I won’t get into this in detail, in part because I wrote about it in my paper The evolution of complex organs in the special issue of Evolution: Education and Outreach about eyes.
It seems the most recent analogy drawn by anti-evolutionists to support the idea of irreducible complexity is a bicycle — yeah, a unicycle works but a bicycle missing a wheel doesn’t, therefore it’s irreducibly complex. You can find out more at The Loom, where Carl shows people riding single-wheel bikes.
I am only weighing in because the discussion seems to be overlooking an important point about exaptation/co-option: the function need not be the same in both instances. So, although riding a bike with one wheel the way you would ride a two-wheeled bike a la wheelie is fine to show that it wouldn’t be totally non-functional, an even more relevant counter-example would involve a bicycle with one wheel that worked well for some other role.
Voila.
This is just the first single-wheeled, functional-for-something-different, co-option example I found. This actually involves one shift and one addition of function. Loss of wheel to make it useful for exercise indoors, then addition of other parts to run a laundry machine. Of course, modern stationary bicycles used for exercise are more specialized, but you can still find gear to convert a normal bicycle into a stationary one, sometimes by taking off the front wheel.
So, basically, bicycles with one wheel provide a very nice illustration of how co-option with shift in function can and does work.
Somebody somewhere mentioned co-option, noting that we can laugh at Luskin without even going that far.
If you like laughing at bad arguments, they’re trying to defend Luskin at Uncommon Descent. Someone has even claimed that Luskin is right because a bicycle with one wheel isn’t a BIcycle.
And in case you haven’t seen it, the DI’s blog has a statement at the bottom which begins