Natural Selection
Variations that help an organism survive pass on the most desirable characteristics by reproduction. This means their offspring are more likely to survive, as they inherit characteristics that help them stay alive.
Peppered moth
Before the Industrial Revolution, most moths were pale coloured which camouflaged them against the pale lichen found on unpolluted tree bark.
Pollution from coal burning factories darkened the trees, making the pale moths more visible to predators. This meant there were fewer pale moths able to reproduce, and their population size decreased.
Meanwhile, the dark moths became more numerous as their markings camouflaged them against the sooty tree bark, meaning more survived to reproduce offspring with similar markings.
Variations that help an organism survive are selected naturally, simply by the organism living long enough to pass on their genes.