What are the mechanisms of natural selection, and how do they lead to changes in
species over time?

Darwin's proposed mechanism for evolution is natural selection. Natural selection allows species to respond to their surroundings through time, or to become progressively excellently-suited. Natural selection is environmentally based and involves established heritable variation within a population.

They lead to changes in  species over time:

Darwin suggested species will evolve over time, new species originate from pre-existing species, and all species share a common ancestor with each other. In this framework each organism has its own special set of heritable i.e genetic variations from the common ancestor that have slowly developed over very longer periods of time. Parroted branching activities that isolate new species from a common ancestor create a multi-level tree that connects all living organisms.