Fossil succession
The horse family's extinct and living members demonstrate "fossil succession." The fossil record shows a sequence of horse ancestors gradually changing and evolving.
If we look at the teeth of different horse ancestors, we can see a gradual change in the shape and size of their teeth over time. The earliest horse ancestors had small, simple teeth for eating soft leaves and fruits from trees and bushes. As horses evolved, their teeth became more expansive and complex, with sharper edges and grinding surfaces because the horse ancestors started to eat more tough and abrasive food.
Gradual changes in teeth shape and size show that the horse ancestors adapted to their changing environment and diet over time, consistent with the idea of evolution by natural selection.