Paul M. Jones
What is life and where do viruses fit in?
At the beginning of any high school biology textbook will be a section characterizing life as having orderly structure, producing offspring, growing and developing, and adjusting to changes in the environment. These four characteristics are for the most part standard in the study of biology, as is the idea viruses are non-living. Yet regardless of the text, students will be quick to see that viruses could easily be considered living things. More important is the role viruses play on life, and how they have shaped and been a part of evolution, not only by their presence and contribution to shared genetics, but by their role as a selection pressure.
Looking at the four characteristics of life, viruses certainly satisfy structure, reproduction and adjustments, and growth and development can be debated. They possess genes, adapt to hosts, and play a role in evolution of other species. For these reasons viruses fit into the web of life and belong in biological study. When one looks at a holistic definition of life, which has been modified over the years to account for new finds and smaller and smaller things, viruses are non-living for many reasons. A more reductionism approach however, would account for only the most critical attributes of the living, the ability to replicate, and clearly consider viruses to be living entities. If viruses continue to be considered non-living, it will be harder and harder to justify this stance as research moves forward.
What is a virus?
Defining a virus is simple when considering what it does not do than to categorize what it can. Simply put, a virus is a biological entity that possesses a genome made form DNA or RNA surrounded by a protein casing. Some have a protective envelope similar to a plasma membrane that protects them when outside a cell. Viruses are the most abundant biological entities on the planet and incredibly diverse. They habituate every niche and habitat on earth. Viruses are parasites in that they use a host cell to replicate at the hosts expense, although it is a broad misconception that all viruses are directly harmful. Upon exiting the host cell, they are complete and full, and do not experience a period of growth. (Edwards, 2005, 504)
Viruses have evolved various shapes and forms of simplicity, though the simplest of viruses behave similar to the most complex. While studying biology, it is easy to make the assumption that the true role of life is to propagate more life, and the virus is no different. Viruses make more viruses, regardless of impact on the host, and the cycle continues.
The minute nature of viruses makes them a particularly difficult entity for students. Sub-microscopic, roughly 1/100th the size of bacteria, makes them impossible to study under conventional microscopes presenting a hurdle for students. Clear images from electron microscopes showing form and structure does make them more accessible.
The gray of living or non-living is best described to students with the example of a seed. A plant seed obviously has a role in life, but is not itself alive, yet it can be killed. It carries genetic information but cannot reproduce in this state. This concept helps illustrate to students that some of these matters are left up to interpretation, while there is little room to argue that they belong in the study of Biology.
Origin
Virus origin is highly speculative. The possibilities of being a descendent of a large pathogen or the remnant of primordial soup are notable hypotheses; though the more widely accepted idea is of viruses being a genetic component that escaped from cells. Regardless of accepted theory, it is known that viruses are ancient forms. Traces of viral DNA in other organism's genomes, roughly 8% of human gnome is made up of virus-derived genes, support several viral origin hypotheses while indicating that viruses are as old as cellular life. (Morse, 1994, 3)
Structure, evolve to interact
Virus morphology is widely diverse ranging from 10 to 1400 nanometers in four major shapes. Helical or tube shaped, icosahedral or spherical, enveloped, borrowing a membrane from a host cell or organelle, and complex. The latter includes the familiar shape of the bacteriophage resembling a lunar landing module, with its capsid holding nucleic acid sitting atop a tail with legs. Commonly this is the shape most students will associate with viruses. Shape is a major portion of the identity of a particular virus and is often a component of classification.
Structure of viruses presents a nice opportunity to apply natural selection. Once students begin to grasp selection, asking them why certain structures persist can invoke quality discussion. Students will opine that certain structures are better than others while certain conditions favor others. This is a fundamental concept that must be enforced throughout the unit, and in doing so will ease the transition to macroevolution later.
Viral Replication
How a virus replicates is an essential concept for students to understand, especially in terms of differentiating between viral and bacterial diseases. Once genetic material has entered the cell, two processes may take place. The lytic cycle, beginning with attachment to cell and injection of the nucleic acid, uses the energy of the cell to replicate the viral nucleic acid and proteins. Once the parts are assembled, complete viruses are released to repeat the cycle. A typical lytic cycle takes as little as thirty minutes and can make thousands of new viruses. Students will equate this cycle with a rapid response time if you relate it to human disease.
The more complicated lysogenic cycle allows for the genetic material to become part of the host cells genetic material in the form of a provirus. Any cell replication that occurs will also replicate the provirus within the chromosome of the cell until it eventually leaves the chromosome to engage in the lytic cycle. The provirus allows for viral latency and delayed reaction and symptoms to disease. This concept always invokes active discussion of disease within a classroom and is ideal to present with information on typical dormancies of diseases. An initiating activity with students choosing which reproductive cycle is likely based on dormancy works well to enforce the topic and procure discussion. Most high School biology texts describe several viral diseases, systems and incubation periods, allowing for students to see the time it takes for a virus to reach levels of symptomatic infection.
