Polyphyletic Taxa

polyphyletic taxon

graphic © Darren Abbey


 
The Polyphyletic taxon is a group composed of a number of organisms which might bear some similarities, but does not include the most recent common ancestor of all the member organisms (usually because that ancestor lacks some or all of the characteristics of the group).  The taxon shares
derived characters which originated several times by convergence.

Illustration: it used to be thought (in Victorian times) that all pachydermous (large stocky hairless thick-skinned herbivores) mammals - e.g. rhinos, hippos, and elephants - were descended from a single large ancestor.  It is now known that each of these animals evolved from a seperate small ancestor, and the common ancestor of all of them was small and slightly built, with presumably thin skin and fur

Another Illustration: Some cladists (e.g. Lovtrup and Gardiner) have argued that because warm-blooded birds and mammals share a number of metabolic and anatomical features in common they must have descended from a single warm-blooded Most Recent Common Ancestor and thus constuitute the clade Haemothermia.  It is now known that endothermy (warm-bloddedness) evolved independently in each group.

Polyphyletic taxa are considered invalid or unnatural groupings, and are not accepted in either the Linnean/Evolutionary or the Cladistics taxonomies.  Metaphysically-speaking they are still valid, in that the members of the taxon, while not phylogenetically related (i.e. they do not share a Common Ancestor with the same chracteristics) nevertheless resonate to the same morphic field.  The Polyphylertic taxon is therefore morphogentically valid, but not biologically or scientifically valid.

Examples:  Pachyderma, Haemothermia, Algae, Arthropoda, Vermes (worms)..
 
 

graphic (above) from UCMP
 
 

Polyphyletic taxon
Paraphyletic taxon
Monophyletic taxon
 
 
 
 
 
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page history

page uploaded 13 November 1998
modified 28 November
and again 18 December