When i first read the voluminous, squishy, frustrating literature on 'what is a species' I thought the worst answer was 'a species is what a good taxonomist says it is'. After decades of working on difficult taxonomic groups (species flocks) using a range of different kinds of data (genetic, morphological, life history, ecological...etc.) AND dealing with the sharp end of the issue in running the ICZN and dealing with practical conservation decisions, I've come to think that definition is probably the best, or the 'least bad'.  What is quite clear is that: 1) much of taxon (e.g., species) definition is conceptual; 2) taxa are human constructs for our own understanding of the diversity of life, 3) there can be no one-size-fits-all definition, 4) part of the strength of the current taxonomic methodology is that taxon concepts can and should change in response to data and need for OUR OWN understanding of the clusters of diversity.

Note that nomenclature is a completely different kind of system of tying a name to a single physical standard, the type specimen. Nomenclature is obviously linked to taxonomy, and provides the handle for retrieving information on the taxonomic concept, but they operate separately. Information on the taxon concept is presented in the published literature, tagged by the name. This is a strength of the system; it allows the taxonomic methodology be a real science to flow and respond to changes in data and interpretation. Nomenclature is a bureaucratic book-keeping system - this is not a pejorative about nomenclature, as it is a very good system, with lots of nuance and sophistication, but it is not an interpretive science as is taxonomy. Ideally good taxonomists should have skills in the interpretive aspects, and be able to make a strong case for why their particular taxon concepts are the most useful, and also be able to provide robust links to the nomenclatural and bibliographic background.

I suppose that you know this paper:

Cornelissen, J. H. C. and Cornwell, W.K., 2014: The Tree of Life in ecosystems: evolution of plant effects on carbon and nutrient cycling. Journal of Ecology 102, 269–274.

That group is worth following. Good luck!

The original definition of a specie at least in mammals is based on a group of individuals who are interfertile. Nevertheless if we take the case   of mice , some strains cannot produced offsprings when crossed with distant strains of mice cannot produce offsprings with distant ...........The Linean is still interesting to know but sbsequent observations and discoveries have changed the lanscape. Studies of mitochondrial DNAs and of genomic DNAs hace contributed to modify somewhat the definitions. The study of regulatory elements (Promoters, enhancers, ..., MicroRNAs and other regulatory RNAs) are changing right now dramatically the landscape of biology and the definition of species

i don't think so. I believe this is a very hard question to solve. In animal, it might be easier since they have behavior limitation, however, in other group like plant the genetic boundary is not so clear.

There are several Problems with many definitions of species

1. the distinction of necessary conditions for different specimens to belong to a species from proxies (i.e. indications) that they do so. One reason for Problems is ...

... 2. the distinction between synchrony and diachrony

Most proxies we observe are synchronic indications, we however assume them to be proxies for diachronic phenomena (i.e. a species is a historic individual, or entity (a) it originates via speciation, leaving two species originating from a common ancestral species in most definitions - in some others only a single species splits from another, and importantly (b) we assume non-reversability of the phenomenon so the two lineages will stay separated in the future - only the latter differentiates species from subspecific entities, since populations may likewise be separated from each other for a certain time )

3. the Problem of generality (in your Definition you have stated sexual and asexual species to be included, but nevertheless you include sexual reproduction in the Definition, which then is applicable only for a subpart of what you think a species might be.

There are a number of other Problems, or you may use different terminlogy to speak about These Problems. But many of what you might call "definite" definitions fall into at least one of These pits.

Species may be defined as given below:

A group of organism in a given area or environment that can produce fertile progeny naturally by random mating within themselves is species.

I think what has not been resolved and may never be solved. 

Kind regard

Species is a group of organisms with a shared, closed gene pool.

Wow, this is not just one question, but a lot of them!!!

The concept 'species' is -in most languages- predating the biological definitions of species. That is why it can be used by chemestrians or whoever...This is the origing of the word 'spice'

I love this question because it is as much a philosophical one as it is a scientific one. Generally, I think it is impossible to find a definition that works for all living entities from the smallest bacteria up to the largest baleen whales in our oceans. Some of the issues related to this where covered by the commenters above. If you need (or at least think you need) operational definitions for subgroups of the entirety of living beings, you should always remember this. 

