1.1 THE SCIENTIFIC METHOD

How do we go about trying to understand the vastness and complexity of nature? For most scientists, studies of the natural world involve the complementary processes of observation and experimentation. Observation is the act of viewing the world around us. Experimentation is a disciplined and controlled way of asking and answering questions about the world in an unbiased manner.

1.1.1 Observation allows us to draw tentative explanations called hypotheses.

Observations allow us to ask focused questions about nature. Let’s say you observe a hummingbird like the one pictured in Fig. 1.1 hovering near a red flower, occasionally dipping its long beak into the bloom. What motivates this behavior? Is the bird feeding on some substance within the flower? Is it drawn to the flower by its vivid color? What benefit, if any, does the flower derive from this busy bird?

Figure 1.1: A hummingbird visiting a flower. This simple observation leads to questions: Why do hummingbirds pay so much attention to flowers? Why do they hover near red flowers?

Observations such as these, and the questions they raise, allow us to propose tentative explanations, or hypotheses. We might, for example, hypothesize that the hummingbird is carrying pollen from one flower to the next, facilitating reproduction in the plant. Or we might hypothesize that nectar produced deep within the flower provides nutrition for the hummingbird—that the hummingbird’s actions reflect the need to take in food. Both hypotheses provide a reasonable explanation of the behavior we observed, but they may or may not be correct. To find out, we have to test them.

Piecing together individual observations to construct a working hypothesis is beautifully illustrated in Charles Darwin’s classic book, On the Origin of Species, published in 1859. In his text, Darwin discussed a wide range of observations, from pigeon breeding to fossils and from embryology to the unusual animals and plants found on islands. Darwin noted the success of animal breeders in selecting specific individuals for reproduction and thereby generating new breeds for agriculture or show. He appreciated that selective breeding is successful only if specific features of the animals can be passed from one generation to the next by inheritance. Reading economic treatises by the English clergyman Thomas Malthus, he understood that limiting environmental resources could select among the variety of different individuals in populations in much the way that breeders do among cows or pigeons. Gathering all these seemingly disparate pieces of information, he argued that life has evolved over time by means of natural selection. Since its formulation, Darwin’s initial hypothesis has been tested by experiments, many thousands of them. Our knowledge of many biological phenomena, ranging from biodiversity to the way the human brain is wired, depends on direct observation followed by careful inferences that lead to models of how things work.

1.1.2 A hypothesis makes predictions that can be tested by observation and experiments.

Not just any idea qualifies as a hypothesis. Two features set hypotheses apart from other ways of attacking problems. First, a good hypothesis makes predictions about observations not yet made or experiments not yet run. Second, because hypotheses make predictions, we can test them. That is, we can devise an experiment to see whether the predictions made by the hypothesis actually occur, or we can go into the field to try to make further observations predicted by the hypothesis. A hypothesis, then, is a statement about nature that can be tested by experiments or by new observations. Hypotheses are testable because even as they suggest an explanation for observations made previously, they make predictions about observations yet to be made.

Returning to the hummingbird and flower, we can test the hypothesis that the bird is transporting pollen from one flower to the next, enabling the plant to reproduce. Observation provides one type of test: If we catch and examine the bird just after it visits a flower, do we find pollen stuck to its beak or feathers? If so, our hypothesis survives the test. Note, however, that we haven’t proved the case. Pollen might be stuck on the bird for a different reason—perhaps it provides food for the hummingbird. However, if the birds didn’t carry pollen from flower to flower, we would reject the hypothesis that they facilitate pollination. In other words, a single observation or experiment can lead us to reject a hypothesis, or it can support the hypothesis, but it cannot prove that a hypothesis is correct. To move forward, then, we might make a second set of observations. Does pollen that adheres to the hummingbird rub off when the bird visits a second flower of the same species? If so, we have stronger support for our hypothesis.

We might also use observations to test a more general hypothesis about birds and flowers. Does red color generally attract birds and so facilitate pollination in a wide range of flowers? To answer this question, we might catalog the pollination of many red flowers and ask whether they are pollinated mainly by birds. Or we might go the opposite direction and catalog the flowers visited by many different birds—are they more likely to be red than chance alone might predict?

Finally, we can test the hypothesis that the birds visit the flowers primarily to obtain food, spreading pollen as a side effect of their feeding behavior. We can measure the amount of nectar in the flower before and after the bird visits and calculate how much energy has been assimilated by the bird during its visit. Continued observations over the course of the day will tell us whether the birds gain the nutrition they need by eating nectar, and whether the birds have other sources of food.

