It seems to me a fair apriori prediction would be that we should find a wheel in many species because it's a relatively simple design............ahh but they would object, "no we shouldn't, because we don't find it and evolution doesn't have to take that path". But how convenient.
Didn't you read the answers given?
Firstly there are some examples in the bacteria flagella and partial ones in the grasshopper's leg. (Also ball and socket joints in mammalian hips).
But there are real reasons why wheels aren't generally used.
They're only really useful on roadways - evolution didn't have roads to work with. You can't climb with wheels and you can't deal easily with broken territory.
Then the wheel needs an axel and a rotating wheel prevents easy inervation and blood supply in the usual way - because cords wind around the axel. A totally different mechanism would need to have evolved. Not impossible, just unecessary and unlikely because it's not a useful feature - because there are no roads.
Now, what's wrong with that?
I'm interested in why there are no tripods - now why do you think that is?
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