This experiment* with yeast seems to me based on a bad methodology for one simple reason:
We used settling selection to select for larger cluster size. Once per day, after ~24 h of growth, we transferred 1.5 ml of culture into 1.5 mL Eppendorf tubes, let them settle on the bench for 3 minutes, discarded the top 1.45 mL of the culture, and only transferred the bottom 50 µl of the pellet into a new 10 mL of culture media for the next round of growth and settling selection. Once the anaerobic populations (PA1-PA5) had started to evolve visibly larger clusters with all biomass settling to the bottom of the tube in under a minute, we decreased the length of gravitational selection to 30 seconds, thus keeping them under directional selection for increased size.
IMHO, this begs the original question of what selects for multicellularity since, individual cell-size being equal, the phenotype of multicellularity is selecting for the same phenotype as “increased size”.
It seems a far more interesting experiment would be to come up with various hypotheses as to the conditions that select for specialization, and then subject long-term experimental conditions to those various conditions. Did I miss something in the Methods section that did this? Or did I miss something else in the paper that explained why specialization is a consequence of aggregation – hence aggregation must be a precondition for specialization and is therefore interesting in its own right?
*Experiments of this kind are incredibly important, and not just for understanding of the deep history of evolution. They are important for the social sciences – particularly as they pertain to specialization – because they permit us to bypass the evolution of virulence among humans. What I mean is that virulent humans such as Popper with his “Open Society” religion, prohibit mutually-consenting parents from imposing experimental controls to exclude confounders. Such exclusion terrifies pathogens like Popper. He claims they are “intolerant” and hence are justifiably the target of mob attacks if they don’t “engage in open debate”. But that’s not what Popper really fears. He fears that use of experimental controls, by permitting societies closed to men like him, would provide strong evidence not only that “Open Societies” are a bad idea, but that they are a bad idea specifically because of men like Popper!
Yes, yeast are far from humans in their characteristics, so we aren’t able to gather much of direct relevance. But that’s what we’re reduced to given that Popper’s attack on science succeeded not only in attacking experimental controls, but, with his “falsification” dogma, managed to deep-six Algorithmic Information Theory’s provision of a model selection criterion that would permit us to decide between causal models even in the absence of experimental controls!
Popper was the quintessential “evil genius”, whose deep understanding of how to destroy science enabled him, along with his other “gifts” to protect he and his kind from exposure for what they truly are – hence maintain inclusion with their food.