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The Problems of Defining a Species

A Voyage from Science to Metaphysics

What is a Species | Some different definitions | Species and Archetype | Some books dealing with the defnition of "Species"


What is a Species?

In the Linnean system and biology in general, a species is the smallest basic taxonomic unit used to define living organisms.   The definition I read when I was growing up was that of Ernst Mayr, the grand old man of modern Evolutionary Biology.  He said that two organisms belonged to the same species if they are able to interbreed and produce fertile offspring.  This of course ignores simple organisms (bacteria etc) that reproduce by fission (asexually).   As external link John S Wilkins points out Mayr's reply to that is that these organisms are not species.  Well if they're not species what are they?  As far as I understand things, every unique biological type (phenotype) is a species.  That includes the amoeba, even though amoebas reproduce asexually.

It's obvious that the issue of what constitutes a species in biology is a sticky one.  In external link A Taxonomy of Species Definitions - Or, Porphyry's Metatree, Dr Wilkins gives a detailed analysis of the problem of defining what constitutes a biological species.  He points out that the same term is used in a number of different contexts and to mean different things.  Fascinating but somewhat heavy going.   The following table sets out his list of definitions of "species" (for those who don't wish to plow through the article).  First of all, the species can be seen either as a theoretical concept used in modelling (simulation) and explanation, or as taxonomic units used to differentiate and classify living organisms (allowing for the fact that many definitions combine both approaches).  That gives us the 1st level distinction.  Each of these two categories can be divided into two, and each of those in turn into, giving the following "meta-taxonomic" arrangement:

1st level distinction 2nd level distinction 3rd level distinction definition examples in the literature units
Species Concepts   

species as a theoretical concept used in modelling and explanation

Horizontal SCs - HSC 
(living organisms)
Reproductive HSCs -RHSC able to interbreed and produce fertile offspring the traditional biological species concept (Mayr) genotype
Ecological HSCs - EHSC use same ecological resources ecolgical species concepts 
(Darnuth, Van Valen)
trophic level
Vertical SCs - VSC 
(fossil: species over evolutionary time)
Process VSCs - PVSC evolutionary speciation 
(one species gives rise to others)
Phylogentic species concepts - cladistic, phenetic and other phyletic reconstructions 
(Hennig, Wiley, Cracaft)
monophyletic group, clade, node, character set
Historical VSCs - HVSC preserved information 
(phylogenetic and palaeontological)
evolutionary species concepts 
fossil organism
Taxanomic Units  

(species as taxa, used to differentiate and classify)

Ontological TUs - OTU Metaphysical OTUs - MOTU species as a  metaphysical entity Plato's_transcendent_Ideas, the Aristotelian notion of Forms,_the_Naturphilosophen concept of Bauplans, and the recent proposals of Ghiselin and Hull for the understanding of species as spatiotemporally 
restricted individuals.
archetype, universal, class, set, subset, member
Causal OTUs - COTU causal relationship between members most species concepts -  
Aristotle's biological 
implementation of Forms  was more causally based on the ability to generate like forms through reproduction.
generally not specified (causal nexus?)
Epistemic TUs - ETU Morphological ETUs - METU  similarity of form Traditional Linnean system 
(also Operational taxonomic units - Sneath & Sokal)
family, genus, species, etc; 
Dynamic ETUs - DETU  similarity of behaviour Game theory (Maynard Smith) strategy, player/agent


Species and Archetypes

It seems to me that the MOTU group (metaphysical ontological taxonomic unit interpretation) in the above table can in turn be divided.  There is the dualistic Platonic theory of "Universals" (Ideal Forms) as transcendent unchanging eternal essences, as opposed to the holistic Aristotlean theory of universals (Forms) as the spirit or soul aspect of an object, the complementary aspect being the body.  In the Platonic theory although the body dies the form is eternal.  In the Aristotlean approach, when the body dies the spirit or form aspect does likewise.  The Creation Science definition of a "species", based on a literal reading of the Hebraic (Mosaic) book of Genesis, is a rather vague and fuzzy version of the Aristotlean position.

The dualistic interpretation of Species/Archetype can in turn be divided into the pure Platonic (the eidos as a transcendent eternal Truth, and the Theosophical/Anthroposophical/New Age interpretation of the species archetype as a sort of group consciousness or group soul that all the organisms of that species participate in.  The idea of an overshadowing group soul - supraphysical but not eternal and transcendent - goes back to the Neoplatonists, and especially to Iamblichus and Proclus who elaborated this concept in great deal.  In a sense modern Theosophy is the true heir to later Neoplatonism, even though Blavatsky was influenced more by Plato, Advaita Vedanta, and Tibetan Buddhism, and Leadbeater more by spiritualism.  I have given my thought son this matter under the heading Evolutionary Platonism.

Traditional (Greek and Medieval Scolastic) archetype species theories were based on what Aristotle called "privation" - specifying key characters by progressively removing qualities that refer to other entities.  The idea of privation - of the diminishing of the plenitude of the Absolute was an important theme in Plotinus's Neoplatonism.  With the late Neoplatonists we have "Porphyry's Tree", which is a comb diagram like a modern cladogram.

Hence whereas biological species come about through physical and biological factors, metaphysical species are determined by metaphysical or ontological factors; i.e. their position in terms of planes of existence, and proximity or distance from the Godhead; the emanationist approach.


Some books dealing with the defnition of "Species"

In Association with

Book Species : New Interdisciplinary Essays by Robert A. Wilson

Book Species Concepts and Phylogenetic Theory ed. by Quentin Wheeler and Rudolf Meier

Book Phylogeography : The History and Formation of Species by John C. Avise

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content by M.Alan Kazlev
most recent update 31 May 2001, links updated 18 January 2010

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