It's hard to see one. Even a nice flat world with ample incentive and taking good "bearings" for granted, how can you evolve a wheel-organ that maintains a biological connection as well as being able to rotate an indefinite number of time?
A few difficult and grotesque endpoints:
* The wheel only rotates a fixed number of times before the creature must pivot and "unwind" in the opposite direction. This one seems most plausible, but it's not a real wheel.
* The main body builds replacement wheels internally (like tooth enamel) and periodically ejects a "dead" wheel which can be placed onto a spoke. This option would make it easier to generate very tough rim materials though.
* A biological quick-release/quick-connect system, where the wheel-organ disconnects to move, but then reconnects to flush waste and get more nutrients.
* A communal organism, where wheel-creatures are alive and semi-autonomous, with their own own way to acquire nutrients. Perhaps they would, er... suckle. Eeugh.
In one of Philip Pullmans His Dark Material novels there is a race of creatures that have a symbiosis with a tree whose huge perfectly round nut can be grasped by their fore and hind limbs and they roll around that way.
Wheels are not balls. Balls are common in nature. Wheels are not. The difference is that wheels need roads, which are not common nature and a large scale artificial objects.
It's hard to see one. Even a nice flat world with ample incentive and taking good "bearings" for granted, how can you evolve a wheel-organ that maintains a biological connection as well as being able to rotate an indefinite number of time?
A few difficult and grotesque endpoints:
* The wheel only rotates a fixed number of times before the creature must pivot and "unwind" in the opposite direction. This one seems most plausible, but it's not a real wheel.
* The main body builds replacement wheels internally (like tooth enamel) and periodically ejects a "dead" wheel which can be placed onto a spoke. This option would make it easier to generate very tough rim materials though.
* A biological quick-release/quick-connect system, where the wheel-organ disconnects to move, but then reconnects to flush waste and get more nutrients.
* A communal organism, where wheel-creatures are alive and semi-autonomous, with their own own way to acquire nutrients. Perhaps they would, er... suckle. Eeugh.