the naturalist man's picture

What is a species?

Following on from the forum "Is a re-introduced species native?"

I've decided to create a new, sister, forum to discuss what is a species. To kick start I've copied some of my postings from the previous forum.



the naturalist man's picture


I hope people don't mind but I've also included resonses from other members where appropriate.

When is a hybrid a species
19 November 2009 - The Naturalist Man
Good question, I guess it would be when the hybrid's genetics and appearance 'stablise' for many generations. I'm not sure if there is a definate answer, anyone else know?

When is a hybrid a species?
27 January 2010 - Derelict

There is no answer. "Species" is a construct that we use for convenience to differentiate life-forms. Unfortunately nobody told the biota so the biota don't know the rules. A viable hybrid is evidence that two of our definitions of "species" (i.e. those for the parents) may be inaccurate. If the hybrid can produce viable offspring then that proves it.

There's a classic case of this from the Drosophila world where two american species with overlapping ranges became one when it was realised that all the id'd specimens of one were male and all the id'd specimens of the other were er female.


Re-assessing species status
20 November 2009 - Gill Sinclair
I agree that species' status should be re-assessed when physical barriers are removed - Abbey is right - before that point, the only thing that stopped interbreeding MAY have been the fact that individuals from two populations couldn't physically get together, and that needs to be 'tested'.

I think hybrid animals should be considered a separate species if the hybrids can breed with one another and produce viable and fertile offspring, and individuals can continue to breed with one another for several(?) generations - effectively a new species has evolved in that case. The genetics could be tested to see if they have 'stabilised', but I don't think waiting for appearance to stabilise would be a good measure - look at how much appearance can differ in humans, dogs (artificially-induced variation I know), American black bears with their different colour morphs and markings etc.

Gill Sinclair
OU Certificate in Contemporary Science

Re-assessing species status
20 November 2009 - AB25426 - abbey
I agree that appearance is an ineffective indicator to use, there can be so much variation within a single species.
And I suppose the additional difficulty (when what are accepted as two seperate species breed e.g. in the duck example) would be if the original two 'species' then bred with the 'hybrids', therefore making a multitude of variations in split between the originals. Also, at some point, the orginal 2 'pure species' may cease to exist. Again I think this supports the case for re-assessing them into the same species, with different subspecies. Of course this could in theory result in countless subspecies. How do you then determine what is a valid subspecies if you have 'hybrid' variations of 99/1, 1/99 and everything inbetween.

Abbey Burn
OU Student BSc Natural Sciences

Graham Banwell

Visit the iSpot Yorkshire forum for information on events, issues and news relating to 'God's own country'

the naturalist man's picture

Baboons as an example of complex taxonomy

Another example of how complex, and difficult taxonomy can be was brought to mind by a recent posting on iSpot of an olive baboon (Papio anubis). The Olive baboon, considered a species since around 15 years ago, was once considered a sub-species of the Guinea baboon (Papio hamadryas), indeed some taxonomists consider all baboons to be nothing more than sub-species of the Guinea baboon; a species which it still readily hybridises with where there ranges overlap.

Olive baboons hybridise with other baboon species elsewhere in Africa to the extent that it can be impossible to be sure which species some animals belong to. To complicate matters some of these hybrids are now considered sub-species of the olive baboon - the International Union for the Conservation of nature (IUCN), the body who produce the red data lists for animals and plants, recognise seven sub-species.

Therefore, considering that all baboons can interbreed to produce viable offspring should we consider them as seperate species even though their DNA is different enough for most scientists to consider them to be so?

For more information visit the IUCN web site:

Also there is a good paper on the subject which has free access:

Be warned the paper has many technical genetic terms. To sumerise, they beleive there have been multiple isolation and re-mixing of species over the last four million years resulting in a very complex situation.

The most interesting aspect of this paper to the discussion here is that they consider there to be 11 species of baboon not the traditional five. Even more interesting these 11 species bear no relation to the five. The authors say in some cases species which look very different and have different social behaviours are, according to the DNA results, the same species, yet others which look identical and behave identically should be different species.

