What is your favorite deep, elegant, or beautiful explanation?

This year, along with 188 others, I was also asked by John Brockman to contribute a response to Edge.org's annual question. Here's my short piece:

My example of a deep, elegant, and beautiful explanation in science is John Maynard Smith's concept of an evolutionarily stable strategy (ESS). Not only does this wonderfully straightforward idea explain a whole host of biological phenomena, it also provides a very useful heuristic tool to test the plausibility of various types of claims in evolutionary biology, allowing us, for example, to quickly dismiss group-selectionist misconceptions such as the idea that altruistic acts by individuals can be explained by the benefits that accrue to the species as a whole from these acts. Indeed, the idea is so powerful that it explains things which I didn't even realize needed explaining until I was given the explanation! I will now present one such explanation below to illustrate the power of ESS. I should note that while Smith developed ESS using the mathematics of game theory (along with collaborators G. R. Price and G. A. Parker), I will attempt to explain the main idea using almost no math.

So, here is a question: think of common animal species like cats, or dogs, or humans, or golden eagles; why do all of them have (nearly) equal numbers of males and females? Why are there not sometimes 30% males in a species and 70% females? Or the other way? Or some other ratio altogether? Why are sex ratios almost exactly 50/50? I, at least, never even considered the question until I read the incredibly elegant explanation.

Let us consider walruses: they exist in the normal 50/50 sex ratio but most walrus males will die virgins. (But almost all females will mate.) Only a few dominant walrus males monopolize most of the females (in mating terms). So what's the point of having all those extra males around, then? They take up food and resources, but in the only thing that matters to evolution, they are useless, because they do not reproduce. From a species point-of-view, it would be better and more efficient if only a small proportion of walruses were males, and the rest were females, in the sense that such a species of walrus would make much more efficient use of its resources and would, according to the logic of group-selectionists, soon wipe out the actual existing species of walrus with the inefficient 50/50 ratio of males to females. So why don't they?

Continued here. All the responses can be seen here.