Change Over Time

Dramatic changes have occurred on Earth over timeClimates have changed, forests have become deserts, and seas have dried up. These changes affect the organisms within these environments. Organisms must respond to changes in order to survive.

Charles Darwin

In the 1800s, Charles Darwin developed a theory about how organisms change over time. He studied different populations of organisms.

PopulationA population is all of the members of a single species living in the same geographic area.

DarwinCharles Darwin studied different populations of organisms.

Darwin noticed variations within the populations.

He did not understand how these variations occur.

Today, scientists know that variations result from changes in an organism’s genetic material.

Variations

Variations are differences among members of the same species.

Natural Selection

Natural Selection is Darwin’s theory.It states that an organism with favorable variations is well suited to its environment. It is more likely to survive and reproduce than other organisms. These variations will be passed on to the next generation. Over time, the offspring of individuals with favorable variations make up a greater percentage of a population.

Natural Selection

Darwin’s theory is sometimes known as “survival of the fittest” because the organisms with the most favorable traits survive.

What causes variations?

Changes in an organism’s genetic makeup.

An organism that has changed over time…Horses have changed over time. Ancestors of

horses walked on several spread-out toes that probably helped the animals move through swamps and mud.

Once land dried, the horses also changed. They developed single hooves that better suited the horses to run on hard ground. This helped them be able to run fast and escape predators.

Extinct

An extinct species no longer exists anywhere on Earth.

Species become extinct because they cannot adapt.

How does natural selection explain how organisms change over time?Organisms with the favorable variations are better suited for survival and are more likely to reproduce and pass on those traits to their offspring. Over time, the organisms with the favorable variation make up a greater percentage of the population, possibly resulting in a new species.

A few bacteria in a population are resistant to an antibiotic. Explain how this bacteria population might change over time.The bacteria that are resistant to the antibiotic will reproduce and pass on this resistant trait to their offspring. Eventually the population of bacteria will consist of the antibiotic resistant bacteria.

What are fossils?

Fossils are the remains, traces, or imprints of organisms that lived long ago.

Fossils

Organisms generally decay quickly after they die. Sediment can sometimes quickly cover an

organism soon after it dies and the hard parts (skull, bones, teeth, shell, etc…) of the organism will be preserved.

These hard parts can become fossils.Minerals in the sediment replace the hard body

parts and the surrounding sediment along with the body harden into rock.

A fossil forms.

FossilsInsects can become trapped in tree sap.

The tree sap will harden into amber and this preserves the insect creating a fossil.

Fossil FormationImprints, like an animals footprint, are made when animals walk across wet soil. If a footprint is covered quickly by sediment, the footprint can be preserved.