The concept of Natural Selection is actually much simpler
than it may initially appear. Thomas Huxley, Darwin's friend and "Bulldog" is said to have angrily proclaimed that he felt...
"extremely stupid not to have thought of that!"
It's basic principles are all truly common sense:
Evolutionary Pressure Selecting for Long Trunks in Elephants
As always, the University of Berkeley's Understanding Evolution website does a great job explaining, as well as correcting common misconceptions about Natural Selection, such as organisms "wanting" to evolve, or evolving to get what they "need."
Here is another simplified example illustrating the possible evolution of elephants with long-trunks from an ancestoral population of elephants with shorter trunks. Notice how the 4 Principles listed above apply to each of the 4 Steps seen here.
Step 1 - There is diversity/variation between elephants trunks
Step 2 - Elephants with shorter trunks are not able to "compete" as well as elephants with longer trunks
Step 3 - Longer trunk elephants are more evolutionarily "fit," meaning they are better adapted to their environment and better equipped to survive
Step 4 - Long-trunk elephants have more kids who, in turn, also have long trunks
Glencoe McGraw-Hill "Visualizing Natural Selection" Video
Buggin' Out Camouflage Adaptation Game
Darwin, Natural Selection, & "Who Wants to Live a Million Years?" Game