Looking at complexity

Complex adaptions: bird wings, insect wings, vertebrate eyes, and insect eyes.

Since many of these complex traits seem to be adaptive, they are likely to have evolved in small steps through natural selection. That is, intermediate forms of the adaptation must have evolved before evolution arrived at a fully-fledged wing, chemical pathway, or eye. But what good is half a wing or only a few of the elements of an eyeball? The intermediate forms of these adaptations may not seem adaptive — so how could they be produced by natural selection?

There are several ways such complex novelties may evolve:

• Advantageous intermediates: It's possible that those intermediate stages actually were advantageous, even if not in an obvious way. What good is "half an eye?" A simple eye with just a few of the components of a complex eye could still sense light and dark, like eyespots on simple flatworms do. This ability might have been advantageous for an organism with no vision at all and could have evolved through natural selection.

  A Planaria flatworm with its light-sensitive eyespots.

• Co-opting: The intermediate stages of a complex feature might have served a different purpose than the fully-fledged adaptation serves. What good is "half a wing?" Even if it's not good for flying, it might be good for something else. The evolution of the very first feathers might have had nothing to do with flight and everything to do with insulation or display. Natural selection is an excellent thief, taking features that evolved in one context and using them for new functions.