Unit One Background

Teaching about evolution in the framework of the nature of science is an important function. Because some people see evolution as conflicting with widely held beliefs, the teaching of evolution offers educators a superb opportunity to illuminate the nature of science and to differentiate science from other forms of human endeavor- or and understanding. However, it is important from the outset to understand how the meanings of certain key words in science differ from the way that those words are used in everyday life.

Think for example, of how people usually use the word "theory." Someone might
refer to an idea and then add, "But that's only a theory." Or someone might preface a remark by saying, "My theory is . . .." In common usage, theory often means "guess" or "hunch." In science, the word "theory' means something quite different. It refers to an overarching explanation that has been well substantiated.

Science has many other powerful theories besides evolution. Cell theory says that all living things are composed of cells. The heliocentric theory says that the earth revolves around the sun rather than vice versa. Such concepts are
supported by such abundant observational and experimental evidence that they are no longer questioned in science.

Sometimes scientists themselves use the word "theory" loosely and apply it to
tentative explanations that lack well-established evidence. But it is important to distinguish these casual uses of the word "theory" with its use to describe - concepts such as evolution that are supported by overwhelming evidence. Scientists might wish that they had a word other than "theory" to apply to such
enduring explanations of the natural world, but the term is too deeply engrained in science to be discarded.

Glossary of Terms Used in Teaching About the Nature of Science

Fact: In science, an observation that has been repeatedly confirmed.

Law: A descriptive generalization about how some aspect of the natural world
behaves under stated circumstances.

Hypothesis: A testable statement about the natural world that can be used to
build more complex inferences and explanations.

Theory: In science, a well-substantiated explanation of some aspect
of the natural world that can incorporate facts, laws, inferences, and tested

As with all scientific knowledge, a theory can be refined or even replaced by an
alternative theory in light of new and compelling evidence.

The geocentric theory that the sun revolves around the earth was replaced by the heliocentric theory of the earth's rotation on its axis and revolution around the sun. However, ideas are not referred to as "theories" in science unless they are supported by bodies of evidence that make their subsequent abandonment very unlikely. When a theory is supported by as much evidence as evolution, it is held with a very high degree of confidence.

In science, the word "hypothesis" conveys the tentativeness inherent in the common use of the word "theory.' A hypothesis is a testable statement about the natural world. Through experiment and observation, hypotheses can be supported or rejected. At the earliest level of understanding, hypotheses can be used to construct more complex inferences and explanations.

Like "theory," the word "fact" has a different meaning in science than it does in common usage. A scientific fact is an observation that has been confirmed over
and over. However, observations are gathered by our senses, which can never
be trusted entirely. Observations also can change with better technologies or with
better ways of looking at data. For example, it was held as a scientific fact for many years that human cells have 24 pairs of chromosomes, until improved
techniques of microscopy revealed that they actually have 23. Ironically, facts in
science often are more susceptible to change than theories, which is one reason why the word "fact" is not much used in science.

Finally, "laws" in science are typically descriptions of how the physical world
behaves under certain circumstances. For example, the laws of motion describe
how objects move when subjected to certain forces. These laws can be very
useful in supporting hypotheses and theories, but like all elements of science they can be altered with new information and observations.

Those who oppose the teaching of evolution often say that evolution should be taught as a "theory, not as a fact." This statement confuses the common use of these words with the scientific use. In science, theories do not turn into facts through the accumulation of evidence. Rather, theories are the end points of
science. They are understandings that develop from extensive observation, experimentation, and creative reflection. They incorporate a large body of scientific facts, laws, tested hypotheses, and logical inferences. In this sense, evolution is one of the strongest and most useful scientific theories we have.

Excerpts from:
National Science Foundation, Teaching About Evolution and the Nature of Science, National Academy Press, 1998, ISBN 0-309-06364-7

Last updated: February 24, 2015

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