Defining Extinction

By Branden Holmes

Extinction is a tricky term to define. Especially since it now has usage outside of a biological context. At the level of the individual organism the term 'death' is invariably used, although 'extinction' is considered a valid synonym. Above the level of the individual, from (sub)population level upwards, the term 'extinction' is invariably used, although 'death' is considered a valid synonym (e.g. the death of the dinosaurs). It seems then that 'extinction' and 'death' are synonymous at every biological level, which begs the question of why we have two terms to describe the exact same range of phenomena.

Other such semantically equivalent pairs of terms can be found in the English language, products of the vagaries of linguistic evolution. The question then becomes, should we make a concerted effort to distinguish them semantically, or allow them to retain their common meaning? Obviously both options have their benefits and their drawbacks, and I as an individual have no vested authority to make the decision myself. However, as nobody is under any obligation to accept anything that I write, I feel free to make a suggestion, which readers (and others) may either choose to accept, reject or ignore.

I submit the suggestion to limit the term death to the level of the individual, and to reserve the term extinction for those biological levels above the individual. The logic behind this submission is that while death and extinction are both natural occurences, the former has a much stricter timeline. Admittedly death and extinction (as I have just defined them) can both occur 'prematurely', which itself needs carefully defining, but the former is restricted to a much tighter chronology. After all, although it varies by several orders of magnitude from species to species, it remains relatively stable amongst the individuals of a single species. And even if the odd individual survives much longer than its conspecifics, its lifespan is not indefinite.

Whereas for (sub)populations there is no defined upper limit to their longevity since they are not tied to any particular taxon, and thus cannot become phyletically extinct (i.e. extinction via intra(sub)population genotypic/phenotypic replacement). Whereas for species and subspecies, although there is technically no defined temporal upper limit to their longevity, there is such a limit genotypically/phenotypically. Although a largely arbitrary delineation, (sub)species can only evolve so much before they cease to remain the same taxon. Thus in one sense they can become extinct simply through evolution, whereby although they have living descendents, all of the members of the population/s no longer express the diagnostic traits that the taxon was described on the basis of (i.e. phyletic extinction). Exactly how many diagnostic traits the (sub)population/s can or must lose before the evolutionarily immediately prior taxon is considered extinct is taxonomically subjective.

Published 7 August 2017.