The million dollar question, therefore, is who is right? Do we seperate species according to differences in their DNA or by the way they look and behave?

Of course the trillion dollar question is, should we care?

Graham Banwell

Visit the iSpot Yorkshire forum for information on events, issues and news relating to 'God's own country'

er2938's picture

Biological definition of a species - response

This post was previously made on native/non-native forum regarding what constitutes a species. I prefer a less stringent approach that takes account of many different factors including sexual reproduction, and should include ecology, morphology and behaviour.

I reposted it at request of Naturalist Man.

Generally I found your comment informed, but assuming. And you know what they say when you assume... you make an ass (hybrid!) out of you and me.

Sexual reproduction is common in fungi, so how you have come to assume I am a mycologist I have no idea.

In the previous post you have made the point that deer species should be re-classified because they can interbreed to produce viable offspring. To this point I annotated that this is a very stringent classification of a species known as the 'biological classification of a species' and not that helpful to taxonomy. Taxonomy is an agreed framework based on a wide number of different factors including the ability to reproduce, but also ecology and behaviour.

Please also bear in mind that mammals, other animals and plants evolved from species without sexual reproduction, including millions of microbes. Your narrow classification of a species has just excluded all your ancestors.

Ignoring other factors and lumping every deer species together because they can interbreed isn't helpful. There must be a continued advantage to their current classification. Naturalist Man's comments regarding baboon species' below also highlight the need to split species even though they can interbreed. Maybe you could share your 'knowledge' of Cervus like a good scientist, rather than telling people with less knowledge of the particular subject off.

Wrapping up Homo in just one species has more to do with politics than taxonomy.

It's good to have a debate about these things within a wider context, that is what this website is about. You may disagree with what I have to say or don't find it relevant, I do... I do find them relevant to the native/non-native discussion, given the increasing divergence in DNA between species in different locations.

You have misinterpreted my point regarding domestic species. My point is, why bother spending millions and millions of pounds classifying and researching domestic species purely designed for commercial purposes? The pursuit of scientific enquiry isn't free and the money could be much better spent elsewhere.

petershirley's picture

What is a 'species'?

I have just been thinking about this in relation to something I was writing. I came across a quote from Darwin - 'It all comes, I believe, from trying to define the indefinable'.

For a lengthy treatment of the subject have a look at Ereshefsky, Marc, "Species", The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.), URL =

Peter Shirley

Derelict's picture

Biological definition of a species - response

A bit laboured. Try having an original thought. See below.

er2938's picture

No need for this

Let's try and keep it to be a friendly forum for discussion. People will and can disagree with your ideas. I am entitled to my opinion and my ideas in the same way as you are entitled to your ideas and opinions. No need to be rude.


er2938's picture

Cervus species or genus - response

Previously posted in mammals forum under native/non-native species discussion.

I neither agree nor disagree with your comments above, however I think it is important to note that that the ability to interbreed and reproduce successful offspring is just the 'biological definition of a species'.

Where does that leave (the very many) species that use asexual reproduction or a mixture of sexual and asexual reproduction based on environmental conditions and habitat?

Ultimately taxonomy is a system of agreed conventions subject to change, not an exact science. I don't know much about the genus Cervus, but imagine there must be some advantage to the current classification.

There seems little scientific value in studying and classifying domestic breeds and genetically modified organisms to me. They are ultimately 'designed' for commercial purposes.

charlieb's picture

I'd be tempted to go on

I'd be tempted to go on similarity of genes.

Say 99.9% shared genes between two animals and call 'em the same. Of course doing things two-by-two could take a while...!

the naturalist man's picture

It's all in the genes

99.9% is all well and good, but where do you draw the line? 99.9% may be little more than a difference in hair and eye colour. So should we make it say 99% ? But would that not cause problems when considering human races? Um... lets go for 97% to be on the safe side. Ah! but that would make chimps and humans less than 1 percent away from being the same species....and that would really upset some people!

