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What is a species? (7 Viewers)

Jon.Bryant

Well-known member
This is probably an old question, but as I am not a trained taxonomist, I was wondering if the definition of a species is universally defined and used by professional taxonomists?

Looking at the latest IOC update, they state that species are added of deleted based on differences in morphology, vocalisation and genetics.

I presume that these differences are assessed and documented by scientists, who then submit papers documenting their assessments and proposals, which are then peer reviewed. It would be interesting if there was any guidance on scientists on the degree of variation required to suggest a new species.

If we were to take this to the nth degree, then perhaps we could score each candidate species based on a formula incorporating parameters for morphology, vocalisation and genetics - visual analysis could score visual differences, audio analysis could score vocalisation and full genome sequencing could be used to score differences in DNA.

The problem I have with this, relates to how to evaluate things approaching the species threshold? As an example let's imagine Green-winged Teal is a subspecies on the verge of separating from Eurasian Teal - under this imaginary scoring system, it scores is X, whereas the to be a species in needs to be >X. Now for any change in morphoplogy, vocalisation or DNA, in theory a new species comes into existence! Firstly this would be messy, as we would now still have Green-winged Teal as a subspecies as well as the new (slightly different) New Green-winged Teal (or New Green-winged Teals if more that one change occurred across the entire population). The new species(s) would exist at the same time as the subspecies. Also, if we believe that genetic change can be detrimental, have no effect or beneficial, the new species may shortly afterwards become extinct, slowly expand in population living alongside the subspecies, or slowly replace the subspecies making the subspecies extinct.

We could therefore add to the definition something about how long the potential species has been in existence, whether the potential species predominates in a defined habitat and whether the hybridization zone is <X percentage of the total population.

If we use time (as Oriental Bird Club seem to do), then if we could time travel, we could imagine going into the field at a future date, and seeing a subspecies change into a species at the strike or midnight (even though no change to the subspecies had physically occurred)! Although highly hypothetical, this does not seem like a good outcome.

If we look at hybridisation as part of the species definition, then if the possible species out-competes it's parent species, the parent species should eventually become extinct - in which case for the last X,000 years, we have in fact actually been watching the spread of a species, and not two subspecies after all! Again, this does not seem ideal.

In any case, with regard to hybridisation, I would have thought that hybridisation zones would be hard to use for species definition and to 'normalise' due to migratory habitats, physical barriers or habitats - i.e. two species of Tapaculo may well have a large hybrid population, if only they didn't live on different mountains!

Perhaps, I am over thinking this. Perhaps, a species is just a hyperthetical human construct for classification purposes, which cannot be fully defined and can only be applied at extremes? But then why do we have so many parties trying to answer an imprecise questions (with the formation of regional, national and international taxonomic committees, who all want to have their own opinions, on something that cannot be universally defined)?
 
Being a taxonomist solves none of your problems because there isn't afaik, a single, unified, definition of what a species is.

'Looking at the latest IOC update, they state that species are added of deleted based on differences in morphology, vocalisation and genetics. I'd probably add habitat preference and behaviour too.

I presume that these differences are assessed and documented by scientists, who then submit papers documenting their assessments and proposals, which are then peer reviewed. It would be interesting if there was any guidance on scientists on the degree of variation required to suggest a new species.'

I think that many birders prefer this holistic approach but, to say that papers are submitted by scientists, isn't always true. There has been some questioning recently of so called 'field guide taxonomy' where authors take it upon themselves to split various things and not all have a science background although most will have vast, field experience.

There has been back peddling recently on some splits in regard to the above.
 
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This is probably an old question, but as I am not a trained taxonomist, I was wondering if the definition of a species is universally defined and used by professional taxonomists?

Looking at the latest IOC update, they state that species are added of deleted based on differences in morphology, vocalisation and genetics.

