When does a species undergoing natural selection, change more?

Does a species change more through natural selection if the weaker, less favored individuals reproduce more, or if the stronger, more favored individuals reproduce more?For an individual of a species to be weaker than the other members of it's species, it must be genetically imperfect in some way, in other words, there is a variation in it's genetic code that is not in the stronger individuals.For an individual of a species to be strong, it must be closer to genetic perfection, in other words there is less variation in it's genetic code than in the weak members.If this is true, then according to evolution by 'survival of the fittest', if the stronger individuals succeed in reproducing more often than the weaker ones (as is claimed by evolutionists), then the genetic code would in fact vary less, as there would be less imperfections/variations in the stronger individuals to pass on to their offspring.Would natural selection then actually narrow the variation of species instead of broadening it, as is claimed?Any feedback appreciatedRegards, aristotleEdited by aristotle, : No reason given.

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