Palaeos Palaeos The Linnaean System
Systematics Species


The Problems of Defining a Species

A Voyage from Science to Metaphysics (and vice versa)

What is a Species?

In the Linnaean 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 John S Wilkins points out (need current url) 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 A Taxonomy of Species Definitions - Or, Porphyry's Metatree (need current url), 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 modeling (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 modeling 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 ecological 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)
Phylogenetic 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
Taxonomic 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 Linnaean system 
(also Operational taxonomic units - Sneath & Sokal)
family, genus, species, etc; 
Dynamic ETUs - DETU  similarity of behavior 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 Aristotelian 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 Aristotelian 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 Aristotelian 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 thoughts on this matter under the heading Evolutionary Platonism.

Traditional (Greek and Medieval Scholastic) 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 definition of "species"

In Association with

Species : New Interdisciplinary Essays by Robert A. Wilson

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

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

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