Now let's talk about speciation. When a spiecies is isolated in two parts, the group A can't reproduce him with the group B, so the two groups will evolve differently. The environment influence also the evolution, Man can also influence the evolution of a spiecies.
There is also a possibility that the two groups can no longer reproduce together. Like wolves and dogs, some wolves have been domesticated by humans and have evolved differently with wild wolves. Now wolves and dogs can no longer breed together.
Mutation and speciation enrich biodiversity because there is more characters and species. However, yhe genetic drift and the natural selection impoverish the biodiversity.