1. What is a theory?

2. Scientific Definition of a Theory
A scientific theory summarizes a hypothesis or series of hypotheses that have been supported by repeated testing. It is a valid explanation of a phenomenon.

3. Theory
In science a theory is:
Explanatory
Predictable
Testable

4. Germ Theory
Using our three criteria for a theory, let’s see if the existence of germs is a theory.

5. In science a theory is… Explanatory A theory explains something
Ex. Germ theory explains that microbes can make us sick.

6. In science a theory is… 2. Predictable
A theory can accurately guess what will occur in an experiment or observation
Ex. A certain set of symptoms can be associated with a certain microbe.

7. In science a theory is… 3. Testable
A theory can be tested by experiments or observations
Must be falsifiable
Ex. If we wash our hands, spread of microorganisms will be reduced.

8. Germ Theory
Yes, it’s a theory!

9. Theory of Evolution
Using our three criteria for a theory, let’s see if evolution is a theory.

10. In science a theory is… Explanatory It explains something
Evolution explains how life changes over time.

11. In science a theory is… 2. Predictable
You can accurately guess what will occur in an experiment or an observation
Evolution predicts how species will change over time.

12. Example: Fossil Record
The theory of evolution predicts more complex fossils will be younger than more primitive fossils.

13. In science a theory is… 3. Testable
You can create experiments to test your hypotheses
Must be falsifiable

14. Example: Diane Dodd Experiment
two groups of a species of fruit fly were separated and bred for 35 generations. One of the groups was given a source of food containing maltose while the other group had fructose instead. After 35 generations the two groups were brought back into contact with one another and yet members of the two groups would not mate with one another or reproduce - even in situations where it was the only way to produce offspring!

15. Never say: Evolution is just a theory.
All of these are theories:
Theory of Relativity
Big Bang Theory
Theory of Plate Tectonics
Particle Theory
Quantum Theory