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

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

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

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

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

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

Exit Ticket What is “spontaneous generation”?

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

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 http://www.sciencekids.co.nz/gamesactiviti es/plantsgrow.html

Setting up a controlled experiment Cont’d

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

Answer The responding variable was whether maggots appeared.

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

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

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

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

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

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

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

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

Quick answer Spallanzai boiled the gravy.

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

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

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

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

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

Quick question Why are controlled experiments sometimes impossible?

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

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

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

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?