Science 21 September 2007:  Vol. 317 no. 5845 pp. 1753-1756

DOI: 10.1126/science.1142490

Biology Direct 2009, 4:34 doi:10.1186/1745-6150-4-34

Species definition is having lots of complexity and contradictions and have school of concepts. 

A group of individuals which is more closely related in comparison with other by any taxonomic measure are species. this is most simpler definitioni thought.  now you can used any concept which ever is suitable for you. and use for species delimitation.

In the past your question could have been easily answered but today with more and more detailed methods of  investigation the question of what is a species is a bit confused. This issue is based on what kind of "approach" you follow: if you are a "splitter" you will find countless number of species and subspecies if you are a "lumper" you will find a smaller number of species.   

I think there is now enough information to structure a fairly comprehensive definition of species. the problem is not getting a satisfactory definition theoretically, if not make that definition has applicability in delineating specific species. And at this point you need to understand the synchronic-diachronic nature of the entity we call species, coupled with the human scale of understanding of the process. Similarly, it requires accepting that as any definition is a simplification of reality; and particularly here a complex and constantly evolving reality. The definition necessarily have to include variations (expressed between  kingdoms,  domains of natural classification, more variants generated by asexual and hybridization, especially in plant). In practical terms, the only option to delimit species is differentiate by a group of the broadest possible characters (morphological, molecular, ecological, physiological, etc) in a phylogenetic framework. Only in cases of species of short life cycle can be carried out tests that establish reproductive barriers in this field (and yet remember the syngameons)

Hi everybody

Species is one of several ranks in the hierarchical system of scientific classification, called taxonomic ranks.

Even though it is not disputed that species is a taxonomic rank, this does not prevent disagreements when particular species are discussed. In the case of the Baltimore Oriole (Icterus galbula) and Bullock's Oriole (I. bullockii), two similar species of birds have sometimes in the past been considered to be one single species, the Northern Oriole (I. galbula). Currently, biologists agree that these are actually two separate species,[3] but in the past this was not the case.[4]

Disagreements and confusion happen over just what the best criteria are for identifying new species. In 1942, Ernst Mayr wrote that, because biologists have different ways of identifying species, they actually have different species concepts.[5] Mayr listed five different species concepts, and since then many more have been added.[6][7][8] The question of which species concept is best has occupied many printed pages and many hours of discussion.[9]

The debates are philosophical in nature. One common disagreement is over whether a species should be defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species an organism belongs to. For many species, all of the individuals that fit the defining criteria also make up a single evolving unit, but it might not be known whether that is the case. These two different ways of thinking about species, as a category or as an evolving population, may be quite different from each other (http://en.wikipedia.org/wiki/Species_problem)

Regards

We have to distinguish between species concept, evolving units in nature and the nomenclature we should adopt for every kind of "species like object". In nature the species are groups of populations of (normally amphigonic) that cover a certain distribution area, exhibit a defined population structure (that may change from species to another), share a common origin through a speciation event and may related in multiple way with neighbouring entities ( semispecies (still having a hybridization zone or strip), still allopatric, already more differentiated or sympatric, splitting in more subspecies (i.e. showing more population groups, etc. Thus, the concepts of Mayr are in my opinion still valid, and also in molecular studies we have Always to cope with features of the populations, their variability (also morphological), and the population structure in the entire distribution area (!), ring species, etc. Both species characteristics as well population structure features should investigated at the same time.

Different fields of study (biology, chemistry, physics, etc...) recycle the same words to different ends. Sometimes it even happens between specialties (botany versus entomology). This is just the way things work. Many words in the dictionary have multiple meanings. This is no different.

How you define species will depend, in practice, on the nature of your study and what you are interested in. As a zoologist, I don't usually have a problem sorting organisms collected or identified at the same time and in the same place into "species" that would be recognised by several definitions. Occasionally, two or more species may be separable only on molecular evidence that suggests that there are more than one distinct sets of genetic constitutions. Also, selfing or asexually reproducing forms require a pragmatic estimate of the degree and consistency of differences between them.

The problems become more acute when you compare faunas separated by great distances relative to dispersal powers. Diachronic issues (time since separation) and degrees of difference come into play, even in conventionally sexually reproducing organisms. In the absence of good cladograms and knowledge of genetic make-up, the distinction between species and subspecies when your populations are patchy rather than continuous becomes blurred, to put it mildly.

