Establishing Controls set of ideas about the nature of the universe, explains ex- Often we are interested in learning about processes that are perimental facts, and serves as a guide to furtl ther questions influenced by many factors, or variables. To evaluate alter and experiments. To a scientist, such theories are the solid ground of sci- native hypotheses about one variable, all other variables ence, that of which we are most certain. In contrast to the must be kept constant. This is done by carrying out two ex- general public, tbeory implies just the opposite--a lack of periments in parallel: in the first experiment, one variable is knowledge, or a guess. Not surprisingly, this difference altered in a specific way to test a particular hypothesis; in the often results in confusion. In this text, theory will always be second experiment, called the control experiment, that used in its scientific sense, in reference to an accepted gen- variable is left unaltered. In all other respects the two exper- eral principle or body of knowledge iments are identical, so any difference in the outcomes of the two experiments must result from the influence of the cL. To suggest, as many critics outside of science do, that lution is"just a theory"is misleading. The hypothesis variable that was changed. Much of the challenge of experi- that evolution has occurred is an accepted scientific fact; it is isolate a particular variable from other factors that might in- supported by overwhelming evidence. Modern evolutionary eory is a complex body of ideas whose importance spreads ce a p far beyond explaining evolution; its ramifications permeate all areas of biology, and it provides the conceptual frame Using Predictions work that unifies biology as a science. A successful scientific hypothesis needs to be not only valid Research and the Scientific Method out useful-it needs to tell you something you want to know. a hypothesis is most useful when it makes predic- It used to be fashionable to speak of the"scientific meth- ns,because those predictions provide a way to test the va- od"as consisting of an orderly sequence of logical"ei lidity of the hypothesis. If an experiment produces results ther/or"steps. Each step would reject one of two mutually inconsistent with the predictions, the hypothesis must be re- incompatible alternatives, as if trial-and-error testing jected. On the other hand, if the predictions are supported would inevitably lead one through the maze of uncertain by experimental testing, the hypothesis is supported. The ty that always impedes scientific progress. If this were in- more experimentally supported predictions a hypothesis deed so, a computer would make a good scientist. But sci- makes, the more valid the hypothesis is. For example, Ein- ence is not done this way. As British philosopher Karl steins hypothesis of relativity was at first provisionally ac- Popper has pointed out, successful scientists without ex- epted because no one could devise an experiment that in- ception design their experiments with a pretty fair idea of validated it. The hypothesis made a clear prediction: that how the results are going to come out. They have what the sun would bend the path of light passing by it. When Popper calls an"imaginative preconception"of what the this prediction was tested in a total eclipse, the light from truth might be. A hypothesis that a successful scientist background stars was indeed bent. Because this result tests is not just any hypothesis; rather, it is an educated unknown when the hypothesis was being formulated, it pro- less or a hunch, in which the scientist integrates all that vided strong support for the hypothesis, which was then ac- he or she knows and allows his or her imagination full cepted with more confidence play, in an attempt to get a sense of what might be true see Box: How Biologists Do Their Work). It is because Developing theories insight and imagination play such a large role in scientific progress that some scientists are so much better at science Scientists use the word theory in two main ways. A"theo- than others, just as Beethoven and Mozart stand out ry"is a proposed explanation for some natural phenome- among most other composers non,often based on some general principle. Thus one ome scientists perform what is called basic research speaks of the principle first proposed by Newton as the which is intended to extend the boundaries of what we "theory of gravity. "Such theories often bring together know. These individuals typically work at universities, and concepts that were previously thought to be unrelated, their research is usually financially supported by their in- and offer unified explanations of different phenomena. stitutions and by external sources, such as the government, Newton's theory of gravity provided a single explanation industry, and private foundations. Basic research is as di for objects falling to the ground and the orbits of planets verse as its name implies Some basic scientists attempt to around the sun. "Theory"is also used to mean the body find out how certain cells take up specific chemicals, while of interconnected concepts, supported by scientific rea others count the number of dents in tiger teeth. The infor- soning and experimental evidence, that explains the facts mation generated by basic research contributes to the in some area of study. Such a theory provides an indis- growing body of scientific knowledge, and it provides the pensable framework for organizing a body of knowledge. scientific foundation utilized by applied research. Scien For example, quantum theory in physics brings together a tists who conduct applied research are often employed in Part I The Origin of Living thingsEstablishing Controls Often we are