The theory of evolution is quite simple to understand, with an open and honest mind. Every living thing mutates, and if you separate populations of the same species, you will observe the physical characteristics change over time.
Each separated population lives in a distinct biome, with different climate, predator, and food characteristics. With each generation, the mutations that work better in each biome will help that organism to survive and reproduce. With time, beneficial mutations will propagate throughout that group.