In a sense, yes, the definition of species has been solved. If it hadn't been solved we wouldn't be able to describe new species. But the definitions vary with the different types of organisms that are described. I am currently describing a new species of fungus purely based on morphological characteristics. In this group of fungi, that's the way we do it. It may not work for other organisms, but it works for this group. And, yes, it gets more technical then what I describe here (especially for fungi that can take many different forms) but we have terminology that has evolved over the years to deal with the issues and, somehow, we muddle through to get the job done.

Thanks for every body I making a new definition in base of all of your are informing contributions, but maybe this future concept is for didactic purposes because is difficult the aplication of this concept to the practical part.

Could any one please show mw some information about this point of synchrony and diachrony?

Please dont do the answers like you are in Yahoo anwers copy and paste, I asking answers from your experience and some helpfull information that you can give to the comunity. Thanks 

Camelus dromedarius and Camelus bactrianus are two different species but they have same number of chromosomes, can interbreed and their hybrids are fertile. In this case it has more or less accepted still to considered them two different species but if the the species are very similar this certainty is in doubt. To further complicate the issue what about "cryptic species" species that look exactly the same morphologically but genetically are very different? Or what about a species that has a strong well defined and seemingly permanent difference in behavior between two populations? The more technical scientific tools will be available the more will be possible to split a "species" into "a different species". Although my answer about "splitters" and "lumpers" has been down voted it is undoubtedly that in the end such approach, as unpalatable to some that can be, has to be considered or at least a reasonable limit to the level of "species splitting" be agreed.     

The word "species" from Latin means essentially "apparition", "form" or "sight". In this sense the question "Is the definition of a species already solved?" seems to contain a somewhat wrong idea. In biology (as also in other fields) a "species" is not something that can be "solved" via the mere observation of nature. It is something that needs to be defined by us humans. And 'definitions' are always something that are subject to debate and revisions in the advent of new developments and newly arising problems. Definitions may also represent a useful working basis in one discipline, but may actually represent a hindrance in another discipline. The early taxonomist described "species" mainly according to visual differences. That is why the camel and dromedar have been named two different species (see example by Maurizio). However, I would not at all be surprised if these will be lumped into one 'taxonomic' species in the future, as many taxonomists already argue it should be done on the basis of genetical/breeding criteria. In fact the naming of the European cattle has just recently changed from Bos taurus to Bos primigenius taurus (you may read up on this on the net). In naming species there is tradition (starting with Linné describing and separating different morphological forms), and new scientists who look at the gene code or proteins. A cow does not evolve quickly into something quite different. So "Bos primigenius taurus" will work quite well for most scientists describing that thing. However, if you work with the thousands of changeable "species" of bacteria found in the cows' guts you may find that genetics is much more useful than binary taxonomy in order to describe "species". You may look up ICZN http://iczn.org/content/about-iczn  

In a recent paper on species concepts in apomicts, I argued (following others!) that species come in kinds. Since the evolutionary pathways in different groups are so different, the term 'species' means something different in these groups! A bacteria species is something else than a bovid species. A species in Plantago might mean something completely different than a Rubus species....

I think biologists have a fair understanding of what a species means in particular groups. As John Wilkins (https://www.researchgate.net/profile/John_Wilkins4) stated, in somewhat different words, it's the task of the biologist to find out what species are within all these groups.

When i first read the voluminous, squishy, frustrating literature on 'what is a species' I thought the worst answer was 'a species is what a good taxonomist says it is'. After decades of working on difficult taxonomic groups (species flocks) using a range of different kinds of data (genetic, morphological, life history, ecological...etc.) AND dealing with the sharp end of the issue in running the ICZN and dealing with practical conservation decisions, I've come to think that definition is probably the best, or the 'least bad'.  What is quite clear is that: 1) much of taxon (e.g., species) definition is conceptual; 2) taxa are human constructs for our own understanding of the diversity of life, 3) there can be no one-size-fits-all definition, 4) part of the strength of the current taxonomic methodology is that taxon concepts can and should change in response to data and need for OUR OWN understanding of the clusters of diversity.

