1. Change Over Time
Chapter 2 Lesson 3

2. What is natural selection?
Dramatic changes have occurred on Earth over time: climate change, forests have become deserts, and seas have dried up
These changes affect organisms within the environments and they must change in order to survive
Charles Darwin developed a theory in the 1800s about how organisms change over time
Population- is all of the members of a single-species living in the same geographic area
Darwin noticed that there were variations in the populations (differences among members of the same species)

3. What is natural selection?
Darwin’s theory is called natural selection
An organism with favorable variations is well suited to its environment. It is more likely to survive and reproduce than others.
Its variations will be passed on to the next generation
“Survival of the Fittest” is when the organisms with the favorable traits have survived.
A horse is an animal that has changed over time
Ancestors walked on several spread-out toes that probably helped them navigate swamps and mud. As the land dried, the horses developed single, flat hooves

4. What is natural selection?
Throughout Earth’s history, many organisms have emerged and many have become extinct
Extinct Species no longer exist anywhere on Earth.
Species become extinct because they cannot adapt.

5. What is evidence of change over time?
Scientists know that environments have changed because the history of change can be found in fossils
Fossils- are any remains, traces, or imprints of organisms that lived long ago.
Fossils can be formed when minerals in the sediment preserve bones, shells, or teeth of an animal
Fossils can also be formed when an insect is trapped in tree sap and the same hardens into amber
Footprints can be made when an animal walks across wet sand or mud and sediment quickly covers it and preserves the print.

6. What Fossils tell us
Provide clues that tell us how environments have changed
For example: Scientists have found fossils of an ancient whale called Basilosaurus in Mississippi which tells us that Mississippi was once covered by water
Scientists can also predict past climate of an area
For example: Fossils of a warm-climate animal found in the arctic indicate that particular areas climate was not always artic like
Fossils provide clues about what organisms were like in the past and how they have changed (Smilodon)
Earth’s history is marked by geological and climate change and have caused some organisms to change over time
As environments change, organisms will adapt, move on, or die out

7. How has Earth changed?
Earth’s history is measured in long stretches of time called eras-each era is divided into periods
Precambrian- Earth first became hospitable to life. Few fossils have been found
Paleozoic- life became abundant. Trilobites, fish, insects, amphibians, and reptiles appear during this era
Mesozoic- reveals the appearance of dinosaurs, mammals, birds, and flowering plants. Rock from this era indicate that life became extinct at the end of this era
Cenozoic- the era we live in. fossils show that some early mammals were very large. The most recent ice age occurred during this era.