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Nomenclature (Scientific ●Common)
When casually discussing nature, whether verbally or in print, we typi-
cally use pretty informal language. Simple, everyday terms for plants and
animals are more than adequate for expressing ourselves. We do not talk
about Malus and Citrus sinensis (taxonomic terms) when referring to
apples and oranges because it is simply not effective communication.
However, we do use such terms when we want to make specific scientific
distinctions—say, between the prairie crab apple, which botanists identify
as Malus ioensis, and the sweet crab apple, which they have designated
Malus coronaria.
Because this book deals with differences, many of them technical and
somewhat detailed, the reader will encounter quite a few such scientific
names, some of which may appear not only needlessly elaborate but also
virtually unpronounceable—although nothing as tongue twisting as
Paracoccidioides brasilensis, a lung-infecting fungus, or Brachyta inter-
rogationis interrogationis var.nigrohumeralisscutellohumeroconjuncta,a
longhorn beetle. As for pronunciation, unless one really feels that the
word will be used in conversation, there is no need to be concerned with
“getting it right.”
But why does science eschew common names and instead encumber
itself with such a seemingly inexplicable vocabulary? Because, in part,
“common” does not necessarily mean “universal.” What may be an
organism’s common name in one part of the country may be quite dif-
ferent in another, making any initial exchange of information suspect. For
example, in the Midwest the thirteen-lined ground squirrel (Spermophilus
tridecemlineatus)is commonly, but mistakenly, called a gopher, which is
really the rightful name of a much larger rodent (Geomys bursarius).Fur-
thermore, some organisms seem to have acquired a different name every
time they were spotted in a new locale, such as the mullein, a plant of the
figwort family that is saddled with almost 150 aliases.
Unfortunately, this is just the sort of predicament that faced the entire
biological science community up until the mid-eighteenth century: scien-
tists from different countries and even individual scientists within the same
country sometimes used different names for the same plants and animals.
Early attempts to rectify the predicament by using descriptive Latin words
to identify each species only led to new problems. Species often ended up
bearing unwieldy labels, such as the red-winged blackbird’s Sturnus niger
alis superne rubentibusand the catnip plant’s Nepeta floribus interrupte
spicatus pedunculatis.Ultimately, it was the Swedish botanist Carolus Lin-
naeus1who remedied the problem by establishing a specific methodology
for naming all organisms. He devised a binomial nomenclature that gave
each species its own, and purposely unique, two-part name.
In accordance with this system, scientific names of species (viruses
excepted) always consist of at least two terms—for example Homo sapi-
ens.The first term (Homo)is the genus (plural, genera) it belongs to, and
the second term (sapiens),called the specific epithet, its species group. An
organism is never referred to by its specific term alone: both terms must
be used or indicated. In print, genus names always begin with a capital
letter and the species name with a lowercase letter, and both are always
italicized. If all the species of a genus are included in a statement, scien-
tists will often indicate only its genus, followed by “spp.” If the genus is
in doubt, a question mark will follow the term. This most often occurs in
classifying fossils or when a family is undergoing reorganization.
Whenever a scientific name is first mentioned in a piece of literature,
both terms are used. Any subsequent use of the name in that work may
be abbreviated by using the initial letter of the genus plus the full species
name—for example, H. sapiens.Other species of the same genus that fol-
low may be likewise abbreviated, but in all cases the references must be
unambiguous. For subspecies, after the genus and species names have
been established, these may be indicated by their initial letters, with the
subspecies name written in full.
While these rules probably appear quite reasonable, the question may
have occurred: why not use more familiar words? Instead of Geomys bur-
sarius,why didn’t scientists just label the animal “plains gopher” or some
other common name, using simple everyday terms? In answer: the foreign
ring to these names is a result of using Latin or Latinized words and word
roots, the language of science that was favored by European scholars at
the time binomial nomenclature was introduced (in some cases, Greek is
also used but in a Latin form).
As a dead language, Latin is ideal because it is free of any nationalis-
tic conceit and bias—averting any potential squabbling over which lan-
guage has the best common name for an organism. It is also functional, in
that suffixes are easily appended, forming terms that carry noteworthy
information.2For instance, the scientific name for the Eastern Hog-Nosed
skunk is Conepatus leuconotus, which comes from four descriptive
terms: konis, “dust”; pateo, “I walk”; leukes, “white”; and notos, “the
back.” Linked together, they tell us that the animal is a “white back that
roams the open or desert-like country.” Unfortunately, scientific names do
not always accurately describe an organism, which, for those who are
familiar with Latin, may be a bit misleading. The pronghorn’s genus
name, for example, is Antilocapra, a name derived from anthlops(for
antelope) and capra(for she goat); the pronghorn is neither. But scientific
names are not always so formal or rooted in the precepts of academia.
