So what makes all the species different? Charles Darwin had the answer! Animals and plants produce too many offspring. Think about how many tadpoles you see at the start of spring, and how few frogs you see at the end of spring. A lot of them die, because there is not enough food to go around. Of course they all try their best to get all the food they need, so they have to compete with each other.
Darwin realised all the members of a species are unique, they are all slightly different. Sometimes this can be the difference between life and death! Think about a bird which eats seeds which have a tough case like a nut. When nuts are in short supply only the really strong birds with big beaks will be able to crack them open and eat them. Since they get more food, they will be less likely to starve or get sick.
Now, since offspring inherit a lot of their characteristics from their parents birds with big beaks will have chicks that grow up to have big beaks too. So over many generations the average beak size in that group of birds which struggle to crack tough nuts will increase. Each generation changes by a really little bit, but all these changes can be added up over time to make a big difference: that’s evolution! Darwin called this process natural selection.