Theory and law (principle): how a tested idea earns its title

From hypothesis to law

When experiments prove a hypothesis correct, scientists use it to make further hypotheses. When those are again proved, the original hypothesis becomes a theory.

A theory is supported by extensive evidence and is repeatedly validated by multiple researchers. Your book's example: the theory of evolution explains how species change over time through natural selection.

Scientists keep testing theories and try their best to disprove them. If a theory is proved again and again, it becomes a law or principle: a uniform or constant fact of nature.

Your book names two biological laws: the Hardy-Weinberg law and Mendel's laws of inheritance.

Hypothesis: tentative answer. Theory: hypothesis proved and validated by many. Law: theory proved again and again, a constant fact of nature.

Key terms

Theory

An explanation backed by extensive evidence and repeated validation.

Law (principle)

A uniform or constant fact of nature.

Natural selection

How the theory of evolution explains species changing over time.

Hardy-Weinberg law

One of the two biological laws your book names.

Short questions with model answers

1. Q1. Trace how a hypothesis becomes a law, using your book's steps.

   • Experiments prove the hypothesis.
   • The new hypotheses are proved too.
   • Scientists try hard to disprove the theory.

   Proved hypothesis, then theory, then law or principle: a constant fact of nature.

2. Q2. Is the theory of evolution a law, according to your book?

   • Check where the book places it.
   • Check the law examples.

   No. Your book uses it as the example of a theory, not a law.

Common mistakes

• ✗ Calling a theory just a guess.

  ✓ A guess-like answer is a hypothesis; a theory is backed by extensive evidence.

• ✗ Putting law before theory, as in "law, theory, deduction".

  ✓ The order is hypothesis, theory, then law.

• ✗ Thinking scientists stop testing once something is a theory.

  ✓ They keep testing and try their best to disprove it.

MCQs

1. 1. A hypothesis whose further hypotheses are proved again becomes a:

   1. (a) Deduction
   2. (b) Theory
   3. (c) Observation
   4. (d) Problem
   Show answer

   (b) Your book, pp. 16–17.

2. 2. A scientific law is:

   1. (a) A tentative answer
   2. (b) An if-then statement
   3. (c) A first observation
   4. (d) A uniform or constant fact of nature
   Show answer

   (d) Your book's definition (p. 17).

3. 3. Which does your book give as an example of a biological law?

   1. (a) Mendel's laws of inheritance
   2. (b) Theory of evolution
   3. (c) Plasmodium causes malaria
   4. (d) Iron deficiency hypothesis
   Show answer

   (a) With the Hardy-Weinberg law (p. 17).

4. 4. In Fig. 1.4, if the hypothesis is not proved, scientists:

   1. (a) Declare a law
   2. (b) Stop the research
   3. (c) Return to hypotheses
   4. (d) Publish it as a theory
   Show answer

   (c) The "if not proved" arrow leads back to hypotheses.

5. 5. The theory of evolution explains how species change over time through:

   1. (a) Experiments
   2. (b) Natural selection
   3. (c) Deduction
   4. (d) Fossils only
   Show answer

   (b) Your book, p. 17.

Quick revision

• A proved hypothesis, confirmed again through further hypotheses, becomes a theory.
• A theory proved again and again becomes a law: a constant fact of nature.
• Examples: theory of evolution; Hardy-Weinberg law and Mendel's laws of inheritance.