Naming is a privilege usually reserved by the first scientist to describe an
organism and, as such, may reflect a personal interest, rather than some
particular character of the organism. One scientist named two new
species of wasps Polemistus chewbaccaand P. yodaafter characters in the
movie Star Wars.And Godzilla, the famous Japanese movie monster, was
the source of the crustacean genus name Godzillius.
As for common names, Robert Bakker, one of the world’s foremost
authorities on dinosaurs, summed up the latitude scientists enjoy in nam-
ing their discoveries. Bakker observed, “[There are] no rule books to
name dinosaurs. It can be Urdu. It can be Hindu. It can be an anagram
[although if it is also to be a taxonomic name it must be pronounceable].
It could be nonsense syllables—as long as they are not overtly insulting to
some colleagues.”3And as if to underscore the point, Bakker named one
of his recent dinosaur finds Big Ed. However, under the codes governing
the spelling of zoological organisms, if “Big Ed” were to be a taxonomic
name, it would at least have to be Latinized with a Latin termination.
Yet as clever as these names may be, most scientific names are not
concocted willy-nilly. The creation of almost all such names is guided by
various international codes, each code determined by the discipline it
serves. Formal terms involving animals are governed by the rules of the
International Code of Zoological Nomenclature,those of wild plants by
the International Code of Botanical Nomenclature, cultivated plants by
the International Code of Nomenclature for Cultivated Plants, and bac-
teria by the International Code of Nomenclature of Bacteria. And
viruses, although not regarded as living organisms, are nonetheless also
named and classified (in an order with families, subfamilies, etc.) accord-
ing to the International Code of Virus Classification and Nomenclature.
These international codes do not regulate all ranks, just the more specific,
lower ones. The zoological code, for instance, concerns itself only with
those names of taxa in the ranks of superfamily down through subspecies.
While all codes follow taxonomic and Latin grammar rules, they are
independent as to the terms they allow to be used. Therefore, the same
term may be used in different fields of science. Corydalis,for example, is
used in botany for a genus of herbs and in entomology for a genus of
predatory insects called Dobsonflies. Likewise, Ricinus is the genus
name for both the castor bean and the blood-sucking bird louse. Such
duplication may also occur among the unregulated ranks within a disci-
pline. In zoology “Decapoda” is used to designate not only one of its
crustacean orders but one of its mollusk suborders as well. Organisms
also have had different designations in separate disciplines. Until just
recently, the troublesome plant/animal-like euglena could be found listed
in the algal family Euglenaceae by botanists and in the protozoan family
Euglenidae by microbiologists.
This illustrates the fact that scientific names, as well thought out as
they may be, are not engraved in stone. While a change in name is not at
all uncommon, it is never done casually but rather because of some com-
pelling rationale. The name may already be in use or an older name may
be found to take precedence, but most commonly, it is because the organ-
ism is reclassified. The echidna, or spiny anteater, is a perfect example.
When first discovered and described in 1792, the echidna was
thought to be a relative of the anteater of South America and was there-
fore classified in the anteater genus under the name Myrmecophaga
aculeta(prickled anteater). However, on closer examination in 1802, the
echidna was found to be not a placental animal like the anteater but a
monotreme like the platypus and was therefore reassigned to the platypus
genus Ornithorhynchus(birdlike snout), with the species label of hystrix
(porcupine-like). But shortly thereafter, scientists decided that this strange
animal deserved its own genus, so they renamed it Echidna hystrixafter
the Greek goddess Ekhidna, who was half reptile and half mammal. Then
in 1876 a new form of echidna was discovered in New Guinea, one with
a much longer snout and shorter spines. These and other features made it
so different from E. hystrixthat it clearly required its own genus. Faced
with creating a new genus name, zoologists decided that some shared fea-
ture should unite the names of both echidna genera. This feature would
be their tongues. So E. hystrixwas renamed Tachyglossus(rapid tongue)
aculeatus (with prickles) and the New Guinea long-snouted echidna
received the name Zaglossus(great tongue) bruijni (from the name of a
noted naturalist).
Although such reclassifications will continue, they will probably
never reach the extent they did during the trend away from the “split-
ting” approach to classification to a “lumping” approach. The splitters,
who held sway into the early twentieth century, often regarded an
organism’s unique characteristics as compelling reasons to separate it,
taxonomically, from similar organisms. This typically resulted in genera
with a seeming overabundance of species (see “Brown Bear,” chapter 7).
