A friend sent me an interesting article from Popular Science on the genetics behind yeast flocculation: http://www.popsci.com/scitech/article/2008-11/beer-brings-yeast-together
Apparently the gene and the protein responsible for yeast flocculation (clumping and dropping) have been identified. The article explains the advantage yeast cells gain by clumping together (protection from environmental threats, including the ethanol their fermentation produces), and why it is a good example of kin selection (self sacrifice for the greater genetic good). The article also explains how a similar gene could have led down the path to the evolution of the first multicellular organism.
It is certainly a solid/interesting article, but it only looks at the benefits of flocculation, and does not mention how humans have impacted the evolution of this gene.
When yeast flocculate they pretty much stop fermenting, so if the cells drop early they will miss out on fermenting more sugar and possibly more reproduction. This is why yeast strains that are more attenuative tend to be the least flocculant. If this wasn't the case yeast cells would have evolved to become more and more flocculant, eventually becoming similar to a an acetobacter mother.
Brewers often select for the more flocculant yeast cells by repitching the cells that drop to the bottom of the fermenter after fermentation is complete. As a result most professional brewers have to start a fresh culture of yeast ever 6-10 batches because the yeast can become too flocculant resulting in lower attenuation. To prevent this (before sterile culturing was invented) brewers generally fermented in open tanks and would take yeast cells off the top of the fermenting beer during high krausen. Some breweries still practice this technique, particularly brewers who specialize in German Weissbier (as a result these strains tend to form large krausens and flocculate very slowly).
This is one of the wonderful things about brewing, no matter what you are interested in beer can tie in. Science, cooking, gardening, building, socializing, writing, travel, etc...