1 Objectives How do scientists test hypotheses 2

1) Objectives: How do scientists test hypotheses? 2) How does a scientific theory develop?

DESIGNING AN EXPERIMENT � Summary �Observe �Identify a problem �Ask a question �Research �Make a hypothesis �Experiment �Record 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

� 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

� What was the responding variable in Redi’s experiment?

� 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

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

� 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

� What improvement did Pasteur make to Redi’s experiment?

� 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 �

� Why are controlled experiments sometimes impossible?

� 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. 1) How do scientists test hypotheses? 2) How does a scientific theory develop?