But as biologists reevaluated their approach to taxonomy, they began to
see the wisdom in lumping together those species with close similarities,
which effectively reduced the number of genera and species. The net
result was that many organisms lost their old designations, sometimes
receiving not only a new species label but a new genus label as well.
One of the basic tools in classifying an organism is the type of words
used. When one forms a binomial name, the genus term is always a sin-
gular noun (e.g., “tongue”), whereas the species name may be a noun, an
adjective, or a Latin rendering of a proper name. Except for the king-
doms, which need only be a plural noun, the taxa (taxa [singular, taxon]
are specific groups within a rank) of other ranks are formed from plural
adjectives used as nouns and are often appended with specific endings.
These endings often serve as an immediate indicator of the rank of the
taxon. For example, within the ranking of the koala bear are the taxa
Phalangeria and Phalangeridae. The “ia” ending indicates that this is its
order name, and the “idae” indicates its family name. Listed as follows
are taxa endings for the plant and animal kingdoms. The parenthetical
endings after “varies” are only typical; often they are found in only one
taxon of a higher rank, and may not be the only such ending in that rank.
Taxa Endings
Rank Plants Animals
Phylum/Division -phyta varies (-a)
Subphylum/Division varies (-phytina)4 varies (-data)
Class varies (-opsida)4 varies (-lia)
Subclass varies (-idae)4 varies (-ia)
Superorder5 varies (-anae) -orpha
Order -ales varies (-formes)
Suborder -ineae varies (-morpha)
Superfamily5 N/A -oidea
continued from previous page
Taxa Endings
Rank Plants Animals
Family -aceae -idae
Subfamily -oideae -inae
Tribe -eae -ini
Kingdom●Phylum●Class ●Order
●Family ●Genus ●Species
These seven terms represent one of mankind’s most ambitious undertak-
ings: the continuing effort to uncover and describe the relationships that
unite Earth’s incredible variety of life. In addressing this task, two scien-
tific disciplines are particularly crucial, systematics and its subdivision,
taxonomy. Systematics deals with the kinds, diversity, and evolutionary
history of organisms, which enables taxonomy to identify, name, and
classify them into related groups. Together, the two work to assign every
living thing—past and present—a position on the tree of life. (Often both
disciplines are collectively indicated by either term.) No simple task, this
immense undertaking assesses a variety of factors that ranges from ances-
try to physical characteristics to gene sequencing.
The working assumption behind this endeavor is that billions of years
ago, all life was born of a common ancestry and that every subsequent form
of life, no matter how unique, is related to all others. Under this principle,
the more temporally close an organism is linked to another in certain shared
characteristics, the more closely related they are apt to be. We, as Homo
sapiens,are more closely related to the gorilla than to the sea squirt, but not
as closely as we are to our 500,000-year-old relative Homo erectus. An
incalculable number of evolutionary links, many unknown and undiscov-
erable, connects these four organisms, but the clues pointing to their evo-
lutionary kinship are unequivocal. This does not mean that certain lower
forms of life such as sea squirts were necessarily our ancestors (sea squirts,
in fact, appear to be an evolutionary dead end), but only that their appear-
ance on Earth arose from a different branch of the same primeval stock.
In their work, taxonomists classify life onto a hierarchical framework
consisting of increasingly restrictive categories, much in the same way one
could rank a deck of playing cards. To illustrate: if we take an entire deck
of cards as the broadest division, all cards would be members. Each also
belongs to a smaller group of either red or black cards. By themselves, the
red cards easily fall into one of two other groups: hearts or diamonds.
Considering just one suit, we can make a distinction between the court
cards and the numbered cards, giving us two yet smaller groupings. Tak-
ing the 10 numbered cards, we can further divide them into two groups
of odd- and even-numbered members. From this point, we have the indi-
vidual cards themselves.
Using such a hierarchical system of classification, we can pick any
card from the deck and assign it a place on the tree of playing cards. If
our lone card happens to be the three of diamonds, we know that it is a
member of an odd card group, which is a member of the noncourt group,
which in this case is a member of the diamond group, which is a member
of the red group, which is a member of the group of all cards. This is a
tree of five ranks, from the rank of deck to the rank of individual card.
An important point to keep in mind in this analogy is that there is no
inherent connection between the various cards. A card manufacturer
could make all the clubs red and all the hearts black and not change the
fundamental nature of the deck.
