Darwin’s finches, those little birds in the Galápagos with beaks of different sizes and shapes, were instrumental in the development of the theory of evolution.

Similar birds had large and small beaks and beaks in between, all related to what kinds of insects and seeds they ate. From one ancestor, it seemed, different adaptations to the environment had evolved, giving the birds that adapted a survival edge in a particular ecological niche — evolution by natural selection.

Biologists who came later went on to identify the genetic changes that had produced different beak shapes.

Now another group of finch-like birds has provided a similar example, but of a different kind of evolution, one driven not by the demands of the environment, but by the demands of female birds. Their preferences in color and pattern caused the evolution of different species of seedeater, all with the same behavior and diet, but with males that look different. That’s a process called sexual selection, which Darwin also wrote about.