The materials that a string is connected too will affect the way it vibrates (which last I checked on a guitar were the bridge saddle and nut/fret), as will the person holding it and the way they are plucking them. Electric pickups don't (or shouldn't) pick up sound waves created by the vibration of the string, they should only collect the disturbances in the magnetic field that are created by the vibration of the string.
This being said, there's no substantive evidence (that I'm aware of) to determine how much different materials effect the vibrations and even if there were, you would need to run these tests on each individual guitar because saying something like 'Basswood gives a bright tone' is a bit of a stretch considering that Basswood is an organic material and therefore subject to a (probably) very large range of standard deviation from the norm.
Even if you could quantify how different materials effect vibration with a small enough level of standard deviation to make the results worthwhile, you would then have to quantify if these differences to string vibration are 'good' or 'bad'. To do that, you would first have to survey enough guitarists (who had passed a double-blind test to prove they can discern such differences in the various materials) to obtain an accepted standard or what is considered 'good' versus 'bad'.
So I'm not going to say that what a guitar is made of
can't effect string vibration, but what I would say is don't get hung up on it. Don't buy some expensive guitar that's made of pink ivory from the guarded vaults of the Zulu chieftain just because the advertisement in Guitar Center says it gives the instrument 'exceptional tone'. Play it and decide for yourself.
I'll also leave this here.
Researchers presented a group of professional violinists with a set of violins and asked them to play and then determine — based on sound alone — which were made by the famed Italian violin-maker Stradivari and Guarneri. The results surprised everyone, including the pros themselves.
www.npr.org