Why is pretty simple:

Mutations.

Every new creature is different from either of it's parents (unless it's parents reproduce asexually, in which case it may end up being identical.).

These difference either help the creature survive and have many children, hinder it's survival and child-rearing, or are neutral.

The vast majority are neutral (in most cases having slightly rounder ears, for example, won't make the difference between life and death.) most of the rest are harmful. The few remaining are beneficial.

The ones with harmful mutations don't survive or don't have as many children as those who have neutral or positive ones. As such they tend to die out quickly, without passing on their bad mutation to more than a generation or two.

Those with good mutations are the opposite. They survive longer and/or have more children than the others. As such their mutation gets passed on to their children, who will also be better at surviving. Given sufficient time they'll replace all the ones who don't have the beneficial mutation, or they'll move to somewhere else where they're better suited to survive, leaving the others behind.

This keeps going, until you end up with a large variety of species.


Now, there's more to it than that, but I'm no biologist, so you get the cliff notes version.

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"Science is the poetry of reality." ~ Richard Dawkins