The origin, and fate, of “sister species”

Warning: etymological nerdery.

 The origin and fate of the phrase, I mean, not the actual species.

In evolutionary biology, a pair of “sister species” (or “sister taxa”, or “sister clades”) are each other’s closest living relatives. I was at lunch last week with an interesting assortment of biologists when the topic of gendered language in biology came up.  I think it started with “daughter cell”, which is routine in developmental biology, and expanded from there.  I was brought up short when the conversation turned to “sister species”.  It’s a term I know well and use often – and it had never occurred to me that it’s gendered.  This is, of course, a nice illustration of how insidious gendered language can be.  Whether or not there’s any real social consequence to our use of “sister species”, the mere fact that I hadn’t noticed the in-hindsight-blindingly-obvious gendered nature of the term was something of a shot across my mental bow.

Once you think about “sister species”, several questions seem obvious. First, when did we start using that term?  Second, does anyone ever use “brother species”?  And third, if we were to decide to abandon the term, what’s a good gender-neutral alternative?  The answers to the first two surprised me; an answer to the third eludes me.

First: the history of the term.  When I said “I wonder who used it first?”, a friend instantly shot back “probably Joe Felsenstein”.  Like most quick guesses at things, this was not dumb (Joe published some extraordinarily important papers taking advantage of sister-clade relationships to test evolutionary ideas), but it was wrong.  The first use I’ve been able to locate* dates from an 1837 issue of The Magazine of Horticulture, Botany, and All Useful Discoveries and Improvement in Rural Affairs”.**  The anonymous author was arguing that British gardeners should cultivate their native flora, which with proper care would produce “as much per pot as some…far-brought sister species, not half so beautiful or gaudy”.  The term here doesn’t quite carry its modern technical meaning, though (and in 1837, pre-Origin of Species, it would be unlikely to).  It just means “a similar or allied species”.  Henry Walter Bates, in 1863’s A Naturalist on the River Amazonas, refers to sister species of beetles, and the term there seems to suggest at least very close relatives.

By 1909, the term had taken on something close to its modern sense: Dewar and Finn, in The Making of Species, ask How is it that species A has given rise to species B, C, and D, or, while itself continuing to exist, has thrown off sister species B and C?”  Uses that more and more match the modern sense crop up through the 1920s, but the term really took off (according to Google Books Ngram Viewer) in about 1970.  (See? The Felsenstein guess wasn’t dumb; Joe started publishing in 1965.)  Now, at least in the technical literature, the term is unambiguous.

Second: does anyone ever use “brother species”?  I was quite surprised to discover that the answer is “yes”.  Here’s Hewitt Cottrell Watson, writing in 1870’s Botany of the Azores“Still more impossible it would be, to name the European Campanula which could be accepted as a brother-species or cousin-species with the Campanula vidalii of the Isles…” .  And since Bates used “sister species” in the Amazon, it seems tidy that Alfred Russel Wallace used “brother species” (in 1891, to compare giant sequoia and redwood in California).  But the term never took off, and despite a sprinkling of modern usages, it would probably draw the same faintly puzzled look from you as it drew from me.  But consider this: in French, it’s “brother group” (but still “sister species”, because “group” is a masculine noun, but “species” is a feminine one.  This is probably just a result of a language difference: common nouns are gendered in French but not in English. (Alex Figueiredo points out that the same is true in Portunguese).

Third: if we wanted to replace the term***, what would we replace it with?  The most obvious possibility isn’t available: Ernst Mayr defined “sibling species” in 1963 to mean ”morphologically similar or identical natural populations that are reproductively isolated”.  That’s not the same as sister species.  Two species may be sibling but not sister (in a rapidly diversifying species complex), and two species may be sister but not sibling (if they’ve evolved obvious differences in appearance).  This is too bad, really: the different technical meanings acquired by sister and sibling block the most obvious path to nongendered language.  “Kin species” doesn’t work either, because kin can be close or distant, but aren’t necessarily closest.  We can go for a longer or more technical phrase, of course: “most closely related species” or “species sharing most recent common ancestor”, but believe me, scientific writing doesn’t need any help in larding itself up with long and complicated ways of saying things.

Can you add any languages to my survey?  Are closest relatives brother or sister in the languages you speak?  And what would be a good non-gendered term?  Leave your ideas in the Replies!

