> The performance of the wheel was based on the final speed achieved by an accelerating imaginary car, vf. The wheel was driven with a constant torque, τ, and no slipping.
There is nothing in this fitness value that defines the shape of the wheel. As described here, there's no clear reason why the shape of the wheel would affect the speed of a car (no slip, no taking into account up/down movement of the car, etc.) Rather this algorithm seems to optimize for a certain radius and the circular shape is the result of the fact that all points converge to the same optimum.
I also find the fact that it converges at a finite radius using this model suspicious. If you look at this optimization problem analytically, the optimum is obviously a wheel with a zero radius: Force on car is torque over wheel radius. If radius goes to zero, then force goes to infinity and acceleration goes to infinity and car speed goes to infinity.