Answer
An easy acronym is ICE
Inheritance
Competition
Environmental pressures
Work Step by Step
For natural selection to occur, there must be inheritance of beneficial traits, competition among species, and environmental pressures. Inheritance is required, or else there would be no point in the most adapted surviving because they won't pass on traits. Competition allows for only the strongest to survive. Environmental pressures allow for a selective pressure, and only organisms with selective advantages will survive.
