Natural selection leads to the increase of certain traits in a population and the decrease of other traits.
Certain qualities become more prevalent in a population and others become less prevalent as a result of natural selection. The reason for this process is that individuals who possess qualities that improve their chances of surviving and procreating are more likely to pass those traits on to their offspring.
While features that are less advantageous in a particular context may become less common over time, advantageous qualities tend to become more common in the population. A basic process propelling the development of species throughout generations is this dynamic.