These four mecanisms have an impact on the biodiversity. The biodiversity is the variety of beings on one place. This impact can be on the one hand positive as the Mutation or the Speciation or on the other hand negative as Natural Selection or Genetic Drift.
The theory of evolution has chaged and progressed in course of time. What we think in the time of Darwin isn't what we know today. Scientific progress allow us to discover new things. For exemple Darwin knew only the Natural Selection and the Speciation.
At the Darwin's time, a lot of people didn't believe his theory. To today, there are still people who don't beleive the theory of evolution. For exemple, 4 americans in 10 don't admit the darwinism as the ex-american prime minister.