more members of the species are born than can survive
selective pressures (predators, competition for food, etc.)
Once upon a time there were three creatures of the earth: those with legs, those with wings, and those with tails.
differential survival & reproduction (fitness)
However, over time, the ocean levels decreased so significantly, that the highest levels only reached a few inches. Thus, the creatures with tails slowly became extinct and those with wings and legs prevailed.
Gradually, as time continued, the earth began to erode until eventually there was nothing left but blue skies. Soon, those with wings were the only creatures left in the world. This trait allowed them to survive while those with more unfavorable traits could not adapt.
shift in composition of the species (or gene pool) because of favorable adaptations.
Now, these creatures with wings lived lavishly for several centuries, mating and producing a variety of offspring. However, their times of peace were short-lived as angry winds began tormenting their world.
As the skies grew angrier, those with shorter wings began dying. This change in environment favored the creatures with long wings and green skin as they could easily battle through the weather and survive the storms Over time, this adaption carried on through generations and those with short wings died out.
Now, the world consists of majority creatures with long wings and green skin and while the winds still turn skies grey, all is well. The long winged creatures thrive and live happily ever after.