Thesis #18 – When such trade-offs can be physiologically tuned within the lives of individual organisms, natural selection may act to produce physiological machinery that provides plasticity which enhances average fitness.

One of the basic features of natural selection is its propensity to favor the exploitation of environmental contingencies.  Thus, in the case of the lactose operon (an operon is a suite of bacterial genes located end-to-end) the genes for the digestion of lactose are “turned on” in the presence of lactose.  Absent lactose, the enzymes that these genes produce are not produced in substantial quantities.  But expose the bacteria to lactose, and its lactose operon responds by producing more transcripts of the genes for making the lactase enzyme.  This is called phenotypic plasticity, where the term “phenotypic” indicates that it is not based on genetic change, and the term “plasticity” refers to change, not the involvement of carbon-based polymers that could be used to wrap food.

In cases of phenotypic plasticity like that of the lactose operon, evolution by natural selection is economizing on resources, where these resources may be material.  Thus the resources involved in producing lactose-digesting enzymes include the nucleotides required to assemble the messenger RNA that carries the enzyme-building instructions from the genome.  Then there are the amino acids required to assemble enzymatic protein, following the RNA-encoded instructions.  There are the additional resources required to execute the protein synthesis process, such as the processing of the mRNA, the diversion of ribosomes to the task of assembling the required enzymes.  Finally, there is just the metabolic time consumed with the task.

The term phenotypic plasticity is sometimes divided into two further subcategories:  adaptive and non-adaptive.  In the non-adaptive cases, there are no specific regulatory signals that modulate the phenotypic interactions between characters.  Thus, for example, there is no specific regulatory signaling involved in the human body’s response to having a hand or a foot cut off.  But there is definitely an extensive plastic response, as many other aspects of the phenotype respond to such dismemberment.  With adaptive cases of phenotypic plasticity, there are hormones and other signaling agents involved in effecting a phenotypic change in response, where these signaling systems generally act to increase the average fitness of members of the population when it is kept in its ancestral conditions.

Note, however, that away from the environment in which those signals were established by natural selection, the effects of such signaling may be counter-productive.  A favorite example of this for many of us who are interested in the human diet is our exaggerated and probably inappropriate appetite for sweetened foods, from ice cream to sodas.  Basically, in our ancestral environments, we retained the general primate taste for sweet foods, perhaps because fruit consumption is such a widespread part of the primate diet.  [Note that adult cats have no such “sweet tooth.”  It is not inherent among all mammals.]  But in our present industrial environment, in which food companies have an incentive to exploit every addictive or otherwise exaggerated preference for particular tastes, consuming “all the sweet stuff” leads many to lives of obesity, type II diabetes, and cardiovascular disease.