What causes species to diversify?

Changes in their habitat (the environment they live in) coupled with a species' possibility to gradually adapt to those changes. That is a very broad term and includes many different factors such as availability of food and water,presence of predators, climate, etc.

The polar bear is a good example as it is related to the normal brown bear. As the ancestors of the polar bear gradually moved to more northern habitats, natural selection made that it slowly but surely adapted to that habitat: thicker fur to withstand the colder temperatures, white coat as camouflage with which it stood a better change to hunt for prey, etc.