I presume that these differences are assessed and documented by scientists, who then submit papers documenting their assessments and proposals, which are then peer reviewed. It would be interesting if there was any guidance on scientists on the degree of variation required to suggest a new species.

If we were to take this to the nth degree, then perhaps we could score each candidate species based on a formula incorporating parameters for morphology, vocalisation and genetics - visual analysis could score visual differences, audio analysis could score vocalisation and full genome sequencing could be used to score differences in DNA.

The problem I have with this, relates to how to evaluate things approaching the species threshold? As an example let's imagine Green-winged Teal is a subspecies on the verge of separating from Eurasian Teal - under this imaginary scoring system, it scores is X, whereas the to be a species in needs to be >X. Now for any change in morphoplogy, vocalisation or DNA, in theory a new species comes into existence! Firstly this would be messy, as we would now still have Green-winged Teal as a subspecies as well as the new (slightly different) New Green-winged Teal (or New Green-winged Teals if more that one change occurred across the entire population). The new species(s) would exist at the same time as the subspecies. Also, if we believe that genetic change can be detrimental, have no effect or beneficial, the new species may shortly afterwards become extinct, slowly expand in population living alongside the subspecies, or slowly replace the subspecies making the subspecies extinct.

We could therefore add to the definition something about how long the potential species has been in existence, whether the potential species predominates in a defined habitat and whether the hybridization zone is <X percentage of the total population.

If we use time (as Oriental Bird Club seem to do), then if we could time travel, we could imagine going into the field at a future date, and seeing a subspecies change into a species at the strike or midnight (even though no change to the subspecies had physically occurred)! Although highly hypothetical, this does not seem like a good outcome.

If we look at hybridisation as part of the species definition, then if the possible species out-competes it's parent species, the parent species should eventually become extinct - in which case for the last X,000 years, we have in fact actually been watching the spread of a species, and not two subspecies after all! Again, this does not seem ideal.

In any case, with regard to hybridisation, I would have thought that hybridisation zones would be hard to use for species definition and to 'normalise' due to migratory habitats, physical barriers or habitats - i.e. two species of Tapaculo may well have a large hybrid population, if only they didn't live on different mountains!

Perhaps, I am over thinking this. Perhaps, a species is just a hyperthetical human construct for classification purposes, which cannot be fully defined and can only be applied at extremes? But then why do we have so many parties trying to answer an imprecise questions (with the formation of regional, national and international taxonomic committees, who all want to have their own opinions, on something that cannot be universally defined)?
The definition of the species is the greatest mystery of biology. For this reason, I will not take this risk.
 
Nature is messy; if we accept that speciation occurs then there must be occasions when the process is in progress, and unless we redefine classification systems to have a “speciation in progress” category, there will be examples that will not fit our categories. AFAIK, there is a form of Darwin’s Finch which has been seen to come into existence as a non-hybridising population in the last 20 years or so, but only has a tiny population. It probably should be a valid species, but equally could be considered as just an aberrant population with an evolutionary life of less than a century (depending what happens next). Basically any classification will merely be one way of labelling the diversity we see.
 
Nature is messy; if we accept that speciation occurs then there must be occasions when the process is in progress, and unless we redefine classification systems to have a “speciation in progress” category, there will be examples that will not fit our categories.
Are you aware of any taxonomies that use a 'speciation in progress' category? I have never seen any taxonomy that sub catogorises subspecies into 'way to go' and 'in the process of speciation'. I suppose a big problem here, is that we cannot generally measure 'progress' in a human life-span, so we cannot know how the course of nature will likely pan out.

If I am thinking of the same finch, as your example, this seems to be a different case, which I think has challenged our understanding of how speciation may occur in some cases - a vagrant finch appeared on Galapagos, which hybridised with a local finch, and rapidly formed a new population, that for all intents and purposes looks like a new species. The article I read suggested that this could be a way that speciation is fast-tracked (within a human lifetime), bypassing the glacial process of natural selection/evolution.

