Why is it that a tiger has stripes, but a lion does not? Why do honeybees live in massive groups, but butterflies don’t? Why are trees large, but herbs small? You may have come across questions like these, especially from children. Many, perhaps most, professional biologists will answer such questions by talking about how embryos lay down tissues to form external features, or how neurons affect behaviour, or how growth hormones influence body size. But this often does not answer the questioner. For many people, these questions are not about “how” tigers end up with stripes, or what makes honeybees social; but rather these are “why” questions: why does it make sense that animals and plants are the way they are?
The distinction between how and why questions in biology is an important one. The answers to the first lie in genetics, developmental biology, neurobiology, physiology, and so on. The answers to the second lie in evolutionary biology. Much debate and misunderstanding has been generated by a belief that how and why explanations compete with each other. They do not. How do I know when to eat? When my brain receives signals that glucose in my blood is depleted and stretch receptors indicate that my stomach is empty. Why do I eat? Because I inherited the desire to eat from my ancestors, who also had that urge and therefore survived; any humans in the past who did not have this desire would have rapidly starved and therefore would have had no descendants to inherit this lack of desire.
Dr. Suhel Quader is a scientist at the National Centre for Biological Sciences (Tata Institute of Fundamental Resarch), Bangalore. He can be reached at [email protected].