Geography 140
Introduction to Physical Geography
Lecture: Classifying Life Genetically
I. Ways of Classifying Life: One of the earliest and critical concerns of
any science is development of a comprehensive and clear classification
system for its object of study. For biogeographers and other life
scientists, there are several systems of classifying living things,
depending on their purposes. In any science, there are several approaches
you can take to classification.
A. For example, you can classify things by their evolutionary
connections. This approach to classification is called a "genetic"
approach for genesis, or origins. If you've studied a foreign
language or two or taken a class in linguistics, you are familiar with
this approach in the classification of languages. Latin languages are
descendants and modifications of Latin; Germanic languages (including
English) go back to a proto-German language; Bantu languages (spoken
in much of Africa) are all related by historical connection. At a
higher level, all the Latin, Germanic, Slavic, Farsi, Hellenic, and
most languages in India, for example, are all descended from a proto-
Indo-European language.
B. You can classify things by their structure or appearance, too. These
are formal, morphic, or structural classification schemes. We saw an
example of this in the discussion of clouds: the stratiform
versus cumuloform clouds, and the cirro-, alto-, and strato-
elevation groups.
C. You can classify things by their function, as well, giving us
functional classification schemes. Car buffs can tell you all about
SUVs, light trucks, sports sedans, sport coupes, sports cars, low
riders, hot rods, muscle cars, etc.
II. The Linnaean binomial nomenclature is the best-known genetic
classification in the life sciences. It is a genetic classification
scheme, based loosely on relationships in evolutionary history. So, it
classifies living things according to how closely they are related to
other living things.
A. It designates all types of life by two part Latin names: the genus and
the species.
1. The genus comes first and is always capitalized.
2. The species comes next and is not capitalized.
3. Since both names are in Latin, a foreign language, both must be
EITHER italicized OR underlined (but NEVER both).
You should do that with any foreign phrase or word that has not
been naturalized into English, by the way. In the old days before
word processors, you couldn't italicize but you could underline, so
you underlined to let a typesetter know s/he should italicize.
B. A few examples to get the hang of it:
1. Human beings: Homo sapiens (which means "people with
'smarts,'" loosely rendered: we're the species doing the naming,
so we get to flatter ourselves!)
2. Street pigeon or rock dove: Columba livia
3. Laughing pigeon or ring neck dove: Streptopelia risoria
4. Wolf: Canis lupus
5. Dog: Canis familiaris (for "familiar pooch," loosely
rendered)
6. Common daisy: Bellis perennis
7. Domestic rose: Rosa domestica
8. Coastal redwood: Sequoia sempervirens
9. Giant redwood: Sequoiadendron giganteum
C. This short, two-part name is part of a very broad system of
relationships, the Linnaean hierarchy.
1. At the species level, the Linnaean system designates a group of
creatures so similar to one another, so closely related, that they
reproduce with one another to make offspring that can survive and
compete long enough to reproduce successfully in their turn.
a. All humans of one gender, for example, can freely interbreed
with anyone of the opposite gender in good reproductive
condition and produce viable offspring that can make babies of
their own in due time. So can all horses (though you might want
to use a couple of generations to get Shetland pony genes mixed
up with those of one metric ton shire horses!). Ditto with all
street pigeons. Ditto with plants of a given species.
b. You know you're dealing with two separate species if, when you
cross one of each, you get a mule (like the famous cross of a
jackass and a horse mare): Now that is one viable offspring
(generally smarter than either parent, which is why they're a
pain to deal with!), but mules can't make baby mules. You want
another mule, you have to find another burro and another horse.
c. Most interbreeding of two distinct species doesn't even get that
far: The offspring might be stillborn or not survive past the
fetal or embryonic stage or not even be conceived in the first
place. Natural selection will pick out those individuals who
refuse to mate with members of another species, because that's a
waste of a perfectly good pregnancy or seed development on a
genetic dead-end. So, animal species often have evolved
courtship behaviors that are incomprehensible to someone from
another species, and flowering plants might do things like shift
the timing of their flowering to avoid being pollinated by a
plant of a different species.
