What are the main ideas of Darwin's theory of evolution by natural selection?

The two key concepts are that slight deviations are introduced in any imperfect replication scheme, such as the way plants or animals are born, and that environmental factors play a role in determining the survivability between individuals with beneficial inheritable traits.

The interplay between these two forces, random mutation and natural selection, ultimately leads to speciation and accounts for all currently observed species, as well as those known only by their fossil remains.