Eyespot
A light-sensitive region in certain protozoa.[1]
D. E. Nilsson and S. Pelger [2] used computer modeling of the evolution of eyes to find out if it was possible for there to be a smooth gradient of change from a pigmented eye spot to an eye with a lens and cornea, and how long such a transformation would take. They used highly conservative figures for the possible change in each generation. Many human traits are over 50% heritable, and they gave their model a lower figure. They also chose very conservative values for the amount of possible variation in a population. Using these figures, they calculated that through the mechanism of evolution an eyespot could evolve into an eye with a lens and a cornea in less than a half a million years.
This mechanism is contested by Young Earth Creationists, who contend that the Earth was created less than 10,000 years ago, and that in that context such a piece of evolutionary development would not be possible.