by Roberto
Fig. 1. A variety of bacterial and fungal species form colonies of very different appearance. Photo credit: Scott Chimileski and Roberto Kolter.
Despite it being fundamental in biology, the species concept has a problem; there is still no universally accepted definition. Already Darwin skirted the issue when he wrote: "Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists..." in "On the Origin of Species" no less. The species concept is particularly thorny when it comes to microbes. In a 2017 STC post I co-authored with Kostas Konstantinidis, we broached the existential question of bacterial species. To get a sense of where the field is going and its continuing debate, I recommend you read these three articles (1, 2, 3). But I'm not here today to solve the species problem; I never could. Rather, I want to share a personal learning experience that has allowed me to come to terms with the problem.
I doubt I stand alone when I say that, as a biologist, I feel an inner tension when thinking about species. On the one hand, I want to be able to classify life forms in a systematic way. For that I rely on naming them and relating them to other life forms. And in naming them, I want that name to represent a stable entity that will always be the same such that when I mention E. coli, I want everyone who hears me to know what species I am talking about. On the other hand, I recognize that the more we learn about microbial evolution, horizontal gene transfer, and pangenomes, the less likely it is that the E. coli I am talking about will be what others think of when they hear E. coli. To assuage this tension, I sought reconciliation – as I often do – in studying the past.
I crossed paths with descriptions of Aristotle's biology and Carl Linnaeus's Systema Naturae (all sound material to write about, but not here, not now) but I found the greatest solace in the etymology of the word species. Aristotle used the Greek words γένος (génos) for general "kinds" of living organisms and εἶδος (eidos) to refer to specific "forms" of organisms. As classification efforts adopted Latin as the lingua franca, "genus" remained similar to γένος. But εἶδος became species, from specere (to look at, to see). In the etymology dictionary, I read: "Latin species 'a sight; outward appearance' had many extended senses, including 'a spectacle; a mental appearance, an idea or notion..." For me, it was as if a light had gone on, an epiphany of sorts, to realize that the species concept in biology was no more than "a mental appearance, an idea or notion." A useful mental invention, albeit an unsatisfactory one, in our attempt to understand natural history. A concept that will never have but working definitions, always evolving.
Judging by the ongoing debates there are still many anomalies to be resolved. Does that mean the field is in crisis, in need of a Kuhnian "paradigm shift"? I would not take it that far, but who knows; we may witness a classification revolution in the future. Until then, I am now much more at ease with freely using species names as I have been for decades and let the debates rage on.
Do you want to comment on this post? We would be happy about it! Please comment on Mastodon, Bluesky, or on 𝕏 (formerly Twitter).
Comments