“One of the principal organizing concepts in biology is the question, ‘What function does this thing serve?”—a question premised on the assumption that everything one finds in an organism serves some function or it probably would not have survived. Other general ideas are related to this question. The student who makes progress in biology learns to ask the question more and more subtly, to relate more and more things to it.”
Excerpted from: Wiggins, Grant, and Jay McTighe. Understanding by Design. Alexandria, VA: ASCD, 1998.