There is many types of dogs in the world.

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There's dogs who can run fast or hunt well and so on.

Some dogs are small and others are big.

Dog's ancestor is an wolf.
The dog's ancestor is a wolf.

People trained the dogs and found out that they were helpful to them so they adaptated to the life.
People trained dogs and found out that dogs were helpful, so dog's were adapted by peolpe.

This is why some kinds of dog look similar.
That is why some kind of dogs look similar.