Scientific Theories

What is a scientific theory?

Evidence from many scientific studies may support several related hypotheses in a way that inspires researchers to propose a scientific theory that ties those hypotheses together. As you read this book, you will often come across terms that will be new to you because they are used only in science. But the word theory is used both in science and in everyday life. It is important to understand that the meaning you give the word theory in daily life is very different from its meaning in science. When you say, “I have a theory,” you may mean, “I have a hunch.” When a friend says, “That's just a theory” she may mean, “People aren't too certain about that idea.” In those same situations, a scientist would probably use the word hypothesis. But when scientists talk about gravitational theory or evolutionary theory, they mean something very different from hunch or hypothesis.

In science, the word theory applies to a well-tested explanation that unifies a broad range of observations and hypotheses and that enables scientists to make accurate predictions about new situations. Charles Darwin's early observations and hypotheses about change over time in nature, for example, grew and expanded for years before he collected them into a theory of evolution by natural selection. Today, evolutionary theory is the central organizing principle of all biological and biomedical science. It makes such a wide range of predictions about organisms—from bacteria to whales to humans—that it is mentioned throughout this book.

A useful theory that has been thoroughly tested and supported by many lines of evidence may become the dominant view among the majority of scientists, but no theory is considered absolute truth. Science is always changing; as new evidence is uncovered, a theory may be revised or replaced by a more useful explanation.