1. 1-2 How Scientists Work Objectives: How do scientists test hypotheses?
How does a scientific theory develop?

2. Designing an experiment
Summary
Observe
Identify a problem
Ask a question
Research
Make a hypothesis
Experiment
Record and analyze results
Draw a conclusion
Publish
Repeat

3. Designing an experiment Cont’d
400 years ago people began to challenge established ideas.
Began to use experiments to answer their questions about life

4. Asking a question
People thought living things could just suddenly appear
Wondered how this happened
Asked question

5. Forming a Hypothesis
Spontaneous generation: the idea that life could arise from nonliving matter

6. Forming a Hypothesis Cont’d
1668, Francesco Redi proved spontaneous generation incorrect
Observed
Inferred
Created a new hypothesis

7. Exit Ticket
What is “spontaneous generation”?

8. Setting up a controlled experiment
Variables: factors in an experiment that can change
ONLY 1 VARIABLE IS CHANGED AT A TIME!
Controlled experiment: 1 manipulated variable, 1 responding variable, all other variables controlled

9. Setting up a controlled experiment Cont’d
Manipulated variable: the variable that is deliberately changed
Responding variable: the variable that is observed and that changes in response to the manipulated variable
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10. Setting up a controlled experiment Cont’d

11. Quick question
What was the responding variable in Redi’s experiment?

12. Answer
The responding variable was whether maggots appeared.

13. Recording and analyzing results
Past
Usually recorded by hand in notebooks or personal journals
Drawings recorded certain kinds of observations more completely and accurately than a verbal description
Present
May record data on computers
Review data any time
Offer a explanation

14. Drawing a conclusion
Use the evidence to decide whether the hypothesis was correct

15. Publishing and repeating investigations
A key assumption is that experimental results can be reproduced because nature behaves in a consistent manner
1 variable is manipulated, the result is always the same

16. Publishing and repeating investigations Cont’d
Publishing results is essential
Other scientists make sure that the design was without flaws
Repeat experiments to be sure that the results match those already obtained

17. Needham’s test of redi’s findings
Anton van Leeuwenhoek created the first microscope which allowed him to see tiny living creatures (animalcules)

18. Needham’s test of redi’s findings cont’d
Mid-1700’s
John Needham
Claimed spontaneous generation could occur under the right conditions
Completed an experiment
Inferred “little animals” came from the gravy juice

19. Spallanzani’s test of redi’s findings
Lazzaro Spallanzani
Read about Redi’s and Needham’s work
Performed an experiment
Sealed jar was microorganisms free

20. Quick question
How did Spallanzani’s investigative procedures improve upon Needham’s work?

21. Quick answer
Spallanzai boiled the gravy.

22. Pasteur’s test of spontaneous generation
1864
Louis Pasteur
Completed experiment
His work convinced other scientists that spontaneous generation was not correct
Proved all living things come from other living things

23. The impact of pasteur’s work
Pasteur made many discoveries related to microorganisms
Considered one of biology’s most remarkable problem solvers

24. Quick question
What improvement did Pasteur make to Redi’s experiment?

25. Quick answer
He used a flask with a long curved neck to allow air, but not microorganisms, to enter the flask.

26. When experiments are not possible
Scientists complete field studies
Study large groups of subjects so small differences do not produce misleading results
Identify as many relevant variables as possible so that most variables are controlled
Scientists can discover reliable patterns that add to scientific knowledge

27. Quick question
Why are controlled experiments sometimes impossible?

28. Quick answer
Ethics prevent most experiments with humans. In field studies, researchers try not to disturb animals that they observe.

29. How a theory develops
Theory: a particular hypothesis may become so well supported
Theory applies to a well-tested explanation that unifies a broad range of observations
Allows scientists to make accurate predictions about new situations

30. How a theory develops cont’d
Sometimes more than 1 theory is needed to explain a particular circumstance
No theory is considered absolute truth
Scientists analyze, review, and critique the strengths and weaknesses of theories
A theory may be revised or replaced by a more useful explanation as new evidence is uncovered
Science is characterized by both continuity and change

31. Exit ticket Answer the following question. Use complete sentences.
You may NOT use your book. You MAY use your notes.
This is a quiz grade.
How do scientists test hypotheses?
How does a scientific theory develop?