The beauty of this system is that at a glance, one can determine how
closely individual cards are “related” to one another (in this particular
scheme). The three of clubs may seem very much akin to our three of dia-
monds—being of equal value and rank (the fifth)—yet it is apparent that
they show no common connection until we move all the way up to the
first rank. That they are both “threes” is trivial, in this case. Had we
ranked the cards by their numerical value before separating them into
suits, their shared status as “threes” would more closely link the two.
This is the same type of disclosure we see in our classification of life.
Bottle-nosed dolphins and pompano dolphins appear very similar in sev-
eral ways: they both swim in the sea, employ the same means of locomo-
tion, eat the same type of food, and have comparable body shapes, tail
fins, dorsal fins, and pectoral fins. Yet the bottle-nosed is a mammal and
the pompano, a fish. And if one tracks their classification back to a com-
mon rank, it would be at the intermediate rank of subphylum, five ranks
away, where the reptiles, amphibians, and birds also come together. So, in
spite of their similarities, taxonomy shows us that the bottle-nosed and
the pompano are only distantly related.
When we ranked the deck of cards, we did so according to a princi-
ple called pheneticclassification, which relies on similarities. We first
considered similar color, then suit, status as a court card or not, and
whether it was evenly divisible by two or not. Phenetically, we could just
as well have first divided the deck into court and noncourt cards instead
of red and black. Yet in the majority of cases (those not involving most
microorganisms), biologists are able to employ a much more precise and
informative technique.
When ranking the higher organisms, scientists most often use phylo-
geneticanalysis,6a technique that while relying on the similarity of shared
structures and functions, also considers whether they are primitive or sub-
sequently derived. Therefore, apparent close similarity does not auto-
matically equal close relatedness. In practice, this prevents us from
indiscriminately taking a trait—such as possession of a tail, for exam-
ple—and classifying animals into tailed and nontailed groups. Such an
exercise would put monkeys with alligators and humans with oysters.
While this classification is logically valid, it tells us nothing about the real
relationships among the four.
Along with shared traits, assessment may also involve patterns of evo-
lutionary descent based on the elapsed time between common ancestors.
Taken together, these factors and other considerations form the basis of
all taxonomic groups, which are called taxa. These are the groupings that
fill out the system of ranks extending from domain down to species, a
reflection of evolutionary progress (or at least kinships), as best we under-
stand it (domain, a new rank, is discussed later on).
Conceived in the eighteenth century by Carolus Linnaeus
(1707–1778), the system originally classified life forms into three ranks:
kingdom, genus, and species. Later on, Linnaeus would add the ranks of
class and order, to which the German evolutionist Ernst Haeckel7inserted
the ranks of phylum and family (botanists and bacteriologists often use
“division” as an equivalent label to “phylum” in their classification of
plants and bacteria).8Today, every form of life, from dinosaur to newly
discovered microbe, is assigned a position within each of the eight ranks.
For humans, lions, and alligators, these rankings are as follows:
Human Lion Alligator
Rank Taxon Taxon Taxon
Domain Eucarya Eucarya Eucarya
Kingdom Animalia Animalia Animalia
Phylum Chordata Chordata Chordata
Class Mammalia Mammalia Reptilia
Order Primates Carnivora Crocodilia
Family Hominidae Felidae Alligatoridae
Genus Homo Panthera Alligator
Species sapiens leo mississippiensis
As a hierarchy, this system of ranks is very simple. A genus is a group
of closely related species whose members have more in common with
each other than with members of species of other genera. A family is a
group of closely related genera. An order is a group of closely related
families, and so on, up to the rank of kingdom.
Notice that there is no rank called subphylum listed, which was men-
tioned as the first common rank of mammals, fish, amphibians, reptiles,
and birds. This rank, along with others, is an optional fine-tuning of the
hierarchical system that is used when a more precise grading of a lineage
is required. If a particular trait within a taxon exhibits a significant split
in evolutionary development, the rank may be subdivided into smaller
units or may be subordinated under an optional higher rank.
In the ranking of Homo sapiens,both types of fine-tuning have been
proposed: between the ranks of phylum (taxon, Chordata) and class
(taxon, Mammalia), the ranks of subphylum and superclass can be estab-
lished. The subphylum rank indicates that Chordata was subdivided, with
the mammals, fish, amphibians, reptiles, and birds assigned to the sub-
phylum taxon Vertebrata. (The other subphylum taxa are Urochordata,
the sea squirts; Hemichordata, the acorn worms; and Cephalochordata,
the lancelets.) The superclass rank indicates that Mammalia, along with
other Vertebrata classes (Amphibia, Reptilia, etc.), were grouped into at
least two higher taxa. There are two: Agnatha, for those without jaws
(e.g., the lampreys); and Gnathostomata, for those with jaws. A more
precise ranking of humans then is Animalia—Chordata—Vertebrata—
Gnathostomata—Mammalia—Primates—Hominidae—Homo—sapiens.
