The Elements of Evolution (38-2) Weak Linkage

Weak Linkage

There is a weak regulatory linkage between genetic code and its expressive effect. A minor edit – either in DNA or epigenetically – can vector a distinctive development. While the core genetic foundation is maintained, an adaptive tweak can produce a profound effect.

Bacteria rapidly adapt their enzymes for available food supply. French molecular biologist Jacques Monod offered a culture of E. coli bacteria a choice of sugars: glucose and lactose. The culture grew for a while on glucose, paused, then started in on the lactose. From repeated exposures, Monod found that enzyme adaptation was not simply a matter of activation, but synthesis of a new working enzyme. Weak linkage allowed quick adaptation.

Monad discovered that living cells selectively control their protein production via a feedback cycle. Most biological processes regulate by feedback, either to accelerate and inhibit. The immune system is exemplary. Relatively simple physiological feedback circuits afford rapid adaptation by gene expression while conserving the core code.

Bacteria and archaea are under frequent attack from foreign genetic elements. These prokaryotes evolved immune responses early on based upon memory.

When encountered, foreign DNA fragments are tucked away for later reference. That way, using specialized proteins, a microbe can recognize something similar by pattern matching previously saved DNA with the new encounter. To thwart foreign gene expression, defense is activated via an RNA interference-like technique.

The prokaryotic immune store-and-compare pattern-matching technique became the primary mechanism for vertebrate adaptive immune systems billions of years later, albeit using cell surface protein patterns rather than genetic fragments. In whatever form, such signature recognition is the only feasible solution.

Nerve cells date back to Ediacaran jellyfish. Nerve cells are all a combination of chemical conveyance at interfaces and electrical conduction within. Neurons are an exquisite example of weak linkage, by being a poised binary communication conduit: on or off, but with wide-ranging room for variation.

There is no physical connection between neurons. No tight fit is required. As the output is basically binary – signal or not – how that signal is obtained or tempered, and how conveyed at the interface, as well as how connections are laid out, allows distinct nerve cells types, with different receptors, different neurotransmitters, and near-infinite variety in connective matrices; a conserved core with an ideal architecture for adaptability in intercellular communication.

The genome of multicellular life is itself structured to facilitate the evolution of new genes as creative expressions. The possibilities are practically infinite, as the sum-total of life’s diversity throughout this planet’s history testifies. This owes to the weak linkage among the protein-encoding domains in the genome. New genes are created by fusing various pieces of other genes, with unlimited possibilities.

Folding as a functional attribute of proteins adds another realm of adaptive flexibility. Moreover, the regulation of gene expression is a related, but altogether separate, aspect of the genomic dynamic: conserving the core mechanics while creating an additional layer of evolutionary elasticity.

The weak linkages found in genes, proteins, and neurons are exemplary of a basic biotic paradigm. Weak linkage is itself a conserved core feature of biology, underlying many organic processes.