In this metaphor, fitness is a landscape, where useful traits are peaks and harmful ones are valleys. Wheels may be a very useful peak, but the valley around that peak is too low or too wide for the gene pool (the set of genes in the population) to move to it by genetic drift or natural selection. Stephen Jay Gould points out that evolution can only build with parts that are already available. He writes that, "wheels work well, but animals are debarred [(prevented)] from building them by structural constraints [(limits)] inherited as an evolutionary legacy."
Natural selection therefore explains why wheels have not appeared. A wheel without one or more of its important parts would probably not help an organism very much. However, the same is not true for the flagellum discussed above. As the flagellum evolved, different parts of other structures were used for a new purpose. The part that is now a rotating motor, called a basal body, may have evolved from a part that was used by bacteria to inject poisons into other cells. This use of existing parts is called "exaptation".
The biologist Robin Holliday has written that, because there are no living things with wheels, creationism and intelligent design (stories of life that claim an intelligent creator) are less likely to be right. Holliday argues that an intelligent creator, free from the limits of evolution, would use wheels in any place they would be useful.
People have found it fairly easy to build wheeled systems, even complex ones, and they have been able to solve problems of power transmission (moving energy from one place to another) and friction. However, the ways new organisms build themselves are very different from the ways humans build things. It is not clear that the processes of life would be able to make a wheel. The reasons for this are talked about below.
The greatest barrier to many-celled organisms with wheels is the interface (meeting point) between the parts that move and the parts that stay still. Wheels, or wheels and axles, need to be able to turn freely relative to the rest of the organism. Animal joints have a limited range of motion (the angle they can move), but wheels need to be able to turn through any angle without needing to be turned back. Because of this, wheels can't be attached to the axle or shaft that they turn on. No true many-celled organism is known to grow solid tissues or organs that are not attached to the rest of the organism.
To drive a wheel (that is, to make it turn), something must put a torque (twisting force) on it. For machines made by people, this torque is usually made by a motor. There are many kinds of motors, such as electric motors, turbines, and steam engines. Torque can also come from a person, as with a bicycle. In animals, movement is usually caused by skeletal muscles. These muscles get energy by metabolizing (breaking down) the nutrients in food. Skeletal muscles are attached with connective tissue to the bones they move, so they would not be useful for driving a wheel directly. Also, animals use their muscles in cycles - joints move in one direction, then the other, instead of moving in one direction constantly. Large animals cannot gain speed very quickly, because it is more difficult for muscles to pull against a large inertia (the resistance of a heavy object to movement).
In most machines, a lubricant (like oil or grease) or a bearing is used to limit friction (rubbing) between moving parts. Keeping friction down is important for limiting wear damage to the parts, and for keeping the parts from getting too hot. The faster the parts move, or the more force between them, the more important it is to reduce friction. In animal joints like the human knee, friction is reduced with a tissue called cartilage that has a low coefficient of friction, and a lubricating fluid called synovial fluid that has a low viscosity (resistance to flowing). Gerhard Scholtz, a professor at the ("Institute for Biology and Comparative Zoology") at Humboldt University of Berlin, has written that a similar lubricating fluid or material from dead cells could help an animal wheel to turn freely, but something like this has never been seen in nature.
Another problem is the flow of materials to and from a living wheel. If the tissues that form the wheel are living, then they must receive oxygen and nutrients and have wastes taken away in order to continue metabolism (the process of using energy to maintain functioning). A normal animal circulatory system (the network of tubes that allow blood to flow), would not be able to provide for the necessary flow of materials. Without blood flowing, materials would have to diffuse to and from the wheel. Diffusion is limited by partial pressure and surface area, by a rule called Fick's law of diffusion. For large animals, diffusion would not be enough to keep tissues alive. Instead of a living wheel, a wheel could be made of dead material like keratin, which makes up hair and nails.
In some environments, wheels are simply not as good as limbs for getting around. So it may be that, even if it were possible for wheels to evolve, they would not be found often in nature. In fact, given the limits of wheels, the main question can be turned around: instead of asking ""Why doesn't nature use wheels?"", the question becomes ""Why don't people build more machines with limbs?"" In engineering, wheels are often used instead of limbs, because limbs are more complex to design and control—not because wheels necessarily do the job better.
On hard, flat ground, like paved roads, hard wheels are more energy efficient than other ways of moving around. However, wheels are not as efficient (they take more energy to move) on soft surfaces, like soils, because they have to deal with "rolling resistance". In rolling resistance, a vehicle loses energy as the shape of the wheel and the surface are deformed (changed), and the surface pushes back against the wheel. Smaller wheels have to deal with more resistance for their size than larger wheels. Softer surfaces deform more than hard surfaces, and they don't come back to their starting shape as well. This causes greater resistance. Compared to concrete, the resisting force on medium-hard soil can be five to eight times greater, and on sand it can be ten to fifteen times greater. While wheels must deform the surface along their entire path, limbs only cause a small deformation at the point that the foot touches the ground.
Rolling resistance is also the reason that some human civilizations didn't use wheels. During the Roman Empire, wheeled chariots were common in the Middle East and North Africa. However, when the Empire fell, wheels became less useful to the local people, who turned to camels to transport goods in the sandy desert climate. Stephen Jay Gould wrote about this strange fact of history in his book "Hen's Teeth and Horse's Toes". He wrote that without maintained roads, camels needed less human power and water than a cart pulled by oxen.
When moving in water, rotating mechanisms are more efficient only when the "Reynolds number" of the flow is very low. The Reynolds number is a way to describe the behavior of a flow—in a flow with a low Reynolds number, like those in which bacteria move, viscosity is the most important effect to consider. In flows with high Reynolds numbers, inertia is the most important effect, and mechanisms that "oscillate", or move back and forth, are more efficient. Although ship propellers usually have an efficiency near 60%, and aircraft propellers around 80% (up to 88% in the human-powered "Gossamer Condor"), oscillating foils like fish tails and bird wings can be as efficient as 96%−98%.