Section 3 Natural Selection in Action
Changes in Populations
• The theory of natural selection explains how a
population changes in response to its environment.
• Well-adapted individuals will likely survive and
reproduce.
Chapter 4
Section 3 Natural Selection in Action
Changes in Populations, continued
• Adaptation to Hunting Scientists think that hunting
in Uganda is affecting Uganda’s elephant
population.
• Because ivory is very valuable, elephants are
hunted for their tusks.
• So, fewer tusked elephants survive to reproduce,
and more tuskless elephants survive. Tuskless
elephants pass the tuskless trait to their offspring.
Chapter 4
Section 3 Natural Selection in Action
Changes in Populations, continued
• Insecticide Resistance Often, insect populations
become resistant to insecticide because the insects
produce many offspring and have a short generation
time.
• Generation time is the average time between one
generation and the next.
Chapter 4
Section 3 Natural Selection in Action
Changes in Populations, continued
Chapter 4
Section 3 Natural Selection in Action
Changes in Populations, continued
• Competition for Mates Survival of individuals
does not guarantee survival of a species.
• For organisms that reproduce sexually,
competition for mates can select for adaptations.
Chapter 4
Section 3 Natural Selection in Action
Forming a New Species
• Sometimes, drastic changes that can form a new
species takes place.
• A new species may form after a group becomes
separated from the original population.
• The formation of a new species as a result of
change over time is called speciation.
Chapter 4
Section 3 Natural Selection in Action
Forming a New Species, continued
• Separation Speciation often begins when a part of
a population becomes separated from the rest.
• Adaptation Populations constantly undergo
natural selection. After two groups have separated,
natural selection continues to act on the groups.
• If the environmental conditions for each group
differ, the groups’ adaptations will differ.
Chapter 4
Section 3 Natural Selection in Action
The Development of Galápagos Finch Species
Chapter 4
Section 3 Natural Selection in Action
Forming a New Species, continued
• Division Over many generations, two separated
groups of a population may become very different
through natural selection.
• Even if a geological barrier is removed and the
groups are reunited, they may no longer be able to
interbreed.
• If they cannot interbreed, the two groups are no
longer the same species.
Chapter 4
Section 3 Natural Selection in Action
Chapter 4
Section 3 Natural Selection in Action
Species
Click below to watch the Visual Concept.
Visual Concept
Chapter 4
Population Changes
Concept Mapping
Use the terms below to complete the concept map on
the next slide.
change
evidence
extinct species
living species
common ancestors
DNA
time
fossil record
body structures
Chapter 4
Population Changes
Chapter 4
Population Changes