“Just-So” Stories

This evening one of my Evolution students asked me to explain “just-so” stories. Since it’s such a fun topic, I thought I would blog a bit about it.

The term comes from narrative myths and legends that explain things about nature – they are also sometimes called “just-so” stories. Here is an illustrated example.

In biology, it refers specifically to an explanation of a trait using an adaptionist framework that may be difficult to test of falsify. I.e. it explains a trait with a story/idea about why it must be an adaptation to serve some purpose that it serves now, or that it served in the environment in which it evolved (which is inaccessible for testing).

One classic example is that noses are an adaptation for humans to wear glasses. It seems pretty far-fetched, but it would be difficult to test… observationally – do humans wear glasses on their noses? Yes. Do they wear glasses anywhere else? Generally… no.

A trait may be an adaptation that evolved as a response to selection. However, this is not the only possible explanation: Drift, indirect selection or preadaptation all could have led to traits in the way they are. Some traits may be the way they are now because of their evolutionary history, but are no longer adaptive in the current environment.

For the record, we probably did not evolve noses to hold glasses; we probably designed glasses to fit on noses. ^.~ In fact, glasses went through a number of different renditions before they developed into the wide range of modern styles and fits we have now.

Read the spandrels debate:

(1) S.J. Gould and R. Lewontin, 1979. The Spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London B. Biological Sciences 205:581-598.

(2) Pigliucci, M., and J. Kaplan. 2000. The fall and rise of Dr. Pangloss: adaptationism and the Spandrels paper 20 years later. Trends in Ecology and Evolution 15:66-70.