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  Science at multiple levels
The process of science works at multiple levels — from the small scale (e.g., a comparison of the genes of three closely related North American butterfly species) to the large scale (e.g., a half-century-long series of investigations of the idea that geographic isolation of a population can trigger speciation). The process of science works in much the same way whether embodied by an individual scientist tackling a specific problem, question, or hypothesis over the course of a few months or years, or by a community of scientists coming to agree on broad ideas over the course of decades and hundreds of individual experiments and studies. Similarly, scientific explanations come at different levels:

Hypotheses are proposed explanations for a fairly narrow set of phenomena. These reasoned explanations are not guesses — of the wild or educated variety. When scientists formulate new hypotheses, they are usually based on prior experience, scientific background knowledge, preliminary observations, and logic. For example, scientists observed that alpine butterflies exhibit characteristics intermediate between two species that live at lower elevations. Based on these observations and their understanding of speciation, the scientists hypothesized that this species of alpine butterfly evolved as the result of hybridization between the two other species living at lower elevations.

example of an hypothesis

Theories, on the other hand, are broad explanations for a wide range of phenomena. They are concise (i.e., generally don't have a long list of exceptions and special rules), coherent, systematic, predictive, and broadly applicable. In fact, theories often integrate and generalize many hypotheses. For example, the theory of natural selection broadly applies to all populations with some form of inheritance, variation, and differential reproductive success — whether that population is composed of alpine butterflies, fruit flies on a tropical island, a new form of life discovered on Mars, or even bits in a computer's memory. This theory helps us understand a wide range of observations (from the rise of antibiotic-resistant bacteria to the physical match between pollinators and their preferred flowers), makes predictions in new situations (e.g., that treating AIDS patients with a cocktail of medications should slow the evolution of the virus), and has proven itself time and time again in thousands of experiments and observational studies.

example of how theories are used


Occasionally, scientific ideas (such as biological evolution) are written off with the putdown "it's just a theory." This slur is misleading and conflates two separate meanings of the word theory: in common usage, the word theory means just a hunch, but in science, a theory is a powerful explanation for a broad set of observations. To be accepted by the scientific community, a theory (in the scientific sense of the word) must be strongly supported by many different lines of evidence. So biological evolution is a theory (it is a well-supported, widely accepted, and powerful explanation for the diversity of life on Earth), but it is not "just" a theory.

Words with both technical and everyday meanings often cause confusion. Even scientists sometimes use the word theory when they really mean hypothesis or even just a hunch. Many technical fields have similar vocabulary problems — for example, both the terms work in physics and ego in psychology have specific meanings in their technical fields that differ from their common uses. However, context and a little background knowledge are usually sufficient to figure out which meaning is intended.

Over-arching theories
Some theories, which we'll call over-arching theories, are particularly important and reflect broad understandings of a particular part of the natural world. Evolutionary theory, atomic theory, gravity, quantum theory, and plate tectonics are examples of this sort of over-arching theory. These theories have been broadly supported by multiple lines of evidence and help frame our understanding of the world around us.

Such over-arching theories encompass many subordinate theories and hypotheses, and consequently, changes to those smaller theories and hypotheses reflect a refinement (not an overthrow) of the over-arching theory. For example, when punctuated equilibrium was proposed as a mode of evolutionary change and evidence was found supporting the idea in some situations, it represented an elaborated reinforcement of evolutionary theory, not a refutation of it. Over-arching theories are so important because they help scientists choose their methods of study and mode of reasoning, connect important phenomena in new ways, and open new areas of study. For example, evolutionary theory highlighted an entirely new set of questions for exploration: How did this characteristic evolve? How are these species related to one another? How has life changed over time?

example of how over-arching theories are used


Hypotheses and theories can be complex. For example, a particular hypothesis about meteorological interactions or nuclear reactions might be so complex that it is best described in the form of a computer program or a long mathematical equation. In such cases, the hypothesis or theory may be called a model.

To see an example of how models of the atmosphere can shape policy, explore Ozone depletion: Uncovering the hidden hazard of hairspray.

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  • The process of science works at many levels — from that of a single study to that of a broad investigation spanning many decades and encompassing hundreds of individual studies.

  • Hypotheses are proposed explanations for a narrow set of phenomena. They are not guesses.

  • Theories are powerful explanations for a wide range of phenomena. Accepted theories are not tenuous.

  • Some theories are so broad and powerful that they frame whole disciplines of study and encompass many smaller hypotheses and theories.

Misconception: Hypotheses are just guesses.

Correction: Hypotheses are reasoned and informed explanations. Read more about it.

Misconception: Theories are just hunches.

Correction: In science, theories are broad explanations. To be accepted, they must be supported by many lines of evidence. Read more about it.

Misconception: If evidence supports a hypothesis, it is upgraded to a theory. If the theory then garners even more support, it may be upgraded to a law.

Correction: Hypotheses cannot become theories and theories cannot become laws. Hypotheses, theories, and laws are all scientific explanations but they differ in breadth, not in level of support. Theories apply to a broader range of phenomena than do hypotheses. The term law is sometimes used to refer to an idea about how observable phenomena are related. Read more about it.

key points
  • You can help students understand the differences between observation and inference (e.g., between observations and the hypothesis supported by them) by regularly asking students to analyze lecture material, text, or video. Students should try to figure out which aspects of the content were directly observed and which aspects were generated by scientists trying to figure out what their observations meant. For example, while watching a video on mammoth fossils, students could be asked to keep lists of scientists' direct observations and the inferences they made from them.

  • Forming hypotheses — scientific explanations — can be difficult for students. It is often easier for students to generate an expectation (what they think will happen or what they expect to observe) based on prior experience than to formulate a potential explanation for that phenomena. You can help students go beyond expectations to generate real, explanatory hypotheses by providing sentence stems for them to fill in: "I expect to observe A because B." Once students have filled in this sentence you can explain that B is a hypothesis and A is the expectation generated by that hypothesis.

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