The one-toed horse of today steadily evolved from its five-toed ancestor. Which theory of evolution can explain this example?

Horses have genes of having five toes, but as they grow, the other four don; t develop. This is an example of patterning and post-patterning modes of evolution. Horses don't hunt preys, so they don't need claws. Horses develops muscles and "the hoof" for them to cover wide areas and long distances.