In the third chapter of The Selfish Gene: Immortal coils, Dr.Dawkins makes the following statement while arguing for the case that ultimately "the gene" as he defines it is the unit of natural selection.
Natural selection in its most general form means the differential survival of entities. Some entities live and others die but, in order for this selective death to have any impact on the world, an additional condition must be met. Each entity must exist in the form of lots of copies, and at least some of the entities must be potentially capable of surviving—in the form of copies—for a significant period of evolutionary time. Small genetic units have these properties: individuals, groups, and species do not.
I am not sure what "impact on the world" he is talking about. Could someone explain this? He then goes on to say..
An individual body seems discrete enough while it lasts, but alas, how long is that? Each individual is unique. You cannot get evolution by selecting between entities when there is only one copy of each entity!
Whereas in his other book, The Blind Watchmaker, he describes a computer drawing program EVOLUTION where he does precisely that, i.e, select between entities (here, computer drawn insect-like figures) of which there is only one copy and allows them to evolve. This seems contradictory to his above statement in The Selfish Gene. Could somebody explain?
tl,dr: Why must there be many copies of entities that are being selected for evolution to be possible?