interested in learning about processes that are influenced by many factors, or variables. To evaluate alter￾native hypotheses about one variable, all other variables must be kept constant. This is done by carrying out two ex￾periments in parallel: in the first experiment, one variable is altered in a specific way to test a particular hypothesis; in the second experiment, called the control experiment, that variable is left unaltered. In all other respects the two exper￾iments are identical, so any difference in the outcomes of the two experiments must result from the influence of the variable that was changed. Much of the challenge of experi￾mental science lies in designing control experiments that isolate a particular variable from other factors that might in￾fluence a process. Using Predictions A successful scientific hypothesis needs to be not only valid but useful—it needs to tell you something you want to know. A hypothesis is most useful when it makes predic￾tions, because those predictions provide a way to test the va￾lidity of the hypothesis. If an experiment produces results inconsistent with the predictions, the hypothesis must be re￾jected. On the other hand, if the predictions are supported by experimental testing, the hypothesis is supported. The more experimentally supported predictions a hypothesis makes, the more valid the hypothesis is. For example, Ein￾stein’s hypothesis of relativity was at first provisionally ac￾cepted because no one could devise an experiment that in￾validated it. The hypothesis made a clear prediction: that the sun would bend the path of light passing by it. When this prediction was tested in a total eclipse, the light from background stars was indeed bent. Because this result was unknown when the hypothesis was being formulated, it pro￾vided strong support for the hypothesis, which was then ac￾cepted with more confidence. Developing Theories Scientists use the word theory in two main ways. A “theo￾ry” is a proposed explanation for some natural phenome￾non, often based on some general principle. Thus one speaks of the principle first proposed by Newton as the “theory of gravity.” Such theories often bring together concepts that were previously thought to be unrelated, and offer unified explanations of different phenomena. Newton’s theory of gravity provided a single explanation for objects falling to the ground and the orbits of planets around the sun. “Theory” is also used to mean the body of interconnected concepts, supported by scientific rea￾soning and experimental evidence, that explains the facts in some area of study. Such a theory provides an indis￾pensable framework for organizing a body of knowledge. For example, quantum theory in physics brings together a set of ideas about the nature of the universe, explains ex￾perimental facts, and serves as a guide to further questions and experiments. To a scientist, such theories are the solid ground of sci￾ence, that of which we are most certain. In contrast, to the general public, theory implies just the opposite—a lack of knowledge, or a guess. Not surprisingly, this difference often results in confusion. In this text, theory will always be used in its scientific sense, in reference to an accepted gen￾eral principle or body of knowledge. To suggest, as many critics outside of science do, that evolution is “just a theory” is misleading. The hypothesis that evolution has occurred is an accepted scientific fact; it is supported by overwhelming evidence. Modern evolutionary theory is a complex body of ideas whose importance spreads far beyond explaining evolution; its ramifications permeate all areas of biology, and it provides the conceptual frame￾work that unifies biology as a science. Research and the Scientific Method It used to be fashionable to speak of the “scientific meth￾od” as consisting of an orderly sequence of logical “ei￾ther/or” steps. Each step would reject one of two mutually incompatible alternatives, as if trial-and-error testing would inevitably lead one through the maze of uncertain￾ty that always impedes scientific progress. If this were in￾deed so, a computer would make a good scientist. But sci￾ence is not done this way. As British philosopher Karl Popper has pointed out, successful scientists without ex￾ception design their experiments with a pretty fair idea of how the results are going to come out. They have what Popper calls an “imaginative preconception” of what the truth might be. A hypothesis that a successful scientist tests is not just any hypothesis; rather, it is an educated guess or a hunch, in which the scientist integrates all that he or she knows and allows his or her imagination full play, in an attempt to get a sense of what might be true (see Box: How Biologists Do Their Work). It is because insight and imagination play such a large role in scientific progress that some scientists are so much better at science than others, just as Beethoven and Mozart stand out among most other composers. Some scientists perform what is called basic research, which is intended to extend the boundaries of what we know. These individuals typically work at universities, and their research is usually financially supported by their in￾stitutions and by external sources, such as the government, industry, and private foundations. Basic research is as di￾verse as its name implies. Some basic scientists attempt to find out how certain cells take up specific chemicals, while others count the number of dents in tiger teeth. The infor￾mation generated by basic research contributes to the growing body of scientific knowledge, and it provides the scientific foundation utilized by applied research. Scien￾tists who conduct applied research are often employed in 8 Part I The Origin of Living Things