The unique attachment protein for individual viral strains presents the next opportunity to introduce natural selection. Students quickly gravitate to the idea that some things work while others do not, and this presents an excellent opportunity to further enforce the idea of selection pressure. Since each virus has proteins for attachment to specific host cells, each virus can only attach to those cells is matches to. Presenting to students the idea of an environment that does not offer an adequate host will lead them to discover the idea that certain traits are selected for, and the shape of a virus is no different. If a virus cannot attach to a host cell, it cannot reproduce.
Genetic Mutation and Natural Selection
Natural selection is the mechanism that drives evolution. The process occurs when a heritable trait makes survival more likely, and becomes more common in a population over time. Preceding this unit, students should have some background knowledge of heredity and genes. The next step the students must make is to understand that some traits that are inherited may make an organism more successful, and therefore more likely to reproduce, and ultimately likely to pass that trait to offspring. A genetic change can result in a mutation and contribute to variation in the gene pool. Less favorable ones are weeded out by natural selection, while beneficial mutations may accumulate and lead to a variation or new species. Students grasp quickly when the trait is visible, while the idea of single gene variation is tougher to grasp. Since a single gene could influence fitness, or reproductive success, any gene can be selected for. Water glasses in an inexpensive furnished apartment that has been repeatedly rented could represent a gene and be explained by selection. Various glasses came into the apartment, fragile ones broke while the attractive ones were stolen. Odd, ugly, or nonfunctional glasses were discarded. The remainders are sturdy, ugly, and functional. Natural selection worked to make a water glass that works best in that particular environment. (Koella, 2008, 8)
The term micro-evolution refers to rapid change, possibly a single generation while macro-evolution refers to long term change over millions of years. High School students relate well to micro-evolution in that there seems to be little resistance in accepting. Although micro-evolution does not equate microbe evolution, presentation of this material as a short term change, happening to microbes seems to flow smoothly into class discussions. It is important to stress that all organisms evolve, some rapidly, and some over geologic timescales; but all are influenced by natural selection.
Many examples should be given to students, and it is most beneficial if the students are allowed to create their own. A simple Google search can provide thousands of images and diagrams showing natural selection, and once the vocabulary and basic concepts are presented, most students will quickly make their own examples. A good example to start with is one that most students will recognize, bacteria and penicillin. Not only does this example offer review opportunity on cells, a chance to introduce the difference in bacteria and viruses, but most students will recognize that there are several types of antibiotics, and that some do not work on certain patients. Students know that bacteria are alive, and antibiotics kill the bacteria cells in our bodies that make us sick. Present to the students that not all of the bacteria cells may be affected by penicillin can lead to penicillin resistance in bacteria because those bacteria unaffected reproduce and likely produce offspring also unaffected by the medicine.
Selection pressure and the staying power of a trait
As mutations happen, they will ultimately be determined to be advantages or deleterious through selections pressures. As environments are complex, pressure can come from various directions. Competition for resources, predation, reproductive ability, and change in environmental conditions are a few examples of pressures that inhibit the ability to reproduce. Mutations that occur can be advantageous, deleterious or neutral. Advantageous traits may seem insignificant, but they accumulate over generations and can lead to increased fitness and possibly to a new species. A single trait that is beneficial can be passed on eventually overtaking the original population. (Domingo, 2008, 66)
Selection pressures such as predation are easiest for students to grasp. An example of a snowshoe hare with longer feet allowing it to run faster on snow avoiding the wolf is common to nearly every high school biology text. Students can grasp that this trait helps avoid capture by the predator and will likely be passed on. Over time, the whole population of hares may have this phenotypic expression.
Viruses are as susceptible to selection pressures as mammals. A change in environement may destroy the virus altogether, while a mutation to a surface protein may inhibit its ability to attach to a cell and render it unable to reproduce. Conversely, viruses may adapt to a host's immune system by the mutation of the epitope, a surface protein, making them undetectable or by a mutation to the reverse transcriptase enzyme that assists in the building of viral DNA. (Freeman & Herron, 2004, 17)
Evidence of Evolution both big and small
Crucial to this unit is evidence for evolution. Most students are familiar with mainstream images of apes turning into man, but lack the information needed to comprehend this concept. It is at this point where resistance from theological views begins to set in.
