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Yet Another FAQ: phylum, order, family



Dear DMLers,

While I await the discussion of the ``how similar are related dinosaur
taxa'' thread to reach a conclusion (yes, I know, I'm deliberately
being optimistic!), I have taken an initial stab at answering one of
the other questions:

        Is there any real signifance to terms like: Phylum, Order,
        Suborder, Family, Genus and Species?

My answer can be found at
http://www.miketaylor.org.uk/dino/faq/s-class/levels/index.html
and is reproduced below because my experience is that while people
don't always follow links from email messages, they can't resist
pointing out errors in messages posted to the DML!  :-)

All feedback to <dinofaq@egroups.com> please, unless you think it will
be of general interest to the DML.

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor | <mike@miketaylor.org.uk> | www.miketaylor.org.uk
)_v__/\  "The secret of success is sincerity.  Once you can fake that
         you've got it made" -- Jean Giraudoux.


--

What do terms like phylum, order, family and clade mean? 

                      10th August 2001 

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QUESTION

Is there any real signifance to terms like: 

      Phylum 
      Order 
      Suborder 
      Family 
      Genus 
      Species 


ANSWER

These terms indicate groupings of living organisms at various levels,
with species being the most specific, and order the most inclusive.
However, these terms have fallen on hard times recently, and many
palaeontologists prefer not to use them. 

To understand why, we need to take a look at the history of biological
classification. 

The science of classification really started with Carl Linnaeus
(1707-1778), a medical doctor from Sweden who is also known by the
variant names Carl von Linné and Carolus Linnaeus. At the age of 28,
he published the first edition of Systema Naturae, his ground-breaking
classification of animals and plants - initially a single thin volume,
although subsequent editions were to mushroom in size. 

Before Linnaeus, classification was a haphazard business. For
example, the common wild briar rose was referred to by different
botanists as Rosa sylvestris inodora seu canina and as Rosa sylvestris
alba cum rubore, folio glabro! There was little organisation of names:
organisms were assigned to species, and species grouped into genera
(for example, lions and tigers were recognised as separate species,
but joined in a single genus, Panthera), but there were no higher level
groupings. 

Systema Naturae introduced two important innovations. The first of
these was the so-called ``binomial system'', whereby each species is
referred to by a two-word name, consisting of the generic name
followed by the specific name - as in Panthera leo, or indeed
Tyrannosaurus rex. 

The second and more far-reaching innovation was the introduction of
higher-level groupings than the genus. In Linnaeus's original system,
genuses (such as Panthera and Canis, the great cats and the dogs)
were joined in orders (such as Carnivora, the carnivorous mammals);
orders (such as Carnivora and Chiroptera, the bats) were joined into
classes (such as Mammalia, the mammals); and classes (such as
Mammalia and Reptilia, the reptiles) into kingdoms (such Animalia,
which I shall not insult your intelligence by translating!) The key insight
here was that life can be arranged into a branching tree-like
hierarchy. The level-names order, class, etc. were known as ranks. 

This simple five-level system (kingdom > order > class > genus >
species) was soon augmented by intermediate levels such as the
family, intermediate in level between the genus and order - for
example, the family Felidae consists of all cats, whether great
(Panthera) or small (Felis). As people found the need for more
precision in locating how inclusive a proposed grouping was, along
came superfamilies, subfamilies, tribes, superorders, infraorders,
hyperfamilies, and ... well, the list goes on. Among these additional
ranks is the phylum, intermediate in level between the kingdom and
order. For example, the phylum Vertebrata, the vertebrates, is a part of
the animal kingdom. 

It doesn't take too much of this sort of thing before the system
becomes, if not actually unworkable, then at least obscene. For
example, in a Dinosaur Mailing List message, George Olshevsky has
listed a hierarchy of thirteen ranks just between order and family! 

      Order 
      Parvorder 
      Nanorder 
      Hyporder 
      Minorder 
      Suborder 
      Infraorder 
      Gigafamily 
      Megafamily 
      Grandfamily 
      Hyperfamily 
      Superfamily 
      Family 

The reaction against this plethora of ranks and the confusion it
engenders (Quick! Which is more inclusive? A grandfamily or a
hyperfamily?) has been a move to abandon named ranks altogether,
simply naming nodes in the tree and making no judgement about how
high or low level they are - and it is a matter of judgement rather than
of fact; one man's parvorder is another man's nonorder. This minimal
approach is particularly popular in the cladistics community, perhaps
in part because the trees generated by cladistic methods have far too
many nodes, and change far too often, to be amenable to labelling
with ranks. 

Proponents of this ``cladistic'' approach to taxonomy, then, would like
to abolish all ranks above the level of species, which they argue is the
only rank with an objective definition (but see ``When is a new
dinosaur erected as a new species or genus?'' ). Under this scheme,
even the rank of genus would be abolished. And therefore, of course,
so would Linnaeus's binomial: every species would have to be given a
new, unique name. Not surprisingly, these radical proposals have not
met with unanimous enthusiasm - see ``Is the PhyloCode a good
thing?'' for more discussion of pros and cons of this approach. 

It remains to be seen how nomenclatural practices will change, but
what seems to be emerging by default is an approach in which some
of the more ``important'' nodes in the large, deep trees generated by
cladistic methods get quietly labelled as families, orders, etc.
according to a blend of historical precedent and what seems to be
useful at the time. For example, most historical dinosaur families seem
to have survived the transition into the cladistic age, and many new
dinosaurs which do not fit into existing families have new families
named after them - or at least, new groupings whose names end with
``-idae'', which is at least very suggestive! 

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