On a similar line, I have thought about what subspecies are likely to form new species given enough time. Say we consider the UK subspecies of Lesser Spotted Woodpecker. The subspecies is highly sedentary. If there is no gene flow (i.e. Europe and Britain do not rejoin to form a single land mass), then statistically (assuming British Lesser Spotted Woodpecker does not become extinct), it would seem that the subspecies will eventually diverge into an new species - OK perhaps in 9.9 million years!. In this case, it seems perhaps a bit wrong for the plight of British Lesser Spotted Woodpecker to be a local concern, which is given less international attention, than if it was a species. Getting rather political, but international conservation's focus on the 'messy' concept of a species, seems to place a greater and unfair onus on some countries rather than others. Just think what the status of UK wildlife might have looked like, if there was international pressure to protect all our endemic subspecies!

I have spoken to BirdLife regarding conservation of subspecies, but the issue is that the problem then becomes far to big to handle and conservation perhaps to fragmented. That said, I am cannot see why local conservation bodies (say RSPB, BB, BTO) do not ignore the importance of the species and do more to champion our unique wildlife (including subspecies) - less focus on Least Concern Black Redstarts in the UK, and more for our Lesser Spotted Woodpeckers, Willow Tits, Black Grouse, etc.
 
I wasn’t suggesting a “speciation in progress” taxonomy; simply that without such a category there will always be examples that could/should be debated; indeed even with such a category what about those that are just starting to split, or just finishing?
 
As others have stated, there is no agreed upon definition of a species, and even when a group of scientists do agree upon the definition, they may still disagree on whether two or more populations have acrued enough described differences to warrant separation. Since speciation is ongoing at any given time you will have different populations which have progressed in different ways towards this, so it become arbitrary to some extent what you define as a species vs a subspecies.

Some people have tried to develop some sort of universal metric for birds: see the Tobias scoring system which is discussed on in multiple threads on this forum. However, the problem with any system is that different groups of birds evolve in different ways. A feature that works well to separate one group of birds may not be useful in another. The Tobias system in general does poorly at identifying more cryptic species, while morphological divergent populations, even if they don't act like species when in contact, are more likely to be identified as distinct species.
 
Are you aware of any taxonomies that use a 'speciation in progress' category? I have never seen any taxonomy that sub catogorises subspecies into 'way to go' and 'in the process of speciation'. I suppose a big problem here, is that we cannot generally measure 'progress' in a human life-span, so we cannot know how the course of nature will likely pan out.
Some people have argued that this is how we should think of subspecies, but subspecies seem to be even more inconsistently defined than species.
 
Rightly or wrongly, I tend to view taxonomy as more akin to jurisprudence than science. Decisions are informed by science, but are ultimately subjective judgements. The fact that within taxonomic committees, members presented with the same data can reach different conclusions tells you all you really need to know.
 
I think that many birders prefer this holistic approach
Probably, but I think layered on top of this is the issue of species status in conservation.

Say for instance we could classify all taxa as 'definitely not a species', 'possibly a species', 'probably a species' and 'definitely a species', then I think for conservation purposes the conservative approach should be to categorize everything from 'possibly a species' and above as a species. Better to label something a subspecies a species and wrongly give it a higher level conservation status, than to accidentally label a species a subspecies, and then loose it for every, due to lack of interest!
 
Probably, but I think layered on top of this is the issue of species status in conservation.

Say for instance we could classify all taxa as 'definitely not a species', 'possibly a species', 'probably a species' and 'definitely a species', then I think for conservation purposes the conservative approach should be to categorize everything from 'possibly a species' and above as a species. Better to label something a subspecies a species and wrongly give it a higher level conservation status, than to accidentally label a species a subspecies, and then loose it for every, due to lack of interest!
Basically I agree, except you now have 3 boundaries to argue about instead of one (species or not)!
 
