Download Some Common Taxonomic Errors

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Some Common Taxonomic Errors
by R. E. BLACKWELDER
Department of Zoology
Southern Illinois University
Carbondale, Illinois 62901
Textbooks of general biology and zoology
are more numerous today than ever before. Many of the recent ones are heavily
or completely slanted toward biochemical,
genetical, and developmental aspects, but
many still retain at least an introduction
to the animal kingdom. Some are still
largely devoted to comparisons of the
groups of organisms.
Among all these, there stand out a small
number in which the taxonomic aspects
are accurately treated. Glaring errors in
the use of taxonomic terms, devices, or
ideas are found in others, being repeated
in the same form from text to text.
Only occasionally do taxonomists find
the time to criticize these practices. In
teaching taxonomy, it is found to be difficult to eradicate the false usages to be
found in the usual beginning text. It is
hardly to be hoped that the unsatisfactory
practices will be stopped if published
criticism is not available to the writers
of such texts, who are usually not experienced taxonomists.
A series of common errors is given below, with explanations of what is wrong
and how it may be put correctly.
(1) Most textbooks contain such a diagram or list as this:
Kingdom
Animalia
Phylum
Chordata
Class
Mammalia
Order
Primates
Family
Genus
Species
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Hominidae
Homo
sapiens
This is called the classification of man.
Although such use of the word classification might perhaps be condoned if it were
appropriate or meaningful, it is in fact
neither. It does not classify or group; it
merely lists. It is in fact a hierarchy of
groups to which this species has been
assigned.
(2) A few texts carry this diagram in correct form, in which the last item is written:
Homo sapiens
Species
This is the only correct form, because
sapiens is not the name of anything; it
is merely a Latin word. It is true that
many persons, including the authors of
the latest code of nomenclature, have
called this word the specific name, but
all are in agreement that it is not the
name of the species. The name of the
species is binominal, a two-part name,
always including the generic name as its
first part. No animal (individual) "belongs to" sapiens, but it is reasonable to
say that all modem humans belong to
Homo sapiens.
There is only one situation in which it
is ever possible to refer to a species by
the second part of its name alone: If two
species are referred to many times in a
work, and they are referred to first as
Fells leo and Fells tigris, they may be
shortened to leo and tigris thereafter with
the understanding that this is merely a
matter of convenience, that the Fells is
understood in every case. Even this is
technically in error, for not even the lessabbreviated form F. leo is recommended
by any rules. (There have been zoologists
who insist that their students must always
use the name of the author after the name
of the species, as part of the name. This
is often appropriate but never required by
rule. It should be done only when this
reference to the original publication would
be helpful to the reader. In almost all
non-technical literature, it is neither necessary nor helpful and may be omitted.)
Vol. 46, No. 2, February, 1968
(c:l) 1968, General Biological Supply House, Inc.
(3) In presenting this diagram, some texts
state that each phylum is divided into
subphyla, each class into orders, each
order into families, and each genus into
species. This is not unreasonable at the
level of phylum and class, but at the
levels or genus and family, at least, the
more inclusive groups are not normally
subdivided to produce the lower ones but
are built up by combining the lower ones
—by grouping them. Genera are made by
combining species, and families are made
by combining genera. There are situations
that produce exceptions, but the general
statement at these lower levels must be
that each is formed by combining ones
from the level below.
There is a large class of exceptions
formed by the sublevels. Subfamilies are
usually thought of as formed by dividing
a family. The family, however, is hardly
ever formed by dividing an order and
never by dividing a superfamily.
(4) It has become common in newspapers
to put a nickname in parenthesis between
the parts of a person's name, as: John
("Pretty-boy") Doe. Perhaps it is this
custom that has led to a resurgence of an
occasional old practice of putting a generic synonym in parenthesis in the middle
of the dual name of a species, as: Clonorchis (Opisthorchis) sinensis. Although
the rules of nomenclature have not specifically forbidden this practice, they have in
effect done so. For fifty years the method
of citing the subgenus, when desired, is
by use of parentheses between the two
parts of the name of the species. Thus,
this device is pre-empted, and a name in
this middle and parenthetical position is
presumed to be a subgenus, not a synonym.
The synonym can be cited after the
full correct name, thus:
Clonorchis sinensis (Opisthorchis
sinensis
(In any case it is assumed that Clonorchis
is the genus accepted by the writer.) It
will be much clearer to a student if the
name is written in the form believed to
Vol. 46, No. 2, February, 1968
be correct, and a note added that it has
sometimes been placed under a specified
other generic name.
