EVOLUTION OF BARBELLUS

Names ______

______

Introduction

To observe changes in organisms that take place only over long periods of time, scientists often use fossils. If a detailed fossil record exists for a group of related organisms, scientists can construct an evolutionary tree. This tree illustrates the relationship between the organisms. It also provides clues about the major speciation events that took place and the rate at which they occurred.

Besides the physical appearance of the fossils, the scientist may also be able to draw upon other information from the fossil record. Most fossils are laid down in rock layers, or strata, above or below other strata. This layering provides a clue as to the relative ages of the organisms being studied.

Sometimes the fossil record may seem to be incomplete. That is, when you look at all the fossils found, sometimes there appear to be BIG changes from one form of the organism to the next form as if there was no transitional form between one type and the next. Given the difficulty involved in fossil formation and discovery, it is not surprising that many “gaps” exist in the fossil record. It takes a very special set of circumstances for an individual organism’s remains to be fossilized in the first place, and another set of circumstances that allows the fossils to be close enough to the earth’s surface to be found by fossil hunters and recovered before eroding away.

The existence of gaps, therefore, does not mean that the transitional forms of the organism never existed, or that the relationships between the fossil organisms found do not exist. We can infer their existence based on the forms whose fossils have been found.

In this investigation, all of the fossil reconstructions belong to the genus Barbellus. These fossils were found in the rock layers along an eroded stream bank as shown in Fig. 1.

Procedure

  1. Cut out the pictures in Figure 2. Make sure to keep the name with each picture.

3. Arrange the pictures in the boxes on the evolutionary tree provided to show the

sequence of changes over time you think may have taken place in the genus

Barbellus. Refer to Figure 1 to see the relative ages of the species based on the

rock layer in which they were found. Use the appearance of the reconstructed

creatures to help derive the correct relationships in descent. Remember, there

could be empty boxes due to missing fossil links.

  1. Glue each picture into position on the Barbellus family tree.

Analysis

  1. Which species of Barbellus represents the common ancestor for all the others?

______

  1. Which two species on the tree are youngest?

______

  1. What is the first speciation event recorded in the fossil record of Barbellus?

______

______

  1. What causes speciation?

______

______

  1. What types of environments did B. opticus and B. dorsalis begin to adapt to

which resulted in their different features?

______

______

  1. What type of environmental change do you think resulted in the transition from

B. quadrapedus to B. hirsutus?

______

______

  1. Does the fossil record of Barbellus show convergent or divergent evolution?

Explain your answer.

______

______

______

  1. Is there any relationship between the complexity of a fossil and its age? Use specific

examples from this activity to explain your answer.

______

______

______