Natural selection is a theory popularized by Charles Darwin. It's a key mechanism of evolution. When a gene or an aspect of an organisms dna isn't up to par with the environment, it starts to change over time. Over time this can result in various groups being much more adapted to a specific environment. It is considered the leading proof for adaptive evolution. An example would be, if there was a population of green bugs and red bugs. The birds in this area were more attracted to the red bugs than the green bugs. Natural selection may take place and the species would the red bugs would get cut out as generations went on and in time, there would be mostly or only green bugs.