Inspired by this question... when making a sourdough starter or similar, does the flour contribute a significant amount of yeast to the final product [either from the flour's origins in the fields, or from the factory where it is milled], as opposed to yeast that is in the environment (floating in the air or whatnot)?  Would two different sourdoughs, both made in the same kitchen, but with different sourced flour, be significantly different due to the yeast (ignoring differences in the flours themselves)?  Or would the natural environment dominate the yeast production? I recognize that some flours might be different than others, so if that is significant (such as bleached flour versus unbleached, more highly processed, etc., please mention that. 
It Depends, but ... First, it depends on the flour. Bleached, sterilized, hot-rolled white flour has the least (possibly none) naturally-occurring wild yeast on it. Cold-rolled unbleached organic whole-grain rye flour has the most. Everything else is in between.  Clearly, if you're using sterile flour, any yeast is going to need to come from elsewhere. Second, it depends on your environment. If your starter is being incubated in a open bakery during the rainy season in San Francisco, it absolutely will pick up some yeast from the environment, more from surface contact than from "the air". In my personal experience as a San Francisco sourdough baker, the primary place that environmental yeast in California comes from is the fruit flies that drown in your starter (nobody wants to say this, but it's true, fruit flies are huge yeast carriers).  But, if you're creating sourdough in the New Mexico desert or on the International Space Station, you're not going to have much environmental yeast available. Within those parameters, for a reasonable starter using non-sterile flour in an average kitchen, where is most of the yeast coming from? The flour. Per the wild yeast blog:  Yeast grow on grain and arrive with the flour. One gram of flour contains about 13,000 yeast cells. I don’t deny that there are a few yeast in the environment that find their way into the starter, but by and large the yeast that will survive in the starter are the ones that like the menu there, i.e, the ones that have a taste for grain.  Given this, why even bother with the whole open-dish sourdough cultivation if you don't live somewhere with ample environmental yeast?  Mostly for the bacteria.  Sourdough is a culture of yeast and bacteria, and benefits from your environmental bacteria (as well as those on the flour) if you get the right ones. 