Basically I agree, except you now have 3 boundaries to argue about instead of one (species or not)!
Not sure I understand? If, as suggested above, species were retained when in doubt, would IOC not have to change the following statement 'Tentatively lump Hume's Whitethroat Curruca althaea and Desert Whitethroat C. minula with Lesser Whitethroat C. curruca following Clements, HBW/BirdLife, Dickinson & Christidis (2014), and Shirihai & Svensson (2018) pending more comprehensive genomic and vocal analysis. Monotypic; includes monticola (Aymí & Gargallo 2006).' to 'Tentatively retain species status for Desert and Hume's Whitethroat'. i.e. when in doubt assume a species! If we are saying that decisions are nuanced, and subjective, this is not the same as saying there are three boundaries - just that the single boundary is set at the division between 'possibly' and 'definitely not', rather than the higher standard of 'possibly' and 'probably' or the even higher standard or 'probably' and 'definitely'.
 
Nobody will ever get this right because one size does not fit all at any level of Nature.

The most to be hoped for is consensus in most areas and stability while areas of doubt are researched in the way most suited to the problem (which will not be the same for all problems.) The thing that shouldn't happen in any area of doubt is change.

John
 
Some people have tried to develop some sort of universal metric for birds: see the Tobias scoring system which is discussed on in multiple threads on this forum. However, the problem with any system is that different groups of birds evolve in different ways. A feature that works well to separate one group of birds may not be useful in another. The Tobias system in general does poorly at identifying more cryptic species, while morphological divergent populations, even if they don't act like species when in contact, are more likely to be identified as distinct species.
This is interesting, and I will look up the Tobias system.

However, I suspect that a big issue will be how genetics has changed our understanding of taxonomy and whether this is correctly factored into any system. I recall that in the 1980's it was the view that there were about 9800 species in the world - most modern lists now are close of over 11,000, so there has been a sea-shift in the classification and assessment of species.

It would seem that me that the optimum goal of the taxonomist would be a thoroughly updated 'Tobias' system, which is not be so easily fooled by cryptic species, when genetics points to speciation. I think, it would be very interesting if subspecies could be 'scored' by some system, with a low score indicating hardly meriting subspecies status and a high score indicating approaching species status.
 
The thing that shouldn't happen in any area of doubt is change.
This is different to my arguement - I was suggesting that when in doubt, air on the side of caution (and of most advantage to species conservation). An example (I think) is Balem Currasow, which I seem to recall is not considered a species by Clements and IOC, but is considered a Critically Endangered species by BirdLife International. If there is any doubt, surely the safe option would be for IOC and Clements to roll over and follow BirdLife International - not to adopt their own status quo.
 
Perhaps, a species is just a hyperthetical human construct for classification purposes, which cannot be fully defined and can only be applied at extremes? But then why do we have so many parties trying to answer an imprecise questions (with the formation of regional, national and international taxonomic committees, who all want to have their own opinions, on something that cannot be universally defined)?

Yes, you are about right. Species is a human construct, an attempt to scientifically nail the laymans observation, that life forms including birds come in different kinds.

The main reason of species discussion is that it is good for generating publications, which are used in academic world for promotion and measuring scientists. So, even if species are not scientific (one cannot verify whether a definition of species is true or false, only it is good or bad for some purpose), there is a pressure to publish scientific papers about species. Splitting a species into more looks especially good in a scientist CV.

The problem I have with this, relates to how to evaluate things approaching the species threshold?

The problem is unsolvable - since evolution is a continuous process, in birds there is no fixed time point which one could point and say that a species appeared now. (curiously, in plants and other lifeforms there can be. It happens that on a day X, a mutant plant sprouts from a seed, unable to cross-fertilize with its relatives, and can start a new species by reproducing vegetatively and between clones of itself).

However, this problem is less commonly found than problems caused by changing definitions of species.
 
Yes, you are about right. Species is a human construct, an attempt to scientifically nail the laymans observation, that life forms including birds come in different kinds.