2. At the top level, kingdom, we're talking about a group of creatures
that can be really dissimilar, related only enough to be considered
all animals or all plants or all fungi.
3. The Linnaean hierarchy traditionally contains seven levels, from
most general to most particular: Kingdom, Phylum, Class, Order,
Family, Genus, Species.
a. There are one or more species in a genus (e.g., our species,
Homo sapiens, is the only current member of the Homo genus; the common
street pigeon, Columba livia, is one of as many as 54 species in the
Columba genus).
b. Similarly, there are one or more genera in a family, one or more
families in an order, one or more orders in a class, one or more
classes in a phylum, and one or more phyla in a kingdom.
c. You are responsible for memorizing this sequence of seven
levels. There are a few amusing mnemonic devices to remember
the sequence: "Kings Play Chess On Finely Ground Sand" or "King
Phillip Came Over For Great Spaghetti" (or Sex in one naughty
version I've heard) or "King Phillip Cried 'Oh, For Goodness
Sake,'" and, for you business majors, "Kindly Produce Credit Or
Furnish Good Security").
d. A few examples of some organisms shown along the seven
traditional levels of the hierarchy:
i. Human beings: Animal; Chordate; Mammal; Primate; Hominid;
Homo sapiens
ii. Street pigeon: Animal; Chordate; Aves; Columbinae;
Columbidae; Columba livia
iii. Band-tailed dove: Animal; Chordate; Aves; Columbinae;
Columbidae; Columba fasciata
iv. Laughing dove: Animal; Chordate; Aves; Columbinae;
Columbidae; Streptopelia risoria
v. American lobster: Animal; Arthropoda; Malacostraca,
Decapoda, Nephropidae, Homarus americanus
vi. Sabertooth cat: Animal; Chordate; Mammal; Carnivore;
Felidae; Smilodon fatalis.
vii. Daisy: Plant; Tracheophyte; Angiosperm; Asteral;
Compositae; Bellis perennis.
viii. Giant Redwood: Plant; Conifer; Pinopsida; Taxodiaceae;
Sequoiadendron gigateum
e. 'Nuff of that. You can determine which species is more closely
related to which other in a list, even if you don't even know
its common name or what the heck it is, by comparing how far
down the hierarchy you can get until you come to a different
name. So, for example, of the seven other species I've broken
out above, which one is most closely related to human beings?
To the giant redwood? To the street pigeon?
D. For most organisms, it is necessary to use more than seven taxonomic
levels to do the job.
1. Some non-traditional levels commonly used include:
a. Domain (sometimes called the Empire or the Super-Kingdom), which
is above the kingdom and is based on the core cell organization
of an organism. There are three domains recognized:
i. Eukaryotes (cells with a "true kernel" or nucleus housing
most of the genes; examples are all plants, animals, fungi)
ii. Eubacteria (aerobic prokaryotes, which have their genetic
material dispersed throughout the cell and which depend on
oxygen for respiration; examples are bacteria and blue-
green algae)
iii. Archaea (anaerobic prokaryotes, which release energy for
their metabolisms by processes other than oxygen-based
respiration. For them, oxygen is a poison. These are the
oldest form of life on Earth and flourished during the
primordial reducing atmosphere of the Archaean Era of 3.8
to 2.5 billion years ago. They are "extremophiles,"
surviving mostly in really extreme, inhospitable anaerobic
places, such as hydrothermal vents, hot springs, volcanic
mud, and even far down below the surface of the earth, and
they are also abundant among the plankton of the open sea).
2. You can get an idea of how hairy this can get by looking at the
expanded classification of human beings: I've put the seven
traditional levels leftmost and indented the extra categories.
