Here's a breakdown:

* Theory of Evolution: This is the overarching idea that life on Earth has changed over time, with new species arising from pre-existing ones. It's a broad framework encompassing several key principles.

* Mechanism of Evolution: This is the process that drives the changes in life over time. There are several mechanisms, but natural selection is the most prominent one.

Natural Selection is the process by which organisms with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This leads to a gradual shift in the population towards those beneficial traits over generations.

So, while the theory of evolution explains the fact of evolution (that life has changed), the mechanism of natural selection explains how that change happens.