(5) When a species has been given two
or more names, there enters the problem
of synonymy. It is sometimes stated that
the rejected names are synonyms. In fact,
the names given to any species are all
synonyms. From among the synonyms,
one is determined according to special
rules to be the correct name. The rejected
synonyms should be called just that.
(6) The International Commission on
Zoological Nomenclature has at various
times had power to interpret the rules, to
suspend them individually, and to propose
new ones. It has never had the power to
lay down rules or to enforce them. The
power to make rules or revise them resides
solely in the International Congress of
Zoology, although the Commission's recommendations are usually accepted by
that body. No one can enforce the rules,
which take effect only through weight of
public opinion among taxonomists. These
matters are frequently misstated in general texts.
(7) It is a very common thing in textbooks at all levels to see a figure of
"Hydra" or "Lumbricus." It is clear that
Hydra, a genus, can have no tentacles
and that Lumbricus, another genus, can
have no clitellum; it is impossible to draw
a genus. It is equally impossible to draw
a species, Lumbricus terrestris. There is
no question that the authors of these
books understand this; they are merely
taking a short-cut to keep from repeating
"Diagram of a specimen of Lumbricus
terrestris." When the generic name stands
alone, the full form would be "Diagram of
a specimen of some unspecified species of
Lumbricus."
No real harm is done by this shortened
form, but it has been found that it is not
self-explanatory to students. A note of
explanation would be worthwhile.
(8) A widespread misuse of terms has
given rise to much confusion in speaking
about groups in a hierarchy of levels. The
common expression "It is in quite a different category. . ." has been carried over
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into taxonomy so that groups are called
categories. For example, "Taxonomic categories are the outcome of changing concepts of classification. . ." Many things
in taxonomy do change, but the categories
are not among them. It is the groups which
change. Categories are merely the levels
in the hierarchy, such as the order category (level). The groups, at this level
are called orders. The level (or category)
doesn't change, but the groups (the actual
orders) may be moved to other categories
or levels. The groups are now often called
taxa. They should never be confused with
the categories of the hierarchy.
(9) Most texts refer to zoological nomenclature as a system of binominal names,
and of course the Linnaean system is binominal. But zoological names are given
to groups placed in at least twenty levels
in the hierarchy; names of groups (taxa)
at only one of these twenty levels are binominal. At one level the names are trinominal, and at all other levels the names
are uninominal. The Linnaean binominal
system consists merely of the names of
species, so zoological nomenclature merely
includes binominal names at a particular
level.
(10) In addition to the above strictly
taxonomic usages, there are some things
that involve features of the animals that
are used in taxonomy. Because it is obviously impossible to deal with all kinds of
organisms, many of the texts give most
of their attention to "typical representatives" or "representative types." These are
unfortunate expressions, because the animals usually are not typical and do not
represent the entire group. They are examples, so why imply more.
For example, in the Nematoda the example cited is usually Ascaris lumbricoides, the common laboratory nematode.
It happens to be a hundred times longer
than most nematodes, a thousand times
longer than many. No parasitic nematode
can be representative of the thousands of
soil-dwelling ones. Helix pomatia is often
cited in the Gastropoda; yet as a pulmonate land snail it can represent only the
one aberrant group among the immense
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diversity of this second-largest class of
animals. Hydra oligactis is almost universally cited in the Coelenterata; yet
even in its class, this species is highly unusual in being solitary, not polymorphic,
and without the usual tetramerous arrangement.
It is, of course, necessary to limit descriptions to a few, but it is highly misleading to imply that those chosen are
representative or typical. It would be better to state, if true, that they show the
fundamental features of their group.
(11) Most recent texts make some use of
the ideas of symmetry: radial, bilateral,
and sometimes biradial. None of these
books succeeds in making any useful point
of these ideas. The reason is simply that
whatever there is that is basic about these
body arrangements is so obscured by diversity, that no story is apparent. A variety of simple definitions are given such
as: radial symmetry—several parts arranged around a central point; biradial
symmetry—several parts arranged opposite each other; bilateral symmetry—two
parts arranged opposite each other. A
nautilus shell has several parts arranged
around a central point, but it is not radially symmetrical. Most "biradial" ctenophores (the only examples) have some
organs clearly bilateral and some clearly
radial. Most diatoms have two parts
(valves) arranged opposite to each other
without being bilateral in this plane, and
to call both a crayfish and a cup-shaped
bryozoan bilateral makes a mockery of
this word as descriptive of body arrangement. The solution is to abandon this useless idea until it is usefully restated.