Other proposed human hierarchies have different or additional interme-
diate ranks.
In a few cases an intermediate rank is established simply as a con-
venience, rather than to express a substantiated relationship among
organisms. The intermediate taxon Glires, a cohort rank, has been used
by some to group the rodent order Rodentia with the rabbit order Lago-
morpha. Lacking any concrete evidence of close kinship, Glires is based
merely on the likelihood that in the distant past rodents and rabbits were
closely related.
As with the eight main ranks, intermediate ranks follow an estab-
lished order, with each scientific discipline determining its own groupings.
For the animal and plant kingdoms, these groupings fill in the mandatory
eight-rank hierarchy as follows.
Domain: Eucarya
Animals Plants
Kingdom Kingdom
Subkingdom
Phylum Division/Phylum
Subphylum Subdivision
Infraphylum Infradivision or Branch
Superclass
Class Class
Subclass Subclass
Infraclass
Supercohort
Cohort
Superorder Superorder
Grandorder
Mirorder
Order Order
Suborder Suborder
Infraorder
Superfamily
Family Family
Subfamily Subfamily
Tribe Tribe
Subtribe
Genus Genus
Subgenus Subgenus
Section
Subsection
Series
Subseries
Species Species
Subspecies; or Race, Breed Subspecies
Form Variety
Infrasubspecies; or Race Subvariety
Breed, Form Form
Subform
Cultivar
Any organism classified in an intermediate rank must be included in
all higher, but not necessarily lower, intermediate ranks within the major
rank. Therefore, if an animal’s classification includes infraclass, it must
also include class and subclass but not necessarily cohort. Those interme-
diate ranks designated as “super . . . ” are not considered an extension of
the preceding rank but of the next major rank. Superfamily, for instance,
is a member of the “family group” of ranks, although it precedes the rank
of family.
No organism has a lineage traceable through all of these intermediate
ranks, but some plants do require quite a few. And whereas intermediate
ranks do serve an important function, some have been of only minimal use,
such as supercohort, grandorder, and mirorder. Certain intermediate ranks,
such as series, are used only in botany, while others, like race, are used pri-
marily in zoology. And not all ranks are officially recognized by taxo-
nomic codes. In zoology, only the eight primary ranks and those prefixed
by “super” or “sub” are officially recognized. In botany, the eight primary
ranks and only those prefixed by “sub” are acknowledged. Moreover, no
taxonomic code officially sanctions the ranks of breed or cultivar9ranks
used for artificially created variants. Also, bacteriology uses the unofficial
rank of strain to classify bacteria populations having a particular charac-
ter, such as the biovars, the morphovars, the serovars, and the phagovars.
Biological classification is a rigorous undertaking, involving great
care in the collection and interpretation of data. Unfortunately, the short-
age, quality, and inconsistency of such data may not allow for anything
more than theorizing. The result is that hierarchical trees are not
immutable facts but assumptions, and the placement of an organism or its
extended lineage often rests on supposition. This is why one will find
instances throughout this book where authorities are in disagreement.
Entomologists, for example, often assign the cockroaches to the suborder
Blattaria and the mantids to the suborder Mantodea, both of which fall
under the order Dictyoptera; however, other authorities eliminate Dicty-
optera as rank and treat Blatteria and Mantodea as independent orders,
with no change in name.10
But an even greater challenge confronts the bacteriologist. Until
recently, this relatively new field was unable to identify any phylogenic
relationships among bacteria; consequently, species were ranked only by
phenetic criteria, which means that many will now be in need of substan-
tial reclassification. Fortunately, other fields of study have eluded such a
broad reorganization, yet because their taxonomy is far from complete,
they are not without their own classification problems and challenges.
The root of such disputes is not that one scientist is less informed than
another—although this certainly could be the case—but that the whole
task of classification is hindered by inadequate and conflicting data. To
deal with these obstacles, scientists constantly wrestle with an assortment
of questions: What traits are significant? What weight should they carry?
Are these primitive traits or are they later modifications? Are they shared
by all members of a group? Such questions typically result in answers that
not only shape the process of classification but also determine the founda-
tion of each taxon. This is particularly true of those ranks extending from
phylum/division to genus. Unlike the rank of species, they are not governed
so much by definition as they are molded by the traits and evolution of
their subjects, which often creates the need for intermediate ranks.
