Life is full of grand complications, such as aerodynamic wings, multi-part
organs like eyes, and intricate chemical pathways. When faced with such
complexity, both opponents and proponents of evolution, Darwin included, have
asked the question: how could it evolve?
Science does not sweep such difficult questions under the rug, but takes them
up as interesting areas for research. The difficulty is as follows.
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
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.