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?
There is no hard and fast rule on that.
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.
Nah man, maybe they just caught a cold.
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.
Genetic perfection is not a thing.
If this is true, then...
It's not true.
...
I see your avatar. Evolution is not religion. I'm religious, and I accept evolution, and they are two different things.