Because the foundation of each taxon is determined by the charac-
teristics of its members, the magnitude and the extent of the defining
traits among various taxa within a rank can vary widely. Within the bat
order, Chiroptera, the family Phyllostomidae is distinguished from the
family Vespertillionidae only by an additional bone in the third finger and
often the presence of a leaflike projection on the nose; whereas in the
Artiodactyla, even-toed ungulates, a considerable number of differences
distinguish the pig family Suidae from the giraffe family Giraffidae. Fur-
thermore, within a taxon not all relevant identifying characteristics may
be applicable to all members. In a particular order, the traits involved in
separating one family from another may not be the same as those that
distinguish it from a third family.
This illustrates an important point: individual taxa of a rank seldom
share a common foundation. For example, within the rank of class the
principles that define the clam taxon, Bivalvia, are nothing like those that
define the millipede taxon, Diplopoda. But, then, what defines a rank
such as class? What is the common denominator that makes a class a
class or a family a family? Well, there really isn’t any. Each rank is only a
position or a level, not a biological construct. Scientists are constrained by
the characteristics and the evolution of their subjects, while at the same
time are bound by taxonomic rules. In practice, no matter how strange or
unique a new organism may be, characteristics are almost invariably
established (usually phylogenetically, as much as possible) to suit each of
the mandatory eight ranks. One current exception is the amoeba-like
animal trichoplax, for which no class or order has been established. Taxa
of higher ranks are generally defined by the more general or gross features
of an organism. Lower-ranked taxa like genus and species depend on
more particular features. When a ranking is completed, there will be eight
specific levels, each defined by increasingly distinctive features as the
species rank is approached.
Yet sometimes, as with a unique organism, formulating the specifics
for each rank is not only irrelevant, but the resulting taxa are essentially
meaningless. As an example, the ginkgo tree is commonly listed as
belonging to the class Gymnosperma, order Ginkgoales, family
Ginkoaceae, genus Ginkgo,and species biloba.It is the only species in the
entire order, which means there is essentially no useful difference between
the characteristics that define its order taxon and those defining its species
taxon (the same is true of the aardvark, the singular representative of the
mammalian order Tubulidentata). In such cases the formation of a
taxon may be no more than a formality as required by the rules of tax-
onomy. And, as with the ginkgo order, in such instances intermediate taxa
often go unmentioned in literature.
Of the eight ranks, kingdom and species are decidedly unique and
deserve elaboration. When Linnaeus devised his system incorporating
genus and species, three great kingdoms were recognized: animal, plant,
and mineral. Biologists, ignoring the mineral kingdom for obvious rea-
sons, decided that if an organism could move about and digest food, it
belonged in the animal kingdom, and if it stayed put and did not digest
food, it went into the plant kingdom. However, with the advent of more
powerful microscopes and exacting biological techniques, it was soon dis-
covered that some organisms, mainly microorganisms such as bacteria
and amoebas, were neither plants nor animals. This led scientists to seri-
ously question the two-kingdom classification as it was then defined.
More important, it became obvious that life required more than two
kingdoms to properly classify its diversity. But the ensuing task of decid-
ing what kingdom an organism belongs to was not an easy one. Conse-
quently, classification systems with between 4 and 20 kingdoms were
suggested, each with varying advantages and disadvantages.
This dilemma was tentatively resolved in 1959 when R. H. Whittaker
proposed a concept employing five kingdoms, a system that soon became
widely accepted. Like others, Whittaker based his kingdoms more on cel-
lular structure and nutritional mode than on the evolutionary principles
that underpin the other ranks. His five kingdoms, with examples, are:
Monera Bacteria and cyanobacteria
Protoctista Amoebas, algae, ciliates
Fungi Mushrooms, mildews, yeasts
Plantae Trees, flowers, dandelions
Animalia Clams, bees, humans
As suitable as this system was, some organisms resisted rigid classifi-
cation. Slime molds, for instance, possessing both animal and plant char-
acteristics, were, until quite recently, claimed in both Protoctista and
Fungi. In a more recent scheme (1978), the five kingdoms were grouped
into two superkingdoms: Eukaryotae, organisms with nuclear mem-
branes, for example, plants and animals; and Prokaryotae, organisms
without nuclear membranes, the bacteria. (A nuclear membrane is a cov-
ering that surrounds the nucleus of a cell.) In effect, this reordering gave
us two major branches of life.
