On the Ori­gin of (the Word) Species

by Roberto

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.

Fig. 1. A va­ri­ety of bac­te­r­ial and fun­gal species form colonies of very dif­fer­ent ap­pear­ance. Photo credit: Scott Chim­i­leski and Roberto Kolter.

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.

 

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