In many cases, experiments provide the most powerful tests of hypotheses because the scientist can ensure that conditions are tightly controlled. We might test whether hummingbirds facilitate pollination by surrounding the flowers with a mesh that allows small insects access to the plant but keeps hummingbirds away. Will the flowers be pollinated? Experimental or observational tests may support the initial hypothesis, in which case the hypothesis becomes less tentative and more certain, or the results might refute the hypothesis, in which case the scientist may discard it for another explanation or amend it to account for the new information.

Figure 1.2: The scientific method.

Using observations to generate a hypothesis and then making predictions based on that hypothesis that can be tested experimentally are the first two steps in the scientific method, which is outlined in Fig. 1.2. The scientific method is a deliberate and careful way of asking questions about the unknown. We make observations, collect field or laboratory samples, and design and carry out experiments to make sense of things we initially do not understand. The scientific method has proved to be spectacularly successful in helping us to understand the world around us.

To emphasize the power of the scientific method, we turn to a famous riddle drawn from the fossil record (Fig. 1.3). Since the nineteenth century, paleontologists have known that before mammals expanded to their current ecological importance, other large animals dominated Earth. Dinosaurs evolved about 210 million years ago and disappeared 65 million years ago, along with many other species of plants, animals, and microscopic organisms. In many cases, the skeletons and shells of these creatures were buried in sediment and became fossilized. Layers of sedimentary rock therefore record the history of Earth. Working in Italy, the American geologist Walter Alvarez collected samples from the precise point in the rock layers that corresponds to the time of the extinction 65 million years ago. Careful chemical analysis showed that rocks at this level are unusually enriched in the element iridium. Iridium is rare in most rocks on continents and the seafloor, but is relatively common in rocks that fall from space—that is, in meteorites. From these observations, Alvarez and his colleagues developed a remarkable hypothesis: 65 million years ago, a large (11-km diameter) meteor slammed into Earth, and in the resulting environmental havoc, dinosaurs and many other species became extinct. This hypothesis makes specific predictions, described in Fig. 1.3, which turned out to be supported by further observations. Thus, observational tests support the hypothesis that 150 million years of dinosaur evolution were undone in a moment.

FIG. 1.3: What caused the extinction of the dinosaurs?

BACKGROUND Dinosaurs were diverse and ecologically important for nearly 150 million years, but became extinct about 65 million years ago.

OBSERVATION

HYPOTHESIS The impact of a large meteorite disrupted communities on land and in the sea, causing the extinction of the dinosaurs and many other species.

PREDICTIONS Independent evidence of a meteor impact should be found in rock layers corresponding to the time of the extinction, and be rare or absent in older and younger beds.

FURTHER OBSERVATIONS

CONCLUSION A giant meteor struck the Earth 65 million years ago, causing the extinction of the dinosaurs and other species.

FOLLOW-UP WORK Researchers have documented additional episodes of mass extinction, but the event that eliminated the dinosaurs appears to be unique in its association with a meteorite impact.

SOURCE Alvarez, W. 1998. T. rex and the Crater of Doom. New York: Vintage Press.

1.1.3 General explanations of natural phenomena supported by many experiments and observations are called theories.

As already noted, a hypothesis may initially be tentative. Commonly, in fact, it will provide only one of several possible ways of explaining existing data. With repeated observation and experimentation, however, a good hypothesis gathers strength, and we have more and more confidence in it. When a number of related hypotheses survive repeated testing and so come to be accepted as good bases for explaining what we see in nature, scientists articulate a broader explanation that accounts for all of the hypotheses and the results of their tests. We call this statement a theory, a general explanation of the world supported by a large body of experiments and observations (see Fig. 1.2).

Note that scientists use the word “theory” in a very particular way. In general conversation, “theory” is often synonymous with “hypothesis,” “idea,” or “hunch”—“I’ve got a theory about that.” But in a scientific context, the word “theory” has a specific meaning. Only if hypotheses have withstood testing to the point where they provide a general explanation for many observations and experimental results do scientists speak in terms of theories. Just as a good hypothesis makes testable predictions, a good theory both generates good hypotheses and predicts their outcomes. Thus, scientists talk about the theory of gravity—a set of hypotheses you test every day by walking down the street or dropping a fork. Similarly, the theory of evolution is not one explanation among many for the unity and diversity of life. It is a set of hypotheses that has been tested for more than a century and shown to be an extraordinarily powerful means of explaining biological observations that range from amino acid sequences of proteins to the diversity of ants in a rain forest. In fact, as we discuss repeatedly in this book, evolution is the single most important theory in all of biology. It provides the most general and powerful explanation of how life works.