Redefining Fitness as Gene Persistence

By J. Sun, published Feb 1, 2017

Currently, genetic fitness fails to fully account for quality vs. quantity of offspring. Compounding this are the high stakes of the term: fitness is meant to be that which the gene strives to maximize. The current definition of fitness posits that the gene aims to maximize the number of copies of itself in subsequent generations, or as a percentage of the population. However, above number of copies, every gene has one higher priority—continued existence. In order to account for such cases where the gene does not maximize the number of its copies, I propose a new definition of fitness, which inherently encompasses the old definition. Under this revision, a gene’s fitness (l) is not its prevalence, but rather its persistence:

Maximize l, where l is the length of time that the gene exists.

Said another way, those genes which did not maximize this value (l) likely no longer exist today — this reasoning constitutes a logical proof. Under this new meaning of fitness, a strategy for creating more copies of the gene is simply a means to the true goal of assuring the gene’s further existence. This new formula has the benefits of being agnostic to the length of generations and the multiplicity of copies of the gene within individuals, which had presented problematic edge-cases of the old formula. More importantly, this new definition of fitness allows for a quality-based offspring strategy, independent of quantity, unlike the previous model. Furthermore, unlike the old definition, this new definition implies that preserving long-term existence is more important than creating more immediate copies, if the two priorities ever clash.

As observational evidence to this claim, we look to pathogens, which have been previously cited for their fecundity under the old fitness model. Upon closer examination, despite their fecundity, pathogens nearly always have a natural reservoir (a separate long-term host in which the pathogen is mostly benign), in the interest of the pathogen’s long-term survival, so they never exhaust or kill all possible hosts. For example, rabies produces the lethal and highly contagious furious behavior in wolves, but rabies lives more passively in foxes and bats. As another example, the bubonic plague multiplies aggressively in humans, but lives on silently in their rodent reservoirs. In these cases, the pathogen forgoes the immediate reward of hijacking certain hosts to multiply further, in favor of the true reward of long-term survival. These examples directly contradict the quantity-based definition of fitness, in favor of the persistence-based definition. As additional evidence, reproductive strategies in several species of animals, including the stereotypical human female, have a goal of quality of offspring, in a way that often precludes quantity. Such species members purposely produce fewer, high-quality offspring, in a way that is largely unaccounted for in the previous theory of fitness. Altogether and in conclusion, the evolutionary goal of the gene is to maximize its persistence, or lifespan, rather than its prevalence.