© Stephen Heard  February 24, 2020

 Image: Sister species of tits: left, azure tit (Cyanistes cyanus) © Jargal Lamjav CC BY 2.0; right, blue tit (Cyanistes caeruleus) © Francis C. Franklin  CC BY-SA-3.0

UPDATED to correct treatment of French terminology – thanks to Flo Débarre for the correction!

UPDATE: Alex Figueiredo (via Twitter) argues that “species” derives from Latin and the “ies” suffix, which denotes an abstract noun, is associated with feminine words.  Although English (mostly) abandoned the practice of having common nouns with genders, it’s possible that the Victorian scholars who started using the phrase “sister species” did so because they were familiar with the Latin etymology.

*^Take everything in this post with a grain of salt.  I didn’t spend days plumbing the depths of the British Library.  This blog is free, and you get what you pay for.

**^The stacks of old magazines in dentists’ waiting rooms used to be more interestingly titled than they are today.

***^There will be people who think a proposal to replace the term is symptomatic of everything wrong with modern society; and there will be people who think that first group of people are what’s wrong with modern society. It is not my purpose here to tell you which stance you should take.

13 thoughts on “The origin, and fate, of “sister species”

  1. Catherine

    In Indonesian “sister species” is “takson saudara,” which means “sibling taxa.” (Kin terms in Indonesian don’t always mark gender in the way an English speaker might expect.) I don’t know what, if anything, sibling species sensu Mayr might be in Indonesian.


  2. gunnarmk

    I wonder how widespread “wrong” (by which I mean noncanonical) use of “sibling species” is, I have definitely heard people use it for species with shared MRCAs. And even if this useage is “wrong”, I don’t think it would be problematic to redefine the term since Mayr’s evolutionary systematics paradigm is pretty much dead. There is already precedence in phylogenetic systematics for using the same terms with different definitions (e.g. monophyly), so if we want to use gender-neutral language we shoulde “sibling species”.
    For the concept Mayr defined as sibling species we today mostly use terms such as “cryptic species”, “pseudocryptic species” or “allopatric cryptic species”.


    1. mooerslab

      I would agree with the above post: little would be lost by switching to “sibling species”, and using the common and obvious “cryptic species” for those damn Dobzhansky drosophilids.
      Great post, Steve..

      Liked by 1 person

  3. Abigail

    After about 8 hours of shaking my brain to follow a thread of half-remembered fragments from grad school, I uncovered that David Starr Jordan used “geminate (twin) species” in approximately the same way that “sister species” is now used. The original paper (Jordan 1908, Am Nat) specifically talks about species on either side of a physical barrier but perhaps this could be applied more broadly.

    “Geminate” itself has a definition in the OED of “Duplicated, combined in pairs, twin, binate. geminate leaves, leaves springing in pairs from the same node, one leaf beside the other” and comes from the Latin for twin.

    Liked by 1 person

  4. James Braund

    German has the word “Schwesterart” (occasionally written early on as “Schwester-Art”), which translates literally as “sister species”. As best I can tell from a quick look through Google Scholar, it was initially used in botany (first reference in the mid-1860s), but did not really start to take off in scientific literature until the 1890s.


  5. Elina Mäntylä

    In Finnish there is a word “sisarlaji” which is sister species but I haven’t seen it used that much. I had to google to see if it even exists. 😉 I have a feeling that most often it’s just said that some two species are closely related.


  6. Caitlyn Cardetti (@CaitlynCardetti)

    I’m down for using sibling species and I am all for getting rid of gendered words. We can easily replace mother/daughter cells with parental/offspring cells. But the gendered word in academia that is pervasive and that really bothers me the most is seminal. We already have plenty of other words for that so why are we still using it?! I think that’s what bugs me the most – it would be one thing if a good synonym was hard to come by but there are plenty! Examples of synonyms to use: pioneering, influential.


  7. Grindle McNarmo


    The reason they are called “sister species” is because only females give birth. Species are “female” because they are capable of giving rise to another species. Ditto for a “daughter cell”, which is capable of giving rise to offspring cells.

    Until males are capable of parthenogenesis, there is nothing wrong with this “gendered language”. “Sister species” is a better analogy than “sibling species”.



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