However, this distinction lasted only until 1996, when the existence
of a third branch of life was said to be proven—the branch from which
all other forms of life are thought to have arisen. Named Archaea, its
microbial members were once known as archaebacteria and were classi-
fied as a distinct group within Monera.11The Archaea, a very peculiar
group of organisms, are distinguished not only by their genetic makeup
but by where they live and how they function. Often found living on
sewage and in sediments and swamps, they exist without sunlight, feeding
on carbon dioxide, nitrogen, and hydrogen and producing methane gas as
a waste by-product. They also live in places hostile to all other forms of
life: 8,600 feet under the ocean at pressures 200 times greater than on
Earth’s surface, in 185°F water (close to boiling), and in extreme cold.
But more important, their distinction rests in the fact that two-thirds of
their genes do not resemble those found in any other form of life.
The upshot of all this reassessment and reworking is that taxonomy
has now been expanded to eight ranks. At the top is the rank of domain,
which consists of three taxons: Bacteria, Archaea, and Eucarya (although
the rank of domain has been in use for several years, it has still not been
universally accepted). In one revision the rank of superkingdom has
been dropped, Eukaryotae was renamed Eucarya, and Prokaryotae has
been eliminated. Also, the kingdom Protoctista is no longer a formal des-
ignation, although it is sometimes used to indicate all members of
Eucarya, other than those belonging to the Fungi, Plantae, and Animalia
kingdoms. Those organisms that once made up Protoctista have now
been divided among as many as eighteen new kingdoms (some have also
been reassigned to the Fungi, Plantae, and Animalia kingdoms), each on
equal footing with Fungi, Plantae and Animalia. All of this reordering is
quite new and much of it is tentative, and, of course, not all scientists are
satisfied with it, so there are certain to be many changes before taxonomy
really settles down.
The following is an example of one newly proposed reordering.
Because the Bacteria domain is quite unsettled, the validity of its king-
doms, listed in the table, is very tenuous. The Archaea domain is also
shaky, with additional kingdoms likely to surface in the future. So, too,
with Eucarya; with continued molecular studies, it will certainly undergo
further revisions.
Domain: Eucarya
Kingdoms
Animalia Plantae Fungi Diatoms
Phaeophyta Chrysophyta Xanthophyta Oomycota
Labyrinthulids Apicomplexa Dinoflagellata Ciliates
Rhodophyta Acrasiomycota Entamoeba Naegleria
Euglenozoa Myxomycota Parabasalids Microsporida
Diplomonads
Domain: Archaea
Kingdoms
Euryarchaeota Crenarchaeota Koryarchaeota
Domain: Bacteria
Kingdoms
Proteobacteria Planctomyces Chlamydia Spirochaetes
Bacteroids Firmicutes Thermotogales Hydrogenbacter
Cyanobacteria and Chloroplasts Green sulphur bacteria
Thirty-four kingdoms!12 This is an incredibly explosive increase in
just a single reordering, and with more likely on the way—not a com-
forting thought for the interested layperson or the beginning biology stu-
dent. And aside from the future revisions that taxonomy is bound to
undergo, most of the kingdoms, besides being saddled with tongue-twist-
ing names, will seldom be remembered or understood by anyone other
than biologists. Perhaps, if the layperson is lucky, most of these kingdoms
will eventually be reduced to subkingdom status or combined with one
another. But this is a very big “perhaps.”
At the species level, the critical differences separating one species from
another often range from the obvious to the obscure. Among the lizards
of Florida, it is easy to spot the external features that distinguish the
bright green–colored green anole (Anolis carolinensis)from the somber-
hued brown anole (A. sagrei),but for the gray wolf (Canis lupus)and the
red wolf (C. rufus)there is little apparent difference except a slight vari-
ation in size and coloring (some authorities have suggested that the red
wolf be regarded as a subspecies of C. lupus). And when one looks at
dogs (domestic canines, C. familiaris),13 such traits are totally ignored,
allowing a range of shapes, sizes, and colorings that is immense. This
illustrates why outward appearances can be deceiving. The Alaskan mal-
amute, for instance, appears to have more in common with the gray wolf
than with the chihuahua, but when their DNA profiles are considered,
and from an exacting anatomical standpoint (mainly, skull features), this
is decidedly not the case. What does separate one species from another
usually rests with reproductive factors—the crux of the species rank and
much of the controversy that surrounds its definition.
Unfortunately, the biological sciences have yet to come up with a uni-
versal definition of “species.” The various species concepts now in use
have arisen out of the need to address classification issues that are often
particular to only one branch or sphere of biology and therefore are sel-
dom widely applicable. These concepts include biological, biosystematic,
evolutionary, genetic, morphological, paleontological, and phylogenetic
definitions.
