The Scientific Method and Hypothesis Testing

Exam Tip: Remember that testability is about predictions. A testable hypothesis is one that allows you to deduce a prediction—a statement of what should or would happen in a specific scenario.

Case Study: The Hershey-Chase Experiment

In the 1940s, scientists were grappling with a fundamental causal question: What causes heredity? At the time, there were two primary competing hypotheses regarding the genetic material:

• The Protein Hypothesis: Genes are made of proteins.
• The DNA Hypothesis: Genes are made of DNA.

To resolve this, Alfred Hershey and Martha Chase designed a way to test these hypotheses using bacteriophages (viruses that infect bacteria). They used radioactive isotopes to label different parts of the virus: radioactive sulfur for proteins and radioactive phosphorus for DNA.

Hypothesis Testing and Predictions

The strength of their approach lay in their ability to turn these hypotheses into testable predictions.

• If the Protein Hypothesis were true: After infecting bacteria and using a blender and centrifuge to separate the viral coats from the bacteria, radioactive proteins should be found inside the bacteria.
• If the DNA Hypothesis were true: After the same process, radioactive DNA should be found inside the bacteria.

When the results were analyzed, the radioactive DNA was found inside the bacteria, while the proteins remained outside. This provided new information that supported the DNA hypothesis and failed to support the protein hypothesis.

Falsification and the Nature of Scientific Truth

A critical takeaway from the Hershey-Chase experiment is the concept of falsification. In science, we cannot "prove" a theory is 100% true with absolute certainty; instead, we work to show that alternative explanations are false.

A hypothesis is considered scientifically valid only if it is possible to show that it is wrong. When the protein hypothesis failed the test, it was not "proven true" that DNA was the genetic material; rather, the protein hypothesis was falsified, and the DNA hypothesis was tentatively accepted because the evidence supported it.

This leads to a cycle of continuous refinement:

1. Make observations.
2. Ask a causal question.
3. Hypothesize (propose alternative explanations).
4. Imagine a test and deduce a prediction.
5. Collect data and compare results with predictions.
6. Draw conclusions: If results match, the hypothesis is supported; if they differ, it is not supported.

Note on Statistical Hypotheses: In advanced studies, you may encounter $H_1$ (the alternative hypothesis, suggesting a difference exists) and $H_0$ (the null hypothesis, suggesting no difference exists). While these are used in statistical testing, they function more like predictions than the broad biological hypotheses discussed here.

Ultimately, science is a changeable field. As new technology and better data become available, even our most "accepted" hypotheses can be re-evaluated. This is not a weakness of science, but its greatest strength.