The main reason of species discussion is that it is good for generating publications, which are used in academic world for promotion and measuring scientists. So, even if species are not scientific (one cannot verify whether a definition of species is true or false, only it is good or bad for some purpose), there is a pressure to publish scientific papers about species. Splitting a species into more looks especially good in a scientist CV.



The problem is unsolvable - since evolution is a continuous process, in birds there is no fixed time point which one could point and say that a species appeared now. (curiously, in plants and other lifeforms there can be. It happens that on a day X, a mutant plant sprouts from a seed, unable to cross-fertilize with its relatives, and can start a new species by reproducing vegetatively and between clones of itself).

However, this problem is less commonly found than problems caused by changing definitions of species.
I know I have replied to this argument of yours multiple of times in various threads, but I will keep replying: The idea that scientists only are interested in splitting species to get more papers is silly. Unless your species is some sort of big charismatic species like an elephant or giraffe, a taxonomic change such as a split will never get published in any high impact journal. The specialist journals where the vast majority of taxonomic changes occur in are often lower impact. Splitting species also won't help you apply for grants. For the most part, taxonomy in general is undervalued and underfunded. If the sole interest of a scientist is to boost there CV for employment/funding reasons, taxonomy is the absolute worst pathway to go down.

I also think it grossly undervalues the type and amount of work that goes into a paper assessing taxonomy, including field work, museum sampling, DNA analysis not to mention the writing and journal submission process itself.
 
The idea that scientists only are interested in splitting species to get more papers is silly.
As also expressed by Thomas Donegan recently:
If you google "taxonomic impediment" there are lots of papers which kind of explain why we don't see much research effort focused on descriptions and taxonomic revisions. These include:
  • Lack of funding for institutions that traditionally do this sort of thing, e.g. natural history museums, versus other biological research areas such as climate change, biochemistry or conservation biology.
  • Taxonomic journals are generally low-rated or not listed at all in leading indices, disincentivising academics, many of whom are subject to pay-related performance reviews and other assessments based on the ranking of journals they publish in, to write on the topic
  • Most people doing taxonomy do it in their spare time as volunteers, whether they are professional biologists or not, since there are so few jobs in the field.

One could add to this list and grumble more over a beer. E.g. , in my experience:
  • It is quite painstaking; any taxonomic study requires not only fieldwork in remote areas which some novelty is found (and related funding, permits, analysis after the event), but then also travel to museums in numerous countries to compare and study the specimen record.
  • Statistical analysis of vocal variation is very painstaking and involves a lot of staring at screens taking measurements.
  • When you do publish on taxonomic topics, there is a lot of resistance to change and criticism. How many discussions here about appropriateness of names, appropriateness of authorships, whether proposed splits are good, etc? [I apologise here for all mine to others and don't mean to put anyone off.] E.g. you have to contend with this sort of crap: Atlapetes blancae, 8 years later, still not found. Wish or Species? – Planet of Birds .]

  • You also have to contend with taxonomic committees (e.g. SACC) whose overall aggressive attitude against non-affiliated research is a real turn-off. [Overall, I've published a few new species, subspecies, splits and lumps, but get the impression many colleagues in the field would prefer none of it had ever been published. ]
  • Difficult to get taxonomic research published anywhere (even in the lower ranking journal group).
  • Peer review for maintstream journals give authors of new taxa a really hard time, expecting the same n= in data samples for very rare species occurring in remote locations as are achievable studying sparrows in your local park.

It's overall a pretty negative environment, whether you are from the global North or global South.

Some of the first listed factors are why many would-be taxonomic papers focus on molecular biology and are published in journals like Molecular Phylogenetics & Evolution, which will publish phylogenetic trees with very wordy and (in my view, generally not very interesting at all, speculative) discussions appended, but apparently will not allow authors to address the taxonomic implications of their findings.
 

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