The most widely used of these definitions, based on the biological
concept of species, usually takes a form akin to that formulated by biol-
ogist Ernst Mayr. Mayr contends that a species is a “reproductively iso-
lated aggregate of populations which can interbreed with one another
because they share the same isolating mechanisms.”14The heart of this
definition lies in the criterion of reproductive isolation.
Reproductive isolation embraces any element of nature that prevents
the reproduction of fully viable offspring. Some of these elements keep
organisms apart before they can mate. These include ecological barriers,
which physically keep populations from meeting; temporal barriers,
wherein species breed at different times of the day or the year; behavioral
barriers, where organisms consciously reject, for whatever reason, other
organisms; and mechanical barriers, such as incompatible physical or
physiological differences in the two organisms. Other prohibitive ele-
ments occur after mating has taken place. These include cases where the
sperm and the egg do not reach each other or fail to fuse; where a hybrid
dies before it sexually matures; and where the hybrid is sterile. The
essence of this definition of species is the ability of two organisms to suc-
cessfully reproduce by mating.
However, not all organisms reproduce by mating. Some asexual
organisms reproduce by fission, where an organism simply splits into two
equal organisms (euglenids); binary fission, in which a parent organism
splits into two offspring (certain green algae); and parthenogenesis, in
which there are no males—the egg always develops into a female (bdelloid
rotifers).
Although most flowering plants reproduce sexually, there are those
that self-pollinate, so the use of the biological definition of species to sort
out these particular plants is questionable. Also, the biological definition
cannot be applied to fossils. Obviously, in such cases other criteria must
be used to determine species. Typically, these will involve external mor-
phology (physical characteristics), chemical and physiological properties,
or genetic makeup. A species that reproduces by mating is also usually
considered to be a naturally occurring group of interbreeding, or poten-
tially interbreeding, organisms that shares a unique gene pool and pro-
duces fertile offspring. Within this somewhat qualified statement are
several key ideas.
“Naturally occurring” excludes offspring resulting from species
deliberately brought together. In the wild, Bactrian camels (Camelus bac-
trianus)and Arabian camels (C. dromedarius)do not mate because their
dispositions, habits, and territories differ; consequently, their gene pools
go unmixed. Yet in Turkey, where they are deliberately interbred, they
produce fertile offspring (some males are born sterile).
Not many crossbreedings between closely related species are this suc-
cessful. If offspring do result, they are most often either deformed or
infertile. (One glaring exception to this stipulation is Canis, the genus of
wolves, coyotes, and dogs—see “Wolf,” chapter 7). Mules are such ster-
ile animals, the product of the mating of a mare and an ass—there are no
mother or father mules. Yet not everyone feels that the fertility/infertility
criterion of species definition is a practical one. Keith Rushford, in his
book Conifers, observes, “The traditional criterion of a species being
interfertile between members of the species and, under natural circum-
stances, at least partially intersterile with members of another species is
subject to too many qualifications to be useful in practice.”15
But “interbreeding” does keep intact those features that distinguish
one species from another. Although some cross-species mating may take
place, usually it is not enough to break down the species’ identity. The
major factors in preserving a species’ identity are the isolating mecha-
nisms mentioned in the biological definition of species.
“Potentially” is used to include within a species those populations
that while not normally interbreeding, could successfully do so. The
Inuits of the North and the Pygmies of Africa do not mate with each
other, yet they are not considered separate species because, among other
reasons, unlike the two camel species, there are no instinctual barriers
inhibiting mating. Humans readily disregard “race” and often find those
of different ethnicities quite desirable. Bactrian and Arabian camels, given
the choice of mating within their species or with each other, prefer to
mate with their own—they are not considered to be potential interbreed-
ers. Within a species, those unique populations of potentially successful
interbreeders are often designated as subspecies or, as was the case with
humans, races.
A subspecies is almost always a geographically isolated group,
which, given enough time (usually, thousands of years), may evolve into
a bona fide new species. Crucial to such changes are shifts in the envi-
ronment and the accompanying genetic mutations that offer the necessary
variations needed for adaptation.
Although “subspecies” sits at the bottom of the hierarchical ladder,
its importance should not be underestimated. “Kingdom” may have
greater status and “species” more recognition, but it is “subspecies” that
is often the laboratory of incipient species, the rank of innovation, and
the genesis of diversity.
Various mnemonic sentences have been devised to help you remember
the order of the eight ranks. Here are three that may come in handy.
Domain KingdomPhylum Class Order Family Genus Species
Daffy King Philip Came Over For Ginger Snaps
Daddy Keeps Putting Cashews Out For Gray Squirrels
Danish Kings Play Chess On Furry Green Squares

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