4 4 What is natural selection?

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4 – 4 What is natural selection?

Explaining Evolution The fossil record shows that living things have evolved throughout Earth’s history. However, knowing this fact does not explain how the changes happened. Offspring are supposed to be the same as their parents. How, then, can a new species develop from an old one?

Lamarck’s Theory Jean Baptiste de Lamarck was one of the first scientists to develop a theory of evolution. According to Lamarck, all organisms developed new characteristics to help them to adapt to their surroundings. Then, they passed on those adapted traits to their offspring. Lamarck used a giraffe as a model for his theory.

Lamarck hypothesized that the first giraffes had short necks and ate grass. As time went on, the grass died and giraffes had to find a new food source. The new food source was tree leaves. In order to reach the leaves, however, the giraffes needed to stretch their necks. This made their necks longer. Parent giraffes then passed on this new trait to their offspring. Lamarck did not have much evidence to support his hypothesis. Eventually, it was proven wrong.

Charles Darwin Galapagos Turtle Darwin drew this picture of the

different finches on the Galapagos

The Voyage of the Beagle Over 150 years ago, the naturalist Charles Darwin suggested a theory of evolution that is accepted by most scientists today. In 1831, Darwin set sail on the HMS Beagle. It was on a five-year expedition to the South Pacific and South America. The purpose was to make maps and to observe and collect specimens of various plants and animals

One of the places the Beagle visited was the Galapagos Islands. There, Darwin saw the Galapagos tortoise. Every island had a different species of tortoise. Some of the tortoises had dome-shaped shells, whereas others had saddle-shaped shells.

Darwin also discovered 13 different species of finches. Darwin observed that each species of finch had a differently shaped beak. Each beak was adapted for eating a certain type of food. Most of the finches’ other traits were similar. Darwin inferred that all the finches had evolved from a common ancestor. Darwin used his observation of the tortoises as the basis for his ideas about natural selection.

Natural Selection Darwin used the term natural selection to describe his theory of evolution. Nature favors the survival of organisms that are best suited for their environment. The theory of natural selection includes the following ideas:

  • Overpopulation Each species produces more offspring than can survive. Not all can survive because there is not enough food or living space for all.

  • Struggle for Existence The offspring of each generation compete for things that they need to survive. Only a few live long enough to reproduce. The others will die.

  • Variation The offspring of each generation are not exactly alike. For example, some organisms are faster and stronger than others. Differences in traits among individuals of a species are called variations.

  • Survival of the Fittest Some variations make organisms better suited for survival in their environments. These organisms are more likely to survive and reproduce than are others.

  • Evolution of New Species Individuals with favorable variations survive and reproduce. They pass their favorable traits to their offspring. Therefore, their offspring are more likely to survive and reproduce in next generation. Unfavorable variations eventually disappear. In this way, favorable variations remain in the species. Over many generations, these changes can result in the appearance of a new species.


1. Natural selection is a theory of .

2. The ide which states that a species produces more offspring than can survive is called .

3. Individuals with variations survive and reproduce.

4-4 What is natural selection?

Lesson Review

PART A Answer the following questions.

1. Who was Jean Baptiste de Lamark?

2. What was Jean Baptiste de Lamark’s theory about evolution?

PART B The main ideas of Darwin’s theory of natural selection are listed below. Explain what is meant by each of these ideas.

1. Overproduction:

2. Struggle for Existence:

3. Variation:

4. Survival of the Fittest:

5. Evolution of New Species:

Skill Challenge

Skills: applying concepts, analyzing

Study the diagrams below. Identify how the diagrams relate to Darwin’s ideas about natural selection.

Concepts and Challenges in Life Science, Teacher’s Resources CD-ROM

(c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Life Changes Over Time: CHAPTER 4

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