Living species offer direct examples of evolution happening in real time. Most notable is the peppered moth reacting to selection pressure by its population darkening in color to match the industrial pollution and lack of light colored lichens on trees common to the suburban areas outside of British industrial zones. Once pollution control was established, pollution decreased, lichens returned, and the moths lightened in color. This famous example sets up many classroom activities and is thoroughly discussed in most texts. Behavioral and microbe evolution offers many short term examples. (Drickamer, 2004, 66)
Examination of the fossil record led to the Law of Succession. Modern living forms are strikingly similar to the most recent fossil record in their locale suggesting a pattern of relation. Current species are descended from ancestors, with modifications that allow them to be successful in current conditions. As the ideas become clearer that species change through time, transitional species that possess characteristics of modern and ancestral forms; further this pattern. Most students are aware of the theory that birds are the modern descendents of dinosaurs, the species Archaeopteryx clearly appears to be a bird like dinosaur, complete with feathers, and will solidify this point of succession. (Freeman, 2004, 43)
Relatedness among organisms does not have to be proven through DNA analysis. Simple study of organisms reveals vestigial structures, such as eye sockets in blind fish and hipbones in whales, suggesting relatedness to organisms that utilize these structures. Virtually unrelated species have homologous structures suggesting common ancestry. Modern genetics have revealed that all organisms share not only the structure and mechanism of genetic material, but large amounts of the material as well. Shared flaws in the genetic material provide further evidence for evolution and common ancestry among living things.
Geology presents vast evidence of environmental change, and the fossil record reflects this. Fossils of aquatic species found in desert areas and tropical plant fossils in polar zones suggest the massive changes Earth has undergone over millennia. The fossil record indicates that life has changed with it.
The law of Uniformitarianism shows that the processes that are undergoing geologically currently, have been for millennia. This allows for the fossil record to be dated accurately and the age of the Earth to be determined. When the age of the Earth is put in context, time is allowed for the slow process of evolution to take effect on the diversification of species. The converse argument of Creationism places the Earth's age at 6000 years, and suggests that all species are created independently and extinct species were wiped out by a catastrophe, probably Noah's great flood. (Freeman, 2004, 57)
Viruses offer evidence for evolution in their ability to mutate and evolve. HIV has evaded drug manufacturers by its ability to mutate rapidly within a host and create versions that are unaffected by medicines. A single cold season in any school building suggests mutating viruses adapting to conditions.
Evolution and medicine
District and national standards call for students abilities to rationalize how evolution affects medical treatment of disease. An in-depth discussion of evolutionary medicine may not be appropriate for most 10th grade classrooms however the students should be able to apply natural selection to pathogen born illnesses. Presenting this concept as the body representing an environment for the parasite (bacteria or virus), with immune response and medicine representing selection pressures can afford the instructor an opportunity to review all concepts in an inter-curricular research/writing assignment.
The high genetic variability of HIV as a result of reverse transcriptase and a rapid replication rate has led to long term failure of medicinal treatments. Inside a single host many variants of the virus form, resulting in a high probability that a version will form that is unaffected by drug treatments. Azidothymidine (AZT) was commonly used to treat HIV infections. It inhibits reverse transcriptase blocking the virus's ability to transfer its genetic material to the host cell. In time the population of viruses evolves so that the virions themselves form enzymes that are not disrupted, functioning normally transcribing DNA within the host cell. HIV evolves to overcome selection pressures presented by the immune system and drug treatment. (Freeman, 2004, 10)
The swine flu and HIV, diseases we all relate to
Current pandemics of the swine flue and HIV can be used to present evolution in medicine. Many articles are available on the two (see student resources) that are reading level appropriate. Currently there is much research on HIV, in particular its behavior inside the host and ability to mutate rendering medicine and immune system response ineffective.
HIV presents an excellent case study for evolution itself. Its origin is becoming clearer as research continues, and it is now known that its many strains have evolved from a family of viruses that infect primates; and leapt to other species, notably humans, more than once. This ability to mutate and infect several species is a contributing factor to the ineffectiveness of medical treatment. Discussion by students can lead to reveal the natural selection process of HIV, using medicine and the human immune system as selection pressures.
The current pandemic of H1NI virus is indicative of a virus's ability to mutate and adapt to hosts. Normally a virus such as influenza has the ability to infect a particular species. One that infects birds does not infect humans because it is not equipped to do so. As a virus mutates or mixes with another strain, it can form a hybrid or new strain that can now infect a new host species. These changes represent the mutations necessary for evolution in all species. Adaptations within the human immune system can represent selection pressure upon this illness causing pathogen.
Evolutionary pitfalls, what the students think and how to change their minds
This unit is intended to afford the instructor a path to teaching evolution that avoids the common pitfalls of the topic. Researching evolutionary history is inappropriate for this age group as the reading material is often to difficult and confusing, and unnecessary for this grade level. The most common resistance from students comes in the form of human evolution and the idea that man evolved from a simpler form. Inner-city students are often challenged with a strong faith base, and perceive evolution as something that refutes their faith. This does not have to be the case, and the material should be adapted to the audience. A common argument for origins of all species of life is that these changes require far too much time for this to be possible as mutations create minute differences over large periods of time. Evidence for the age of the Earth sometimes works best at the beginning of the unit and is infallible to most students. Looking at evolution as a process that all living things undergo satiates this problem somewhat, and focusing on microbe evolution and micro-evolution makes this transition smoother, removing morality and ethical arguments. Teachers should be prepared for the argument that man did not come from apes, and have a retort ready that disarms while not provoking further argument. A former student once stated in class "If we came from monkeys, there wouldn't be monkeys anymore." A better retort might not be found.