[Taxacom] Species Definition?

Norbert Holstein holstein at uni-bonn.de
Tue May 23 03:48:32 CDT 2017

I always wonder why people cannot accept that due to the 
enormous diversity of organisms a single clear-cut 
definition of "species" must be complex and 
multidimensional. Why don't we simply go back to the 
question why we are talking about "species" in the first 

The idea of species (and all classification) was created 
to facilitate handling diversity. Initially, species were 
nothing else than coherent clusters of individuals that 
can be differentiated through characters 
(morphology/anatomy, behavior, sexual isolation, life 
cycles, ecological roles, nowadays also genes/allels 
a.s.o.), and each in strongly varying degrees depending on 
the organism (esp. on how they handle gene flow). To 
handle this diversity, a single individual (or few very 
similar individuals) from such a cluster is picked (a.k.a. 
the "type specimen") to represent this cluster and given a 

The taxonomical ("classical") approach to the "species 
definition" revolves around the question which characters 
(morphology/anatomy, behavior) represent such a cluster in 
the best way to sort any given individual to such a 
cluster. Mayr thought about the reasons how such clusters 
come into existence (sexual isolation) and defined 
"species" to be clusters being coherent solely by their 
potential ability to reproduce sexually. Consequently due 
to their common ancestry (according to Hennig), species 
would accumulate apomorphies that may be detected 
genetically. This is then the theoretical basis for 
barcoding. However, this has nothing to do with the 
diversity of morphology, anatomy or behavior, which are 
reduced to proxies. In a Mayr-based classification, it 
might make sense to place all tigers into two subspecies. 
However, does that concept really reflect the 
morphological and behavioral diversity of tigers? In the 
end, it shouldn't matter to scientists, but it does 
depending on what (other) people make out of it.

The clash seems to come about the consequences, esp. in 
conservation. If we only protect clusters with sexual 
coherence (Mayrian species) as a whole but not whole 
diversity, which may be dealt with in subspecies/variety 
level, then the diversity is disregarded. Conservationists 
and classical taxonomists often recognize diversity much 
stronger, and it does not matter per se on which 
classification level it is represented. However, when 
morphological or behavioral clusters (that don't fit 
sexual clusters 1:1) are threatenened to extinction, we 
lose factual diversity. The disregard of morphological, 
behavioral and ecological diversity entails many problems, 
not only the loss of the romantic idea of diversity. If we 
want to predict the reaction of organisms to environmental 
changes or if we want to study the organims' interactions 
with each other or with humans or potential applications 
for humanity, we might obtain very wrong results. E.g., we 
now know that medical drugs do not work the same in all 
humans, although we are one species. If we want to study 
the impact of climate change, which humans do we think of? 
All humans? People will be affected very differently 
depending on where they live, how the live etc. How can 
anyone believe that all individuals of a Mayrian "species" 
act and react the same? If we want to handle diversity to 
predict and study organisms (for curiosity or for 
applications), we have to deal with the true diversity 
(morphological, behavioral, ecological), not simply the 
average of a sexually/genetically coherent cluster sensu 
Mayr. How we deal with diversity, as species or subspecies 
or varieties, should not matter as long as politicians and 
users know and realize that a species name does represent 
a simple all-are-the-same entity. And that seems to be the 
problem. We try to simplify a complex world, maybe too 
complex for the human mind. Applying only a single binomen 
to a complex and morphologically and behaviorally diverse 
Mayrian "species" is intelligible, but ignoring the 
complexity beyond the binomen is not. If 
people/politicians are too unaware or ignorant to realize 
that (some are, some are not), then the "taxonomic 
community" has two options: adapting Mayr's/Hennig's 
definitions loosely and thus concentrating more on 
morphology and behavior to reflect diversity by binomens. 
Alternatively, we stick the evolutionary reality and its 
results on how we handle diversity on a classification 
level. Then, we must explain things to make 
people/politicians aware of the complexity of the true 
diversity apart from the binomen/Mayrian species. This is 
part of the job of taxonomists (that's why taxonomy 
matters, anybody can describe names). Making people 
understand complex matters is idealistic, and it seems 
quite optimistic to bet on this horse in these days.


> I was curious if anyone read this:
> http://www.smithsonianmag.com/science-nature/what-does-it-mean-be-species-ge
> netics-changing-answer-180963380/
> And if so, whether you agree with the subtitle, "As DNA 
>techniques let us
> see animals in finer and finer gradients, the old 
>definition is falling
> apart"?  We all know that people are (ab)using genetic 
>data to draw
> ever-finer lines among groups of organisms and labelling 
>them with
> Linnean-style names (binomens, no less). But it's not 
>clear to me whether
> this represents an evolving consensus on what we mean by 
>"species", or if it
> is really more reflective of using a shiny new toy to 
>boost one's CV/tenure
> prospects/etc.?
>For the record, as far as I'm concerned, the de-facto 
>definition of a
> "species" hasn't changed since Darwin's time, 
>paraphrased as "a species is
> what a community of taxonomists says it is".  But my 
>question is about
> whether the baseline for what the community "says it is" 
>has changed/is
> changing)?  Or, in the long run will we retain roughly 
>the same
> within-taxon-group gestalt that we've generally had for 
>a while now?
> And if the consensus really is evolving, is that a good 
>thing (more
> recognition of biodiversity)? Or a bad thing (increasing 
>incongruence with
> historical knowledge)?  My vote is to keep the best of 
>both worlds and have
> a massive increase in the use of trinomials, but I 
>imagine that statement
> will be seen by many as a troll.
> Aloha,
> Rich
> Richard L. Pyle, PhD
> Database Coordinator | Associate Zoologist | Dive Safety 
> Department of Natural Sciences, Bishop Museum, 1525 
>Bernice St., Honolulu,
> HI 96817
> Ph: (808)848-4115, Fax: (808)847-8252 email: 
>deepreef at bishopmuseum.org
> http://hbs.bishopmuseum.org/staff/pylerichard.html
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Dr.rer.nat. Norbert Holstein
Nees-Institut f. Biodiversität d. Pflanzen
Venusbergweg 22
53115 Bonn
Phone: +49-228-73-6530

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