Why do races look different?

These are illustrations for a book proposal, showing the differences between European and North American barn swallows. Female European swallows prefer males with longer tail streamers, and have selected for males with streamers that are longer than is aerodynamically optimal. In contrast, North American female swallows don't care much for long tails, but prefer their males bright orange below. As a result, American barn swallow males have shorter tails but brighter bellies than their European counterparts. In the Descent of Man, Darwin first proposed this idea of the arbitrariness of female mating preferences as an explanation for most of the racial variation within humans.