Section 1 Change over Time

Objectives Identify two kinds of evidence that show that organisms have changed over time.

A characteristic that helps an organism survive and reproduce in its environment is called an adaptation.

2 kinds of evidence Some adaptations are physical, such as fur or scales. Other adaptations are behaviors that help an organism find food, protect itself, or reproduce.

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.

Do Species Change over Time? Since life first appeared, 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.

Objectives Explain how comparing organisms can provide evidence that they have ancestors in common.

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.

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.

How Do Population Changes Happen? Section 2 How Do Population Changes Happen?

Objective Describe the four parts of Charles Darwin’s process of 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.

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.

Section 3 Natural Selection in Action

Objectives Give two examples of how natural selection can result in an adaptation that helps an organism survive.

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.

Examples:Scientists think that hunting in Uganda is affecting 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.

Examples:An insecticide kills all but a few insects in a population Examples:An insecticide kills all but a few insects in a population. The survivors have a gene that makes them immune. They pass it to their offspring and after a few generations, the population is mostly all immune.

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.

Objectives Outline the process of speciation.

Speciation often begins when a part of a population becomes separated from the rest. 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.

Over generations, two separated groups of a population may become very different through natural selection due to the division. 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.