Differential reproduction in simple terms is the survival of certain individuals within a given species up to the reproductive age and thus, their genes are carried forward through progression of future generation and their reproduction further. However certain other individuals with in a species are not able to survive up till the reproduction age and thus they do not reproduce and die before reproducing. In this case their genetic information is not passed on to the future generations.
Therefore, limited reproduction with in the same kind of individuals of a species (due to inability to survive till reproductive age) would lead to reduced genetic diversity.
Evolution is a process of elimination. the traits that are successful and help them survive are kept for the next population, because they are abet o survive to repopulate. So its improvement.
