Species contra subspecies
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The definition of species is still under debate within scientific circles. This is what Tim Gamble, Chris Durden, and Ron Gatrelle have written.
I hope this helps.
Here is a quick review:

The morphological species definition:
A species has several features that are not shared by other groups. This is the oldest definition which is solely based on "visible" features. The main problem with this definition is the question how different to groups have to be before they can be considered as separate species. Also creating problems is the fact that there often can be considerable differences in appearance among members of the same species, just look at the variation among Homo sapiens.

The biological species concept:
A species is a group of interbreeding individuals that are reproductively isolated (in place, time or behavior) from other groups. All members of the group share a common ancestor. The species is the fundamental unit of evolution, while higher taxa (families, orders, etc.) are artificial groups reflecting a possible evolutionary relationship. Thus, species that may interbreed in the unusual circumstances of captivity (such as the large cats) do not qualify as belonging to the same species since they normally would not interbreed in the wild.

The Genotypic Cluster definition:
A species is a common genetic pool that are identified by their genetic differences from other groups. This is the latest, and probably most accurate, definition which is based on differences in DNA sequences. The problem is that this is also the least practical definition. Gene sequencing is complicated, costly and time consuming. It cannot be done in the field, where biologists must still relay on morphological characteristics for quick identifications.

-- Tim Gamble Charlotte, NC


Mayr’s biological species concept – ‘groups of actually (or potentially) interbreeding natural populations which are reproductively isolated from other such groups’. (defining species in relation to other species)
Paterson’s recognition concept of species - ‘that most inclusive population of individual biparental organisms which share a common fertilization system ’. (defining species as having a specific-mate recognition system; and not in relation to other species)
These definitions can only relate to sexual species, and also are not immediately helpful to taxonomists.
Taxonomy only DESCRIBES species, and does not attempt to DEFINE species.
Species concepts like Patterson’s and Mayr’s, are attempts to DEFINE species (in nature).
So I guess it is difficult to say what a DEFINITION of species is in taxonomic terms.
Should we try to group together taxa based purely on morphology, or should we try to incorporate as much biological data into revisions and classifications?
Could separating a species from a genus be a valid decision if based purely on biological differences (if the difference is radical). (I dont have a specific example, but am curious if anyone knows of such a case, or can comment in general)
Is species biology an unrealiable character for taxonomy, because the 'whole' biology cannot be observed. And, if incorporating biological data into taxonomic work, what should one be wary of?
I am really just starting out as a taxonomist and am asking these questions. Does anyone have any advise for a beginner who wants to get started on the right path.
Justin Bartlett


This a meaningful point of view.
Anyway, the problem of the relationship between 'theorical species' (definition) and 'taxonomical species' (identification) comes only after another bad problem: the biological concept of species does not easily apply to all organisms (think to not-sexuate organisms, or to species generated by the hybridisation of other species - more common in plants than in animals).
So it seems almost impossible giving a single DEFINITION of species fitting for all organisms.
Coming back to the identification of species, it is obviously impossible to group , as a rule, populations in species by studying their capability of interfecundate...the most easy way to study the single specimens or populations is the morphological or molecular approach. Let's remember that, nowadays, the classification of organisms should be based on phylogenetic trees. Consequently, wheter a single species belong to a genus or to another is not depending from its characters (morphological or biological), but from its position among its relatives. Of course, this can lead to strange situations... for example, now we know that birds are not a class, or at least, they are not at the same rank of the class Reptiles. They are just a group emerged from the Reptiles... very different from all other reptiles, but included in the radiation of them and having the same rank of crocodiles and turtles. So, we have not to look at the single taxon, but at his position in the phylogeny of his group. Obviously, when phylogenetic data are absent, or poor, we are forced to take provisionary decisions based on morphological or biological traits.
Hope this help, and hope my english is clear enough.

Marco Uliana


All higher categories (family, genus, etc.) are artificial and man-made and you may never get a finite definition. Obviously, even Orders (e.g., Orthopteroidea) have had drastic shifts in definition recently.
On the other hand, "specie" is defined in the dictionary as "Minted money; coin", although in Latin it means "kind". The word you really wanted is "species" which is the same in singular or plural. Although various definitions have been proposed (especially in biology), they usually include " a category below genus, which consists of organisms capable of interbreeding". Although there are many definitions, and hundreds of papers have been written on "what is a species", the main belief is that they exist in nature (in possible contrast to the man-made higher categories). The only problem (and that is a taxonomist's job, and why we write "revisions") is how to distinguish them from each other! Bob Woodruff, Emeritus Taxonomist, Florida State Collection of Arthropods.


For those who are interested in such topics I can recommend two excellent papers:
Mayden R. L. 1997. A hierarchy of species concepts: the denouement in the saga of the species problem. In: Species: the units of biodiversity: [pp 381–424.] (M. F. Claridge, H. A. Dawah and M. R. Wilson, Eds.). Chapman & Hall, Melbourne.
Ereshefsky, M. 1998. Species Pluralism and Ant-Realism. Philosophy of Science 65: 103–120.
There are references to many other studies that discuss the species concept in many different ways and contexts. If anyone is interested in the Mayden paper, I have it here as a PDF. Email me privately (not via the list) and I can send you a copy.

Best wishes,
Christopher Majka

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Last modified on Tuesday, 12 November 2019