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Biology · Chapter 2: The Scientific Method (Solving a Biological Problem)
Theory, Law and Principle
The difference between a hypothesis, a theory and a law or principle, with biological examples such as cell theory and Mendel's laws.
Key termRememberExampleWatch out
01Concept
When a hypothesis is tested again and again and always passes, scientists gain confidence in it. The textbook describes the ideas that come out of this as theories, laws and principles.
Theory
A theory is a hypothesis that has been tested many times by different scientists and is supported by a large amount of evidence. A theory explains a wide range of observations.
- Cell theory: all living things are made of cells, and new cells come from existing cells.
- Theory of evolution (Charles Darwin): living things change over many generations by natural selection.
- Germ theory of disease (Louis Pasteur and Robert Koch): many diseases are caused by microorganisms.
Law and principle
A law or principle is a statement of a fact of nature that has been tested over a long time and has never been found wrong. In the words of the textbook, a theory that keeps passing every test over a long time becomes a law or principle.
- Mendel's laws of inheritance: how characteristics pass from parents to offspring.
- Law of biogenesis (principle of biogenesis): living things come only from other living things.
- Hardy–Weinberg principle (A Level): gene frequencies stay constant in a large population if nothing disturbs them.
| Hypothesis | Theory | Law / principle | |
|---|---|---|---|
| What it is | A possible explanation | A well-tested explanation | A well-tested statement of what always happens |
| Evidence | Not yet tested | Supported by many experiments | Never found wrong over a long time |
| Can it change? | Easily | Rarely, if strong new evidence appears | Very rarely |
| Example | Seeds need water to germinate. | Cell theory | Mendel's laws |
02Real World Examples
Why we boil drinking water
The germ theory of disease explains why boiling or filtering water protects us from typhoid, cholera and diarrhoea: heat kills the microorganisms that cause them.
Hospital hygiene
Hand washing, sterilised instruments and clean operation theatres in every hospital come from the same germ theory, tested countless times since the 1860s.
Family resemblance
Why a child has her father's dimples or her mother's eye colour is explained by Mendel's laws of inheritance, which you will study in genetics.
03Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define theory and give one example.Show answer
Answer
A theory is a hypothesis that has been tested many times and is supported by a large amount of evidence.
Example: the cell theory.
Q2.What is a law or principle? Give one example from biology.Show answer
Answer
A law or principle is a statement of a fact of nature that has been tested over a long time and never found wrong.
Example: Mendel's laws of inheritance.
Q3.Differentiate between a hypothesis and a theory.Show answer
Answer
| Hypothesis | Theory |
|---|---|
| A possible explanation | A well-tested explanation |
| Not yet tested | Supported by many experiments |
| Changes easily | Changes rarely |
Q4.State the cell theory.Show answer
Answer
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- New cells come only from existing cells.
Q5.Why can a theory still change?Show answer
Answer
Because science is always open to new evidence. If new, repeated observations do not agree with a theory, it must be changed or replaced.
Long questions
Q6.Explain how a hypothesis can become a theory and then a law or principle. Give examples.Show answer
Answer
- A hypothesis is a possible explanation, for example "seeds need water to germinate".
- It is tested by experiments. If the results support it again and again, done by many scientists, it becomes a theory. Example: cell theory.
- If a theory passes every test over a long time and is never found wrong, it is called a law or principle. Example: Mendel's laws of inheritance; the law of biogenesis.
- At any stage, new evidence can show the idea to be wrong. Then it is changed or replaced.