(12) The adjectives "true" and "false"
are used in naming organs of several types.
When an animal with a solid body, like a
flatworm perhaps, evolved a body cavity
to surround the viscera, could this new
invention, a superior development, be
called a false coelom or false body cavity?
Whatever term we wish to use for it, it
was a true one, a real one. It is unfortunate that the term chosen was pseudocoel,
by persons familiar with a still-moreadvanced body cavity already known as a
Vol. 46, No. 2, February, 1968
coelom. The pseudocoel is not a false coelom, nor a false body cavity; it is a true
pseudocoel. The words true and false are
completely inappropriate in all such terms.
(13) There is probably no end to the
general statements that are made in spite
of the fact that exceptions are known to
exist. This is not always bad, if no conclusions are drawn. In the classification
of animals, there is one subdivision of the
coelomate Metazoa made on the basis of a
feature that once appeared clear-cut. This
is the method of formation of the coelom,
giving rise to the groups Enterocoela and
Schizocoela, depending on whether the
coelom arose from the archenteron or by
a split in the mesoderm.
Without going into the developmental
sequences, it is now clear that the enterocoelous condition occurs in many species
in one group always listed as schizocoelous (Brachiopoda), and the schizocoelous
method of formation occurs in one of the
classes (Ayes) always listed as enterocoelous. There are many groups in which
the condition is unknown.
At present this feature cannot be used
for the purpose of setting up two subgroups of coelomate animals. It should be
dropped completely from beginning texts,
unless it is to be used to illustrate how
an unfounded generalization can lead one
astray.
These remarks are not intended to be
critical of the efforts of the writers of
beginning textbooks, whose problems are
already legion. They cannot be specialists
in all the fields on which they must write,
and there is frequently no apparent reason
why they should not use an expression or
a device used by many previous writers
of the same sort of book. It will make
their book more accurate, more understandable to the student, and more useful to him in later courses if these things
are dealt with in defensible and meaningful manner.
Summary
1. A list or diagram of the groups into
which a certain species is placed is not a
"classification" of that species but a hierarchy of its groups.
Vol. 46, No. 2, February, 1968
2. At the base of this hierarchical list,
the name of the species must always be
a binomen—the genus name must be repeated.
3. Genera are never subdivided into
species. They consist of one or more species, but in this context they are made by
assembling species into a group. The same
goes for families.
4. Never put a synonym in parentheses
between the two parts of the name of the
species. Only the subgeneric name can go
there.
5. All names given to any one taxon are
synonyms. Refer to the rejected ones as
rejected synonyms.
6. Don't overstate the power of the
International Commission on Zoological
Nomenclature. The rules of nomenclature
originate only in actions of the International Congress of Zoology.
7. If a single generic name is used in
referring to a figure, be sure that it is
understood that it is not a figure of
Drosophila, for example, but of a specimen of some one species of this genus.
8. Species are assembled into groups
called taxa. Each taxon is assigned to a
level in the hierarchy. Only the levels are
categories. The category "order" contains
the taxa considered to be orders.
9. Names at only one of the twenty or
more levels are binominal. Thus zoological nomenclature is not a binominal system but includes a system of binominal
names for species.
10. Don't label any species as typical
or representative.
11. Avoid the subject of symmetry,
unless it can be made meaningful.
12. Never use the adjectives "true"
and "false" in describing features of organisms.
13. Use schizocoel and enterocoel in
classification only if you are sure the
facts you give justify such use.
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P5.125
P5.12I
P5.123
Interesting Slides
Fasciola hepatica, probably the best known
of all flukes, was the first digenetic trematode in which the life cycle was completely
worked out. In a sense the cycle is unusual because the encysted metacercarial
stage occurs on vegetation (in water) and
not within one of the hosts. Eggs from
54 • Turtox GNews
the adults in the liver pass into the alimentary tract via the bile ducts and pass
out with the feces. After being in water
from about four days to two weeks the
ciliated miracidium escapes from the shell
and swims about and penetrates a compatible snail.
Vol.
46,
No.
2,
February, 1968