In biology, we often approach a question by making a related hypothesis or hypotheses and then seeking to evaluate these hypotheses based on experiments, field observations, etc.  We assess the validity of the hypotheses using our data from these sources using statistical tests.  Each statistical test has an associated null and alternative hypothesis that differ from the biological hypothesis in specific ways.

For instance, Professor Tyrone Hayes (Ch 1 in your textbook) is interested in asking questions about the decline of amphibians.  Specifically, he hypothesizes that atrazine (an herbicide) is causing a decline in amphibian abundance in California.

Let us imagine a possible student of his, Furaha, who plans to study amphibians in streams in two areas, one area has high levels of atrazine and the other does not. She begins by collecting data on frog condition and survival over a period of thirty days in each area. With her data set, we can conduct a statistical test to determine whether there is a difference in the survival of leopard frogs (Rana pipiens) in each area.

The statistical null hypothesis would be that there is no difference in the survival of frogs by area and the alternative hypothesis would be that there is a difference in the survival of frogs by area. If there is a statistically significant difference in survival by stream, we reject the statistical null hypothesis.  This provides some evidence that atrazine may be influencing abundance of amphibians in these areas, but Furaha recognizes that there are other factors she needs to consider. For instance, would other species of frogs respond in the same way? Could the results be repeated under laboratory conditions and other areas?  Does the age of the frogs make a difference? Does fecundity play a role?  How does atrazine influence reproduction of frogs?  Is there another variable that may explain the lower survival in the area with atrazine?  As she found, exploring alternative explanations for her findings would be important to evaluating the biological hypothesis.