The Scientific Method Tutorial

Theories in Science

Theories play an important role in the scientific approach to understanding the universe. A theory can be defined as "a testable explanation of a broad range of related phenomena, one that has been subjected to extensive experimental testing and can be used with a high degree of confidence" (Starr, Cecie, Biology Concepts and Applications, 4^th ed. [Pacific Grove: Brooks/Cole, 2000], glossary). In everyday language, the phrase, "it's just a theory," is used to imply there is considerable doubt about an idea. In science, things are different; the word "theory" specifically applies only to proposed explanations of phenomena that are supported by a great deal of accumulated evidence. Let's take a closer look at the characteristics of theories.

Characteristics of a Theory

1. A theory explains a great deal of experimental observations and is consistent with these observations. A good theory will tie together and account for a wide range of observations, sometimes from different fields. For example, the theory of evolution is consistent with observations as diverse as the animals and plants (living or extinct) found on different continents, the comparative DNA compositions of different organisms, the fossil record, and the way genetic traits are passed on from parents to offspring.

2. Theories account for observations or data that were previously unexplained. For example, the theory of plate tectonics (continental drift) "offers an explanation of such diverse phenomena as the presence far inland of fossilized marine life, the emergence of volcanoes, and the shape of the continents" (Hatton, John, and Paul B. Plouffe, eds., Science and Its Ways of Knowing [Upper Saddle River: Prentice-Hall Inc., 1997], vii).

3. Theories allow researchers to make predictions about data or observations that have yet to be discovered. Take the example of Dimitri Mendeleev. Mendeleev, a Russian chemistry teacher of the 19^th century, created the periodic table of elements by simply listing the elements that were known and their properties. While arranging the elements in order of their atomic weight, he noticed that there were patterns of specific properties among the listed elements. For example, every eighth element would bond with exactly one chlorine atom. He arranged the elements in columns according to their properties and was able to predict the existence of as-yet-undiscovered elements where there were gaps in the table. Many of these were discovered in his lifetime (Trefil, James, and Robert M. Hazen, The Sciences: An Integrated Approach, 2^nd Edition, [New York: John Wiley & Sons, Inc., 1998], p. 9–10).

4. Theories provide a context for interpretation of new facts, while at the same time pointing toward areas of uncertainty where further testing is warranted. For example, the theory of evolution has provided an effective means of interpreting the vast number of fossils uncovered since the theory was proposed in the mid-nineteenth century. The mechanism of evolution, natural selection, has been tested on many populations of a variety of different organisms to determine its validity.

5. Theories, like hypotheses, are subjected to continual testing and the possibility of revision, despite the large body of evidence that supports them.

6. Finally, new theories must be consistent with the existing body of scientific knowledge. If some aspect of a newly proposed theory does not harmonize with a well-established principle or natural law, the theory may be flawed or, if correct, a reexamination of the apparent inconsistency may lead to a broader understanding of nature.

(Source for part of this list: Hatton and Plouffe, eds., vii-viii.)

Question: Summarize the characteristics of theories.

Theories Versus Hypotheses

Theories share some elements in common with hypotheses. For example, both theories and hypotheses are subject to the possibility of revision based on experimental evidence. However, there are some important differences between the two. Maxine Singer, a molecular biologist and president of the Carnegie Institution of Washington, writes:

"A scientific hypothesis is sometimes a hunch, although it can't be an off-the-wall idea and has to be testable. But a hypothesis lacks the scope and solid factual foundation of what scientists honor with the name "theory"—such concepts as the chromosomal theory of heredity or the general theory of relativity. The theory of evolution, for example, is supported by myriad physical, chemical and biological observations and experiments. A theory in science is not a hunch or "just a theory" as some say" ("Believing Is Not Understanding," Washington Post, Aug. 18. 1999, A19).

One important distinction between a theory and a hypothesis is that a hypothesis is often proposed without any experimental evidence to support it, but a theory has been tested again and again in a variety of situations and has not been falsified. The more a theory is tested and supported by research, the more accepted it becomes. Also, a hypothesis is a specific statement about a particular phenomenon or observation while a theory is broader, and applies to a wider range of natural phenomena.

The table below outlines the differences between a hypothesis and a theory.

Here is a comparative example:

Hypothesis: The tobacco plant secretes nicotine in response to leaf feeding by Manduca sexta caterpillars. (How might you test this hypothesis?)

Theory: Many plants produce chemical substances as a result of insect feeding. These substances serve as defense mechanisms to protect plants from damage by leaf-feeding insects.

Note the differences between the two statements above. The first is quite specific, dealing with a single plant species and a single insect, the second suggests a general principle that applies to many plants and insects. Also, note that the second, the theory, provides a framework for the first, the hypothesis. Once a theory is accepted, it suggests a myriad of experiments or observations that could be done to gather support for or refute the theory.

Question: Is the following an example of a theory or a hypothesis? "Maple trees always lose their leaves before oaks do."

Theories Versus Facts

A theory is not the same thing as a fact. A fact is "something that can be shown to be true, to exist, or to have happened" (Encarta World English Dictionary [New York: St. Martin's Press, 1999], 636). Facts are beyond dispute and can never be shown to be wrong, by definition. A theory, by contrast, is a descriptive model constructed to explain a large body of facts (accumulated observations). The aim of a theory is to explain general principles of cause-and-effect that will account for all of the known observations and allow predictions of future observations in similar circumstances.

Although every theory is built around a large number of supporting observations, a theory will never become an absolute fact because it is never practically possible to examine every observation related to a phenomenon. A new set of observations (facts) that are inconsistent with an existing theory may arise some time in the future. Once verified, these new observations will result in a revision of the theory to account for all the facts. In this way theories evolve to present ever clearer and more detailed descriptions of reality.

In his essay "Does Theory Ever Become Fact?" Charles Wynn uses the example of atomic theory to explain this distinction between theories and fact:

"Even though atomic theory has been able to explain the behavior of all matter studied thus far, and, even though in all samples scanned by microscopes, the existence of the postulated atoms has been verified, it must be considered at least conceivable that entities other than atoms might be discovered, particles which also explain the laws of Conservation of Mass, Constant Consumption, and Multiple Proportions. Scientific theories can never become facts, because a scientific theory deals with all instances of a phenomenon; i.e., it is a universal theory. While the behavior of all matter may indeed be explained by atomic theory, there is no way of being certain that this is the case. Such is the open-ended nature of science" (Hatton and Plouffe, eds., 62).

Theories and Natural Laws

A theory may be elevated to the status of a natural law. A natural law has been tested millions of times and explains an even broader range of observations than a theory. It says something very fundamental about how nature works. An example of a natural law is the general law of gravity.

The general law of gravity may well have evolved from the hypothesis "An object falls when dropped." This hypothesis was tested repeatedly and found to be true with some exceptions (certain gases). This eventually led to Newton's laws of motion, which were later incorporated by the much broader general law of gravity, which explains the behavior of falling objects on Earth and the motion of planets around the stars, as well as many other observations (Trefil and Hazen, 6).

Natural laws, like theories, are subject to the possibility of revision if new information becomes available. Revision in this case would typically be due to the emergence of a broader understanding of the principles at work, as in the above example.

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