User:Jtwsaddress42/Projects/Project 2/Sections/Chapter 1/Crossing The Selection Threshold From Unicellar to Multicellular Individuality

In 1987, Yale professor Leo W. Buss published The Evolution of Individuality (1987). Buss was thinking about the early conditions surrounding metazoan origins with regard to selection and the development of metazoans. The blurb on the back of his book summarizes Buss' take on the problem elegantly:

"'Leo Buss expounds a general theory of development through a simple hierarchical extension of the synthetic theory of evolution. He perceives innovations in development to have evolved in ancestral organisms where the germ line was not closed to genetic variation arising during the course of ontogeny. Variants that favor both the proliferation of the cell lineage and the organism harboring them were sequentially incorporated in an increasingly sophisticated epigenetic program.  In contrast, variants that favor the replication of cell lineage at the expense of the individual were eliminated and ultimately favored the fixation of variants that limited the production and/or expression of subsequent variation, creating a stable developmental system.'"

This vision of the evolution of the developmental process, by Buss, is very compatible with the concepts of Neural Darwinism and Somatic Selection put forth by Gerald Edelman around the same time- each authors work informing the others in an indirect manner. Perhaps, even offering the opportunity to develop a clear understanding of how a bottom up process can lead to the organized diversity and complexity we see around us in the animal kingdom. As we will see, many unicellular organisms took the first path to multicellularity, but it is animals that took the later path by "creating a stable developmental system."

Multicellular colonies following the first pathway where variants that favor both proliferation of the cell and the colony that harbors it - are capable of organizing themselves through genetically constrained epigenetic routines that organize it into a multicellular whole - but, they are reversibly committed to such endeavors, defaulting to unicellularity for each cell in the colony for sexual reproductive purposes. They are also intracellularly digesting via phagocytosis.

To protect the species integrity of colonies where variants that are parasitic to the colony as a whole arise, the second pathway eventually constrains the preexisting epigenetic routines of the cells via somatic selection and topobiology. These constraints become increasingly stringent and inhibitory on the ancestral pattern of eternal cell division as they are required to produce stable and reliable ontogeny. In these colonies the ontogenetic development of a sacrificial somatic population that supports the reproductive germ line arises and protects it from damage. Responsibility for carrying the genome into the gene pool is relegated to the germ line at this point.

Once a stable and adaptive ontogenetic program of epigenetic behaviors has been established, further variation that favors uncontrolled proliferation takes us away from the adaptive solution that successfully results in the genetic recombination and proliferation of the germ line. Further variation often results in positive selective pressure for the acquisition of masking agents that buffer the system by suppressing or masking the expression of non-lethal, but destabilizing variants. From the perspective of a somatic selective systems and population biology, this store of variation can be unmasked under stress conditions to instantaneously offer a much broader range of adaptive options in the moment of need than could be acquired by gradual mutational pressure over time.