Evolution and Fossil Record

Does natural selection still occur today?

•YES…an example: cockroaches

Interpreting the evidence•Fossils- scientists can infer the structure of ancient organisms. Fossils show that organisms that lived in the past are very different than organisms that live today.•Similarities in early development

•Similarities in Body Structure

• An organism’s body structure is its basic body plan, such as how it’s bones are arranged.• Similar structures that related species have

inherited from a common ancestor are known as homologous structures.

Inferring Species Relationships • Along with fossils, early development patterns, and body structures, scientists also use similarities in DNA to show how organisms are related to one another.•Genes are made of DNA, DNA is made of a sequence of nitrogen bases. The more similar the DNA sequence, the more closely related the species are.

Did you know that an elephant and a tiny elephant shrew are closely related?

Combining Evidence• In most cases, evidence from DNA has confirmed conclusions based on fossils, embryos, and body structure.•Sometimes new evidence changes scientists hypothesis.•Branching trees are used to show how scientists think different groups of organisms are related.

Branching Tree

How do new species form?• A new species can form when a group

of individuals remains isolated from the rest of its species long enough to evolve different traits.•Rivers or other bodies of water, a volcano, or a mountain range can separate group members.

Fossil Record

Meet, “Sue.” She was discovered in 1990 in South Dakota and resides in the Field Museum in Chicago.

How do fossils form?• *Most fossils form when organisms that die

become buried in sediment.•Some remains that become buried in sediments are actually changed to rock. These are called petrified fossils.•Sometimes the remains dissolve and a hollow space is created, this space is called a mold. This mold becomes filled with hardening minerals, forming a cast. •Some organisms have been preserved in substances other than sediment…like ice. Ex. mammoths

Fossils

Determining a Fossil’s Age•Scientists can determine a fossil’s age in two ways: relative dating and radioactive dating.•Relative dating is used to determine which of two fossils is older. This is done by looking at the layers of rock in which the fossil was found.

•Radioactive dating allows scientist to find the actual age of a fossil. The rocks that fossils are found near contain radioactive elements, which are unstable that break down or decay into different elements. The half-life is the time it takes for half of the atoms in the radioactive element to break down. By comparing the amount of radioactive element in a sample, scientists can calculate the age of the rock, thus the age of the fossil.

What do Fossils Reveal?• The millions of fossils that scientists

have collected are called the fossil record.• A species is extinct if no members of

that species are still alive. Most of what scientists know about extinct species comes from the fossil record.• The fossil record provides clues on

how and when new groups of organisms evolved. This calendar of Earth’s history is called the Geological Time Scale.

Unanswered Questions????????•What causes mass extinctions? •Scientists are not sure. They hypothesis major climate changes caused by an asteroid or volcanic eruption.

• At what rate does evolution occur?•Scientists are not sure of this either. One theory is called gradualism. Evolution occurs slow and steady