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Studying Living Organisms

Viruses and Host Evolution

Don Chamberlain
Woodrow Wilson Biology Institute
1995


Target age or
ability group:
Regular or honors biology.
Class time
required:
Four days (assuming a 55-minute class period)
One day of group work to discuss the worksheet and questions.
Two days for group research of an assigned topic.
One day for group presentations.
Materials and equipment: The student handout, paper and pen/pencil.
Summary of activity:
Students are organized into groups of four. On Day 1 they are given a worksheet about viruses and their effects on the evolution of a rabbit population. After about 30 minutes of group work, a class discussion of the material begins.

On Day 2 we finish our group discussion and each team or group is assigned a research topic. After groups are organized and all questions answered, students begin the research process.

On Day 3 the research is completed. Each group organizes what has been learned and compiles a copy of the "Research Notes"

On Day 4 each group makes an oral presentation on their assigned research area.

Prior knowledge, concepts or vocabulary necessary to complete activity

This activity assumes that students have received information about viral structure, the viral replication cycle, differences between RNA and DNA viruses, and some effects of viruses on living tissues. It also assumes general student knowledge about evolution, including concepts such as variability and natural selection.

Teacher Instructions

References on viral diseases should be available to students for the research part of this assignment. In addition to biology textbooks, anatomy and physiology textbooks, and virology texts would be helpful. This activity could be used as a summary activity in a unit on viruses or as an activity within an evolution unit.

Viruses and Host Evolution

A. Review of Viruses

What are viruses? Viruses are submicroscopic, obligate, intracellular parasites.

Are viruses alive? The debate rages concerning this question. According to Aristotle, an object has life if it "has a plan" for survival. All viruses contain either DNA or RNA. The molecules could constitute such a "plan." Thus, Aristotelian philosophy suggests that viruses do live.

Viruses require living cells for replication or reproduction. Many viruses require a specific cell type to act as a host. Once the virus locates its cell host, it attaches or adsorbs to that cell. It must then enter the cell host. Methods of entry into cells differ in plant cells, animal cells, and bacterial cells. When viruses enter the host cell, they take control of the cell's replication machinery and convert it into a "virus-making factory." Copies of nucleic acids and protein capsids are produced and assembled into new virus particles called virions. These virions seek out new hosts and attempt to repeat the cycle.

Questions for Review

1.Explain the meaning of each of the italicized terms in the definition of viruses above.

2.After a virus enters a host organism, it must find a particular type of host cell. How do viruses locate and "recognize" the host cells?

3.Explain how the mechanism of viral entry is different for the following types of host cells:

    A. Bacterial Cells:



    B. Animal Cells:



    C. Plant Cells:



Viruses and Evolution

When a parasite population infects a population of host organisms, both the host organisms and the parasitic organisms are influenced by each other. Since viruses evolve faster than all other organisms (20-30 generations per day), they have a greater potential to change the host population.

Evolution Of A Viral Host

In 1859, European rabbits were introduced into Australia for hunting purposes. Since there were no natural predators for rabbits in Australia, the rabbits soon multiplied in an uncontrolled fashion and became a major problem. The rabbits attacked and ate crops intended for human consumption.

In 1950, two virologists introduced a virus into the rabbit population of Australia. This virus called a myxoma virus, had been shown effective in killing rabbits found in America. Though the actual viral effects on the European rabbit were not known at the time, it was hoped that it could be used to control the rabbits of Australia. Mosquitoes were used as a vector to spread the disease throughout the rabbit population. Some of the results of the program are shown below:

After Year 1 --90% of the rabbits were killed.
After Year 2-- 90% of the rabbits were killed.
After Year 3 -- 50% of the rabbits were killed.

4. Hypothesize why only 50% of the rabbits were killed after the third year. By the seventh year after the introduction of the virus, 99% of the original number of rabbits were present. In 1957 the original viruses were injected into new rabbit hosts; only 25% of rabbits died (as opposed to 90% initially).

5.Explain why the viruses originally inoculated in 1950 killed only 25% of its rabbit hosts in 1957?

6.Rabbits continue to be a problem in Australia. Can you think of other ways of attempting to control the rabbit population? What negative consequences might your suggested solution have?

7.When the Ebola virus, discussed in newspapers and the subject of major motion pictures, infects a host, it kills over 90% of the victims it infects within 3 or 4 days. Is this a good survival strategy in terms of viral evolution over time? Why or why not?

8.The HIV virus infects a patient and kills its host 100% of the time. It may remain in incubation for 8-10 years before doing so. Is this a good survival strategy in terms of viral evolution over time? Why of why not?

9.There have been flu outbreaks for many years (sometimes killing thousands of people on a worldwide scale). We are all aware of flu shots that are available to people to combat influenza. Why do we still have outbreaks of flu?

10. Why have many diseases developed in human populations whose members live closely with domesticated animals?

"Viruses And Host Evolution":Worksheet

Group Members:
1.
2.
3.
4.

Answer The Questions From Your Group Worksheet Below:

1.

    a. Submicroscopic:
    b. Obligate:
    c. Intracellular: d. Parasite:

2.Entry into-

3.

    a. Bacterial cells:
    b. Animal cells:
    c. Plant cells:

4.



5.



6.



7.



8.



9.



10.



Viruses And Host Evolution: Research Notes

Your team will be assigned one of the following viral diseases to research:

Smallpox

Herpes Simplex Virus

Yellow Fever

AIDS and the HIV virus

Viral Hepatitis B

Influenza Viruses

Poliomyelitis

For the disease you are assigned, you are to find answers for the following questions:

1. How is the disease contracted?


2. What are the signs/symptoms of the disease?


3. How is the disease treated or prevented?


4. What effect has this disease had in human history?


5. What effect has this disease had in human evolution (think about this one!)?


6. What effect have humans had on the evolution of this virus?


7. Is there an appreciable level of natural immunity for this disease?


On to Studying Species by Examining
the Evolution of the Canidae Family

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