Seminar Two
Evolution of sexuality
Natural selection versus sexual selection

Are the two images above of the same frog? Absolutely—it is the same individual, a male marbled tree frog in the Peruvian Amazon. The photo above showing cryptic coloration was taken when the frog rested on the bark of a tree, and the photo showing bright underside colors was taken after he had been captured and placed in a small lucite aquarium, where we photographed him from below.

Why is tree frog so differently colored on top and bottom? To my knowledge no research has been done, but a reasonable hypothesis is the following: the cryptic patterning evolved through natural selection as a useful camouflage, and the bright coloring underneath evolved by sexual selection as a display either to females or to males, visible when the frog sits up.

Now we are able to evaluate the abbreviated comparison made by the psychologist Geoffrey Miller in the quotation at the beginning of this section: “Natural selection is about living long enough to reproduce; sexual selection is about convincing others to mate with you.”

This is a good initial comparison, but what is conspicuously omitted? Intrasexual selection—the competition of males with other males, and of females with other females.