Unit 5: Natural Selection and Evolution Assessment

Darwin’s Finches and Natural Selection

The Galápagos Islands are a group of volcanic islands far off the coast of South America. These islands are home to 13 different species of birds called Darwin’s finches. At first, the finches look plain, but their beaks are very different. Each species has a beak shaped for eating certain kinds of food.

Charles Darwin visited the islands in 1835. Later, he wondered why there were so many kinds of finches. He believed the finches all came from one original group that arrived from South America long ago. Over time, the finches changed as they adapted to different environments and foods on different islands.

More than 100 years later, scientists Rosemary and Peter Grant studied finches on a small island called Daphne Major. They measured the finches carefully, especially their beaks. They also studied the seeds on the island. Some seeds were small and soft, while others were large and hard.

One year, a severe drought hit the island. For over 500 days, very little rain fell. Many plants did not produce new seeds. The finches quickly ate most of the small, soft seeds. Soon, the only seeds left were large, tough seeds. The Grants noticed that finches with larger, stronger beaks were better able to crack these seeds open. Many finches with smaller beaks could not get enough food and died.

After the drought, the finch population was much smaller. But the birds that survived were mostly the ones with larger beaks. Since beak size can be passed from parents to offspring, the next generation of finches had more large-beaked birds than before.

This is an example of natural selection. When the environment changes, some traits help organisms survive better than others. Over many generations, these helpful traits become more common in a population. This is one way evolution happens.

Read the passage below, and then answer the questions.

Anatomy

Adapted from RocketLit

The bones in the limbs of a human, dog, bird and whale all share the same overall way that they are built. These similarities comes from their beginning in the appendages of a shared ancestor and are called homologous structures. Over time, changes in the shapes and sizes of these bone occurred in the different species, but they have kept the same general layout. There is even evidence that their are structures present in living things that have no clear function at all! Scientists infer that these are remaining parts from a past ancestor. Some snakes have pelvic bones even though they do not have legs. Whales have been found with leg bones that they do not use anymore. These are called vestigial structures, and they come from a past ancestor that used that body part. They still remain on the modern organism because they do not hurt the animal's ability to survive and reproduce.