Rotating locomotion in living systems
There exist two distinct modes of locomotion using rotation: first, simple rolling; and second, the spinning of parts relative to a fixed axle or body, in the manner of a wheel or propeller. Several organisms move by rolling; however, despite the ubiquity of wheels in human vehicles, true wheels do not appear (with the exception of certain flagella) to play any role in the movement of organisms. Biologists have expounded on the reasons for this apparent lack of biological wheels, and wheeled creatures have often appeared in speculative fiction.Given the utility of the wheel in human technology, and the existence of biological analogues of many other technologies (such as wings and lenses), it might seem odd that wheels do not appear in nature, but there are two main factors which explain this phenomenon. First, there are several developmental and evolutionary obstacles to the advent of a wheel by natural selection (addressing the question ""Why can't wheels evolve?""). Secondly, wheels are often at a competitive disadvantage when compared with other means of propulsion (such as walking, running, or slithering) in natural environments (addressing the question ""If wheels could evolve, why would they be unlikely to do so?""). Incidentally, this disadvantage in some environments also explains why some civilizations have abandoned wheels.