THE LIVING WORLD
Unit One. The Study of Life
1. The Science of Biology
1.7. Stages of a Scientific Investigation
How Science Is Done
How do scientists establish which general principles are true from among the many that might be? They do this by systematically testing alternative proposals. If these proposals prove inconsistent with experimental observations, they are rejected as untrue. After making careful observations concerning a particular area of science, scientists construct a hypothesis, which is a suggested explanation that accounts for those observations. A hypothesis is a proposition that might be true. Those hypotheses that have not yet been disproved are retained. They are useful because they fit the known facts, but they are always subject to future rejection if—in the light of new information—they are found to be incorrect.
We call the test of a hypothesis an experiment. Suppose that a room appears dark to you. To understand why it appears dark, you propose several hypotheses. The first might be, “The room appears dark because the light switch is turned off.” An alternative hypothesis might be, “The room appears dark because the light bulb is burned out.” And yet another alternative hypothesis might be, “I am going blind.” To evaluate these hypotheses, you would conduct an experiment designed to eliminate one or more of the hypotheses. For example, you might reverse the position of the light switch. If you do so and the light does not come on, you have disproved the first hypothesis. Something other than the setting of the light switch must be the reason for the darkness. Note that a test such as this does not prove that any of the other hypotheses are true; it merely demonstrates that one of them is not. A successful experiment is one in which one or more hypotheses is demonstrated to be inconsistent with the results and is thus rejected.
As you proceed through this text, you will encounter a great deal of information, often accompanied by explanations. These explanations are hypotheses that have withstood the test of experiment. Many will continue to do so; others will be revised as new observations are made. Biology, like all science, is in a constant state of change, with new ideas appearing and replacing old ones.
The Scientific Process
Joseph Farman, who first reported the ozone hole, is a practicing scientist, and what he was doing in Antarctica was science. Science is a particular way of investigating the world, of forming general rules about why things happen by observing particular situations. A scientist like Farman is an observer, someone who looks at the world in order to understand how it works.
Scientific investigations can be said to have six stages as illustrated in figure 1.7: 1 observing what is going on; 2 forming a set of hypotheses; 3 making predictions; 4 testing them and 5 carrying out controls, until one or more of the hypotheses have been eliminated; and 6 forming conclusions based on the remaining hypothesis.
Figure 1.7. The scientific process.
This diagram illustrates the stages of a scientific investigation. First, observations are made that raise a particular question. Then a number of potential explanations (hypotheses) are suggested to answer the question. Next, predictions are made based on the hypotheses, and several rounds of experiments (including control experiments) are carried out in an attempt to eliminate one or more of the hypotheses. Finally, any hypothesis that is not eliminated is retained. Further predictions can be made based on the accepted hypothesis and tested with experiments. If it is validated by numerous experiments and stands the test of time, a hypothesis may eventually become a theory.
1. Observation. The key to any successful scientific investigation is careful observation. Farman and other scientists had studied the skies over the Antarctic for many years, noting a thousand details about temperature, light, and levels of chemicals. You can see an example in figure 1.8, where the purple colors represent the lowest levels of ozone that the scientists recorded. Had these scientists not kept careful records of what they observed, Farman might not have noticed that ozone levels were dropping. Observations usually generate questions, such as: Why were ozone levels dropping?
Figure 1.8. The ozone hole.
The swirling colors represent different concentrations of ozone over the South Pole as viewed from a satellite on September 15, 2001. As you can easily see, there is an "ozone hole" (the purple areas) over Antarctica covering an area about the size of the United States. (The color white indicates areas where no data were available.)
2. Hypothesis. When the unexpected drop in ozone was reported and questioned, environmental scientists made a guess to answer their questions—perhaps something was destroying the ozone; maybe the culprit was CFCs. Of course, this was not a guess in the true sense; scientists had some working knowledge of CFCs and what they might be doing in the upper atmosphere. We call such a guess a hypothesis. A hypothesis is a guess that might be true. What the scientists guessed was that chlorine from CFCs was reacting chemically with ozone over the Antarctic, converting ozone (O3) into oxygen gas (O2) and in the process removing the ozone shield from our earth’s atmosphere. Often, scientists will form alternative hypotheses if they have more than one guess about what they observe. In this case, there were several other hypotheses advanced to explain the ozone hole. One suggestion explained it as the result of convection: the ozone spun away from the polar regions much as water spins away from the center as a clothes washer moves through its spin cycle. Another hypothesis was that the ozone hole was a transient phenomenon, due perhaps to sunspots, and would soon disappear.
3. Predictions. If the CFC hypothesis is correct, then several consequences can reasonably be expected.
We call these expected consequences predictions. A prediction is what you expect to happen if a hypothesis is true. The CFC hypothesis predicts that if CFCs are responsible for producing the ozone hole, then it should be possible to detect CFCs in the upper Antarctic atmosphere as well as the chlorine released from CFCs that attack the ozone.
4. Testing. Scientists set out to test the CFC hypothesis by attempting to verify some of its predictions. We call the test of a hypothesis an experiment. To test the hypothesis, atmospheric samples were collected from the stratosphere over 6 miles up by a high-altitude balloon. Analysis of the samples revealed CFCs, as predicted. Were the CFCs interacting with the ozone? The samples contained free chlorine and fluorine, confirming the breakdown of CFC molecules. The results of the experiment thus support the hypothesis.
5. Controls. Events in the upper atmosphere can be influenced by many factors. We call each factor that might influence a process a variable. To evaluate alternative hypotheses about one variable, all the other variables must be kept constant so that we do not get misled or confused by these other influences. This is done by carrying out two experiments in parallel: In the first experimental test, we alter one variable in a known way to test a particular hypothesis; in the second, called a control experiment, we do not alter that variable. In all other respects, the two experiments are the same. To further test the CFC hypothesis, scientists carried out control experiments in which the key variable was the amount of CFCs in the atmosphere. Working in laboratories, scientists reconstructed the atmospheric conditions, solar bombardment, and extreme temperatures found in the sky far above the Antarctic. If the ozone levels fell without addition of CFCs to the chamber, then CFCs could not be what was attacking the ozone Carefully monitoring the chamber, however, scientists detected no drop in ozone levels in the absence of CFCs.
6. Conclusion. A hypothesis that has been tested and not rejected is tentatively accepted. The hypothesis that CFCs released into the atmosphere are destroying the earth’s protective ozone shield is now supported by a great deal of experimental evidence and is widely accepted. While other factors have also been implicated in ozone depletion, destruction by CFCs is clearly the dominant phenomenon. A collection of related hypotheses that have been tested many times and not rejected is called a theory. A theory indicates a higher degree of certainty; however, in science, nothing is “certain.” The theory of the ozone shield—that ozone in the upper atmosphere shields the earth’s surface from harmful UV rays by absorbing them—is supported by a wealth of observation and experimentation and is widely accepted. The explanation for the destruction of this shield is still at the hypothesis stage.
Key Learning Outcome 1.7. Science progresses by systematically eliminating potential hypotheses that are not consistent with observation.