In my opinion we are too obsessed with trying to force species into boxes. The Darwin quote above says it all. If we must create arbitrary boxes to put species into, and I think we must to better understand the world around us, then lets use some common sense and base the species on genetics first, being fine tuned by observable differences. Therefore in the baboon example, the two groups which look and behave differently yet are almost identical as far as their genetics are concerned, would be two species, or at least sub-species; as would the ones which are identical but have different genetic makeups.

One thing to consider, it is very expensive to map a whole genome, therefore, in most studies (baboon one included) only a small portion (considered to be a key area) is studied. Therefore, they may be the same in that segment but different elsewhere.

Graham Banwell

Visit the iSpot Yorkshire forum for information on events, issues and news relating to 'God's own country'

Derelict's picture

Phylogenetics not a perfect match for species.

Molecular phylogeneticists are the new kids on the block and they tend to dismiss old-style zoologists (e.g me) as people obsessed with 'variation in superficial phenotypes'. Actually we do rather more than that. What they don't say (or rather what gets forgotten) is that their own work is really focussed on the evolution of mitochondria, not animals.

The analogy I use is that mitochondria are the batteries that power the cells that make up all animals. Thus molecular phylogeneticists are really studying batteries and classifying animals according to which sort of battery they come supplied with. Mitochondria - like AA batteries - are pretty simple and don't vary very much. They do so by random mutation in their constituent genes. You can plug any brand of AA battery into a whole range of electrical goods. I suspect that if you could remove the mitochondria from a human cell and replace them with mitochondria from a sea slug that the human cell would continue to tick over. I really ought to check to see if this has been done... Fortunately for the molecular phylogeneticists animals don't swap their batteries - they use the same ones their mums gave them.

However mitochondria don't impact the organisms they're in which are free to diversify as they choose and there is always the possibility that a species will evolve into two species without the internal batteries changing. All mammals are made of essentially the same building blocks and mechanisms (skeletons, reproductive systems etc) and genetic variation between them is surprisingly low. For animals to reproduce successfully the genomes have to be very, very similar indeed. We don't mate with chimpanzees and much play has been made about how similar our genomes are. The classic example is red-headed v white-headed ducks. To us they look really different but in fact they are probably not that many genes apart, one of which is a gene that is expressed in an already genetically predetermined area of the body. You get greater variation in domestic chickens. So if you protect white-headed ducks you're putting a lot of effort into preserving one or two genes which mean something to us, but not to the ducks, a process not dissimilar to domestication. The rest of the ducks' genome is doing very nicely thank you very much.

Cervus suffers from this - the molecular phylogenists can identify different battery users within those groups identified by those dealing with 'variation in superficial phenotypes', But the important question to ask is "what would Bambi do?" and Bambi doesn't really care about what batteries a potential mate is using so long as they're not dead. In Cervus they don't care much about 'superficial phenotypes' either (unlike 19th century taxonomists) cf. Red deer x sika deer. They go on chemical stimuli. If you smell right at the right time you're in.
(This is why in artifical insemination programmes you can get bulls to mate with dexion trolleys.)

Hopefully I haven't used the word 'species' anywhere in the above! Now for species...

The fallow deer provide an even better example of this. Before the last Ice Age they were everywhere in Europe. Afterwards they found themselves stuck in the middle east and had to rely on humanity to restore them to their old ranges, There are two forms of fallow deer Dama dama and D. mesopotamica. They look rather different. D. mesopotamica didn't make it to Europe and is now very rare. Everyone wanted it to be a different species (even me) and this picture can be confirmed if you look at their batteries. They're different. Hooray! We can justify expensive protection programmes for D. mesopotamica However if you put the two species together (why?) and ask "what would Bambi do?" they get on like a house on fire, producing viable hybrids. The variation between them isn't great enough to put them off and they don't give a toss about which batteries they use. So here's an example where an evolutionary change by random mutation in batteries isn't really reflected in the functional biology of the animals using them.

Molecular phylogenetics can provide us with very useful information about how groups of animals arise. It cannot tell us what they will do when they get there. The strength of the biological definition of species is that it tells you something about what that animal will (or will not) do. If we get it right that is.

