Natural selection (1 of 4) Variation
There is variation in traits. For example, some beetles are green and some are brown.
Natural selection (2 of 4) Differential reproduction
There is differential reproduction. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
Natural selection (4 of 4) End result
End result: The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.