Where do you draw the line between one species and another?

As the title suggests folks, I’d like to know from you all where one draws the line between separating one species from another?

Allow me to give an example using the following observation: https://www.inaturalist.org/observations/71814674

I have given this plant (Family: Scrophulareaceae - Genus: Zaluzianskya) a tentative ID as Zaluzianskya elongata. This was done after a number of days of intense comparison of this specimen with others in South African Herbaria and Databases as well as consulting experts

That observation does indeed conform to almost every single characteristic of the species Z. elongata, except for 3. Those are:

  1. This plant flowers only at the onset of total darkness as opposed to other specimens flowering at dusk and in dim light
  2. This plant has a heavily decumbent habit (i.e. the main shoots creep along or very close to the ground with only the flowering extremity beginning to grow upwards) as opposed to other specimens with a straight, ascending growth habit
  3. The corolla lobes are highly reflexed

After careful consideration, I can potentially(!) attribute these differences to the geographic location and conditions in which this observation was made - a high altitude peneplain criss-crossed with very broad and meandering river networks which is prone to diabolically low temperatures even on some summer days. I have caught glances of similar looking plants growing in very close proximity to this

So what I want to know from you all is: Do you think these differences can be regarded as sufficient enough to separate this specimen as a unique species? Or do you think that it still falls under the taxon of Z. elongata and is merely in the process of speciation? Please substantiate your reason’s, as science based as possible, for why you think so, and please note that I am NOT asking for an ID here, I just want to know if you see this as 1 or 2 different species and why

I’d really appreciate the help guys, thank you!


If this was indeed representative of a new species, then you would expect to find other individuals with the same characters; is this individual the only one you’ve found, or are there 10, 50, 100, etc others that also conform to that phenotype?


But the same is true if it is an ecotype.

Maybe unintentionally, you have asked the question of the ages for taxonomists the world over. One person could only scratch the surface of all that has been written and thought about the topic. But for what it’s worth, here is one perspective. (Bear with me - I’ll get to your example eventually…)

Some will insist that species are merely artificial and arbitrary constructs of human thought (like any other taxonomic rank), with no external reality. I think (and hope) that is still a minority view, at least among taxonomists.

To the extent that we still consider species to have a “special” biological significance different from other taxonomic ranks, that significance generally focuses around genetic lineages that “breed true” among themselves (by whatever reproductive method(s) they have acquired), and rarely if ever breed successfully outside their species. And yes, there can be gray areas, where lineages have become partially isolated reproductively, or have re-acquired partial interfertility. (That is where subspecies ranks are best employed.) And there are special cases, like asexual reproduction, where different biological criteria are often applied.

This is all fine in theory, but determining what is actually going on among populations in nature is much more fraught. Until the biochemical and DNA revolutions of the past few decades, phenotypic features such as those you noted with your observation served as the best “proxy” we had for assessing genetic isolation (or lack thereof) among lineages. Broadly generalizing, consistent and persistent differences between populations of individuals were a strong indicator that a species barrier existed between them. (And often this is still the best information available to us.) Such barriers could be genetic in origin, and/or could be enforced by external geographic, habitat, or pollinator barriers, etc. Often species that breed true in nature can be artifically crossed with relative ease when their natural reproductive barriers are other than genetic.

Biochemistry, genetics, and DNA have since provided much sharper tools for assessing the genetic and evolutionary relationships among lineages, but these tools are not a panacea and have their limitations too. Again to generalize broadly, genetic differences between individuals or populations do not necessarily indicate that they are separate species. And conversely, failure to detect genetic differences (unless one is sequencing the entire genome) does not necessarily indicate membership in the same species. It all depends on how much of which part(s) of the genome are sampled, and how intensive and extensive the sampling is across populations in nature. And correlation with the results from other tools still provides the most confidence.

So that’s a long answer to your question. Which enables a short answer: it all depends… :sweat_smile:


My point wasn’t that more than one = is definitely a new species, it was that not more than one = less likely to be a new species

I also think you’d want to grow seeds from it to see if really those differences consistent.


Odd individuals are just odd individuals and you get them for a bunch of reasons. With very rare exceptions they do not indicate speciation.

If you think this is (or could be) a new-to-science species or ecomorphotype take lots of photographs over a period of time with good representation of the entire plant’s anatomy and share them with experts. Look for other individuals that share the characteristics and document them if you find any. If you know someobody who can sequence genotypes, take samples and preserve them appropriately.


There are at least 26 different species concepts in use in modern scientific literature:

  • Phylospecies
  • Agamospecies
  • Autapomorphic species
  • Biospecies
  • Cladospecies
  • Cohesion species
  • Compilospecies
  • Composite species
  • Ecospecies
  • Evolutionary species
  • Evolutionary significant unit
  • Genealogical concordance species
  • Genic species
  • Genetic species
  • Genotypic cluster (aka Polythetic species)
  • Hennigian species
  • Internodal species
  • Least Inclusive Taxonomic Unit (LITUs)
  • Morphospecies (aka Linnaean species)
  • Non-dimensional species
  • Nothospecies
  • Phylogenetic taxon species
  • Phenospecies
  • Recognition species
  • Reproductive competition species
  • Successional species
  • Taxonomic species

Each of those concepts is a different answer to the question “Where do you draw the line between one species and another?” As you can tell, there isn’t much consensus on the answer. From your post, it sounds like you are wanting to apply a species concept similar to morphospecies or phenospecies. These species concepts do not include consideration of genetics or reproductive isolation, but rather depend on observable characteristics. Morphospecies is more about a species being consistently “distinguishable”, while phenospecies is more about sharing a cluster of characteristics. Under either of these concepts, it sounds like you would have a good argument for these plants being a unique species. Note, however, that both of these species concepts have largely fallen out of favor with taxonomists since the 1960s. That said, they both still have their proponents.


Thanks for the replies everyone. Certainly some very interesting insights offered by @jdmore ; @pmeisenheimer ; @zygy and others

As it turns out, I have found a healthy population of this plant just next to where I found the first. They all seem to share those same 3 deviant characteristics as the type specimen. I agree with the general consensus here that this is likely to be a morphotype of the common species Z. elongata, that is to say it is likely very similar in genetic lineage but has differed from its ancestor due to its presence in this environment with very unique environmental and climatic characters. With this being the case, I have begun the long and involved process of pressing a specimen, and taking photographs and measurements which I will use to consult experts on the Genus and see exactly what it is we have here.

Even if its just a new subspecies of the nominate, this would be incredibly exciting as this would be a significant botanical find in an area which most people believe to be botanically explored to exhaustion

Cheers guys!