Joe Botting's picture

A couple of other considerations

Huge topic, but a few thoughts:
1. The standard biological definition of interbreeding populations falls flat when discussing things like bdelloid rotifers (they've done without males for several million years, so don't interbreed at all)... and as for bacteria, I stay well away. I've heard rumours of thirty different genders, and as far as I'm concerned, that's overdoing it.

2. There are now many species complexes that co-exist, are morphologically inseparable, and yet genetics has shown them to consist of several independent species. Examples include some rock wallabies in northern Australia, and the ubiquitous European sponge Halichondria panacea. We're now finding out that identifiable species of some UK leafhoppers also appear to be complexes, which from a conservation perspective should be considered separately... which is a pain for recorders!

3. On the other hand, there are some species with a huge range of morphotypes (see the Common Froghopper, Philaenus spumarius, for example) that simple field observations could easily classify as separate in the absence of breeding studies.

4. Palaeontology is a whole new kettle of coelacanths, because we're dealing with evolving lineages that couldn't possibly have interbred due to having lived in different times... also, we can have no idea of their genetics (with the exception of some extremely recent examples). Species of fossils are defined purely on morphology - a recognisable series of of characters with a range of variation observed in populations from a relatively short length of time. (Dimorphic males and females, and fragments of a whole organism, have introduced lots of synonyms that are slowly being weeded out.)
From a biological recorder's point of view, morphology is the only practical approach - we work from keys that tell you what the morphological differences are, and what each one means. Going into the nitty-gritty of what those "species" are involves a different sort of study entirely...

In other words, what a species is depends on who you're asking, and what they want to do with it! :)

the naturalist man's picture


Some excellent comments on this forum, glad to see it progressing.

I love your analogy Derelict, I to have described mitochondria as a cell's battery when teaching but never so eloquently or taken the analogy so far; I will in future.

You also make some good points Joe, I have to say if I need a microscope to see it I don't usually bother with it so didn't realise bacteria were so, erm, strange. I thought humans had, in some cases, a confusing enough time with two genders how would we cope with thirty!

Graham Banwell

Visit the iSpot Yorkshire forum for information on events, issues and news relating to 'God's own country'

RoyW's picture

Wherever someone draws the line!

If we accept the view that one species can gradually evolve into another (or diverge to form two or more) through natural mutation and natural selection, then can there really be a defined point when one species becomes another?
I would think that it is theoretically possible for a relatively small genetic change to make it impossible for two populations of a species to successfully produce offspring, even if they do meet (eg. if that change means that there is a 'mechanical' incompatibility, or the two populations are not sexually receptive at the same time). Conversely, a high degree of genetic divergence does not necessarily mean that viable offspring cannot be produced.

Even without any obvious genetic difference, two co-habiting animal populations can effectively be segregated by behaviour, and this can be very effective in preventing interbreeding. If neither group can recognise that the other is receptive for breeding, mating is very unlikely to occur even if they are entirely compatible genetically. For plants genetic compatibility is generally more important, but details like the time that flowers are produced may seperate species that are otherwise compatible.

I doubt that there will ever be a concensus on this topic - how can you agree where to cut a line that is continually, and gradually, changing?
The thing to remember is that all classifications that we give wildlife are entirely for our own benefit. Whether a population is defined as a sub-species or species is irrelevant to the organism concerned. 'Species' and 'subspecies' will continue to be defined by taxonomists studying the relevant groups of organisms, and these will continue to be revised at later dates by others working with the same groups.

One final point:
It may be important to recognise that even though two populations that can apparently produce sexually viable offspring, the fertility of these offspring may be limited in certain sex/species combinations, so the continued fertility of the 'hybrids' may be in question (eg. a study that I have read on Ruddy x White-headed Duck hybrids suggests that there may be a fertility problem for some of the crosses or back-crosses - I can't remember the exact details, and the need for further study was clearly stated). This suggests that although two populations may be able to interbreed the long term viability of the hybrids could potential be in question in some cases.