HYPOTHESIS, THEORY, AND LAW

What is a scientific hypothesis? A scientific hypothesis is not an educated guess! Hypotheses are based on: Prior experience Scientific background knowledge Preliminary observations Logic In this class, we CAN state hypotheses as “If, then, because” statements. If the type of vehicle is changed, then a car will have a higher mpg rating than a truck, because lighter vehicles use less gas per mile than heavier vehicles.

Scientific Hypothesis The format of a good hypothesis: If {manipulated variable} is changed, then {prediction about responding variable}, because {research that supports the prediction}. The hypothesis should clearly state the predicted relationship between the manipulated and responding variables, and it must include the background knowledge you use to make that prediction.

Scientific Theory Theories are not educated guesses either! Theories are explanations for how nature works. Theories explain a major phenomenon of nature. Phenomenon: any observable event Theories are well-supported with evidence, and are thought to be the best explanation of a phenomenon.

Scientific Law Scientific laws describe what nature does under certain conditions. If those conditions are present, then the law can be used to make predictions about what will happen. Phenomenon A always happens when conditions 1 and 2 are met. Scientific laws are well-supported with evidence and have been tested repeatedly over time.

Relationship between theories and laws Theories DO NOT become laws with more evidence. Laws describe what happens and theories explain why it happens. Theories are used to explain why a law can be used to predict what will happen under certain conditions.

How broad is the explanation? Hypothesis: explains a narrow group of phenomenon Theory: explains a broader group of phenomenon Law: describes the basic and broadest principles of science; might even be expressed as a math equation.

Cell Theory 1665: Hooke observed dead cork cells and coined the term cells. 1680’s: Leeuwenhoek looked at saliva from men who never cleaned their teeth and was the first to observe living cells: bacteria and protists.

Cell Theory continued … 1855: Virchow proposed that living cells come from other living cells. Went against the concept of spontaneous generation that most scientists thought was true. Cell Theory today: 1. All living things are composed of cells. 2. Cells are the basic units of structure and function in living things. 3. All cells are produced from other cells.

The Theories of Evolution 1937: Dobzhansky’s Synthetic Theory tied together genetic mutations and natural selection. 1809: Lamarck proposed the theory of acquired characteristics. He thought that giraffes acquired long necks through generations of reaching for leaves on trees. 1858: Darwin and Wallace developed the Theory of Natural Selection, but there was no explanation of how it worked because no one understood genetics. 1856: Mendel began his experiments with pea plants and developed his Laws of Inheritance. Sadly, his work went unnoticed until 1900.

Evolution continued… In the late 20 th century, technology became advanced enough to allow scientists to study the process of evolution on a genetic level. Now they can analyze the genetic similarities between species and make hypotheses about shared common ancestors among species. 1972: Eldridge and Gould proposed the theory of punctuated equilibrium, where periods of rapid evolution are followed by periods of very little change. Other scientists held the theory that the evolution of species happens very slowly.