Note that nomenclature is a completely different kind of system of tying a name to a single physical standard, the type specimen. Nomenclature is obviously linked to taxonomy, and provides the handle for retrieving information on the taxonomic concept, but they operate separately. Information on the taxon concept is presented in the published literature, tagged by the name. This is a strength of the system; it allows the taxonomic methodology be a real science to flow and respond to changes in data and interpretation. Nomenclature is a bureaucratic book-keeping system - this is not a pejorative about nomenclature, as it is a very good system, with lots of nuance and sophistication, but it is not an interpretive science as is taxonomy. Ideally good taxonomists should have skills in the interpretive aspects, and be able to make a strong case for why their particular taxon concepts are the most useful, and also be able to provide robust links to the nomenclatural and bibliographic background.

Definition of  species is never ending.....introduction of new methodolgy..new definition !!!

But Santhosh, there is a good, evidence- and interpretation-based reason for that! New methodologies can be either 1) new data, which in turn is either more data of a kind already in hand, or new kinds of data; and/or 2) new ways of interpreting data. Its important to be specific about these things. Personally, i think all papers describing new taxa should be required (by their editors) to explicitly discuss their taxon concepts. To some degree this happens by listing the methods for data collection and analysis...but often that final step is missing of saying exactly why the author things a new taxon is justified by the data and interpretation. If that were all a bit more transparent it would not seem 'never ending'; it would be clear what the specific details are in interpretation of taxon concepts and those could be addressed - or not, and just seen as alternative interpretations of the same data.

By the way, this is a strong argument for making data open access. 

In ecology every difference matters. Every individual at every location at each instant in time. Mostly we can't deal with this level of detail, so we back up a little and we get populations, or groups of individuals. Somehow a group containing a cormorant and an oak tree doesn't seem useful. We name them differently so that we can gain a better picture of how a group of individuals interacts both with each other and with other groups. So one might ask, does creating another grouping of individuals improve our understanding of the world we live in?

A more cynical definition might be "whatever a taxonomist can get through the peer review process of a journal." It also must withstand the scrutiny of other scientists using techniques that are not available to you.

It really comes down to a question that you are best able to answer: What definition of species helps you better understand the system that you are studying and helps you communicate with other scientists?

In my opinion, for what it is worth, species definitions tend to focus on one or more sptatio-temporally universal criteria (e.g. reproductive isolation, mate recognition etc) which is, ironically, anti-evolutionary since species are mutable.

Another way of looking at species in the practical world is that they are diagnosable entities where a species identification is a matter of providing a diagnosis that is spatio-temporally bounded - that for a given time and place particular characteristics are sufficient to indicate a discontinuity. This is a bit like how diseases are recognized - as a diagnosis of characteristics rather than an innate essence (such as a disease organism).

John Grehan

There is no solving the definition of species as a universal concept. One problem is that many species arise from existing species. The end points are different species, the intermediates are ????? Other problems include that some fungi have 17 different sexes. The sexual stage of some insects and fungi have never been found. What is sex for a virus? What do I do if the organisms will only fight, but artificial insemination shows that they are genetically compatible? What defines solved for this problem? How do I know that I have not solved the problem? Are you hoping for a universal answer, or something more limited?

Can you define something that is missing from the existing answers where inclusion of that missing element will solve the issue? Please use a concrete example.

The systems of taxonomy were established at a time when sophisticated lab facilities linked with efficient software were not available. Traditional taxonomy is useful for simplifying a complex world, but their rigid structures might give rise to many questions. I remember, when working in the Knysna forests in South Africa, that several species of the Celastraceae family, including the genus Maytenus and Pterocelastrus, had visibly different traits (e.g. serrated or rotund leaves) in individuals of the same species. The interest in this particular topic shows that there may be a need to develop entirely new concepts (involving different disciplines), without discarding the existing taxonomies

I support the points of view of e.g. Ellinor and Timothy, the 'species' is just a concept, an abstraction in our mind and there is no reason why this should be 'real'. However it is true that definitions are useful for our own work and understanding; unfortunately only those concepts with an obious and immediate 'practical' (i.e., economic or sanitary) implication reach a neat definition in a short term. Even when the concept of species has many practical implications, it is obvious that fron the applied point of view, in general, no strict definition is required.

In response to the comment  by Zoltán Czier I would agree that the diagnosis does not 'solve' the definition as it is only a definition of how individual species are delineated. As for the concept in general, there is a definition that covers all circumstances - that a species is just a link in a chain of differentiation. Where one chooses to place that link - well that is where the diagnosis criterion comes in. I'll duck now.

John Grehan