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ScienceNews: “Multicellular life arises in a test tube” Selecting for snowflakes or evolution of multicellularity?
The Society for the Study of Evolution was all abuzz with the news: a multicellular organism has evolved from a single-celled ancestor right before our eyes. William Ratcliff, presenting the paper, reported, “The evolution of multicellularity was one of the most significant innovations in the history of life. Its initial evolution, however, remains poorly understood largely because all known transitions are ancient. Using experimental evolution, we demonstrate that key steps in the transition to multicellularity evolve far more easily than previously thought.”1
Ratcliff’s group coaxed brewer’s yeast to do what evolutionists speculate may have happened to make multicellular life possible. They “subjected the unicellular yeast Saccharomyces cerevisiae”2 to a daily centrifuge ride, drawing the largest organisms to the bottom. They saved this sludge and pitched the rest. Brewer’s yeast reproduces by budding. Yeast which had not yet pinched off their buds were thus repeatedly selected to survive. These multicellular forms eventually resembled snowflakes.
Ratcliff “expected multicellularity to be adaptive”3 in his centrifuge, and it was. Only bigger clusters were allowed to survive and reproduce. “The key step in the evolution of multicellularity is a shift in the level of selection from unicells to groups,” says Ratcliff. “Once that occurs, you can consider the clumps to be primitive multicellular organisms.”4
But the excitement didn’t stop there. Eventually, these yeast snowflakes developed what the researchers called “division of labor,”5 a necessary leap toward being a bona fide multicellular organism. “We observed the evolution of programmed cell death (apoptosis) among cells within multicellular clusters,” they wrote. “Cellular suicide, while costly to the cells that express this behavior, is nevertheless adaptive, benefiting viable cells within the multicellular cluster by regulating propagule size.”6 They believe they have observed that “key aspects of multicellular complexity . . . readily evolve from unicellular eukaryotes.”7
So what about it? Have we witnessed an evolutionary steppingstone like that which supposedly happened millions of years ago? The writers admit this evolutionary step is “poorly understood” because there are no transitional forms for us to look at. But if evolution has been observed under their guiding hand, who can say it didn’t happen randomly and repeatedly millions of years ago?
But let’s take a closer look. Like many fungi, brewer’s yeast is known to engage in “dimorphic switching.” Programmed into its genome is the ability to change forms depending on conditions. Brewer’s yeast is considered unicellular because it usually is. However, under certain conditions—nitrogen starvation, for instance—it becomes a multicellular filament. A study8 in 1993 identified three genes responsible for this branching growth. Thus, genetic information to become multicellular did not evolve in Ratcliff’s laboratory; the information was in the genome all along.
Due to this dimorphic nature, some evolutionists are skeptical about Ratcliff’s interpretation. They believe this yeast has a multicellular evolutionary history with “a vestigial ability to become multicelllular, rather than evolving into something entirely new.”
Thus, brewer’s yeast already had the genetic ability to stay hooked together after budding. Repeated culling allowed only yeast with the tightest grip to survive and reproduce. In botany and animal husbandry, this process is called selective breeding. And not only were these yeast still yeast, the cells in each snowflake were “genetically identical,”9 “clonally related cells.”10 They had not evolved anything new genetically. They had been selected for abilities they already had.
If unicellular organisms that are not clonally related can communicate to coordinate cellular suicide, why should such behavior in connected clones signify evolution to a new kind of multicellular organism?
Finally, what about Ratcliff’s claim that he saw “reproductive division of labor” evolve? When rows of cells died, the clusters reproduced by fracturing. Did those cells acquire the genetic ability to die as a group? Brewer’s yeast has a known genetic mechanism for programmed cellular death.11 The capacity of single-celled organisms to coordinate group suicide and other activities is commonly seen in biofilms, cooperative communities of microorganisms like dental plaque and slime mold. So if unicellular organisms that are not clonally related can communicate to coordinate cellular suicide, why should such behavior in connected clones signify evolution to a new kind of multicellular organism?
Ratcliff plans to try his experiment on “Chlamydomonas, [a] single-celled organism with no multicellular ancestry.” Though he is optimistic about gaining insight into “one of the most crucial phases in our evolutionary history,” we maintain that multicellular behavior in Chlamydomonas, if it occurs, will be a manifestation of that organism’s underlying genetic ability. Phenotypic switching is quite common among both prokaryotic and eukaryotic microorganisms. (This ability allows many to become pathogenic12 and escape host immune systems.) Evolution of a new kind of organism will not be observed, but with enough selective pressure and culling, perhaps the reawakening of a dormant ability will be.
Selective breeding is selective breeding, not evolution.
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