-------------------------------------------------------------------
Domain -- Eukarya
Kingdom -- Animalia
Phylum -- Chordata
Subphylum -- Vertebrata
Superclass -- Tetrapoda
Class -- Mammalia
Subclass -- Theria
Infraclass -- Eutheria
Superorder -- Archonta
Order -- Primates
Suborder -- Anthropoidea
Infraorder -- Catarrhini
Superfamily -- Hominoidea
Family -- Hominidae
Subfamily -- not used in our family
Genus -- Homo
Subgenus -- not used in our genus
Species -- sapiens
Subspecies -- sapiens (vs. neanderthalensis)
-------------------------------------------------------------------
3. While I'm sharing all this with you, I am not holding you
responsible for anything more than the seven traditional levels,
remembering their sequence, and being able to use it to recognize
evolutionary affinities among pairs of organisms (the way you did
in II.C.3.e). Whew!
E. The Linnaean hierarchy is under fire these days by a group of
taxonomists (people who specialize in classifying organisms) who are
developing an even more explicitly genetic classification approach:
Cladistics or phylogenetic classification.
1. Their premise is that the only solidly defensible classification in
the Linnaean system is the species (review definition of species
above). Every other taxon (level in the Linnaean hierarchy) is
kind of subjective, depending a lot on the judgment of taxonomists
who specialize in one group or another of organisms. For example,
the pigeon genus, Columba, variously includes 11-54 species,
depending on whether a taxonomist is more of a splitter (more
genera) or a lumper (put 'em all in the same box).
2. So, what they've been working on is a completely binary system.
The key principles of cladistic taxonomy are:
a. Groups of organisms are descended from a common
ancestor: Such groups are called "clades," hence, "cladistics."
b. At each node (divergence of a population), there are two and
only two branching lines of descendants (though there remain
controversies with some triplets as to which is the older and
which the derived lines).
c. Evolution results in modifications of characteristics over time,
and these modifications accumulate through time at pretty
predictable rates, which allows you to time when two species in
the same clade last shared a common ancestor.
d. You can't use paraphyletic taxa in the cladistic system, or
lineages that include some but not all of a common ancestor's
descendants (e.g., Reptilia seen as the ancestor of mammals and
birds but not including either). The Linnaean system does allow
this for convenience.
e. This system has the disadvantage of massively reproducing levels
in the hierarchy (you thought the 17 used in the expanded
Linnaean classification of our own species was bad? You should
see a full-blown cladistic hierarchy!)
f. So, results are often shown, not like the table above but as a
"cladogram," a branching tree graphic. Here's a simple one,
showing birds and dinosaurs as part of the reptile group:
![[ cladogram of Reptilia ]](http://www.zoomwhales.com/rgifs/Reptilecladogram.GIF)
If you're curious, you can visit this site, which shows an
expanded (and I mean expanded proposed cladogram for
Dinosauria, including modern birds.
3. So, taxonomy or systematics (the science of classifying organisms),
which was a pretty sleepy subject when I went to college, is now
hot with debate and has seen a great expansion of information that
can be used for analyzing lineages, which is where this concern
with cladistic analysis comes from:
a. Traditional Linnaean taxonomy relied on structural and
functional differences among organisms to group the most similar
and most related organisms together.
b. Now, taxonomists can use really detailed chemical information,
things like the molecular structure of very particular proteins
or enzymes to sort out who is related to whom.
c. They can make use of massive amounts of new genetic and
chromosomal information, including nuclear and mitochondrial DNA
(mitochondria are the power plants of cells and they carry
smidges of their own bacteria-style DNA, which can be used for
cladistic analysis. Some geotrivia for you that I won't test
you on: Mitochondria resemble certain species of bacteria and
may in fact once have been independent creatures that took up a
symbiosis with some other species of one-celled organism: They
provide food for the cells in which they reside in exchange for
shelter).
c. Cladistic taxonomists can also use biogeographical data on the
distributions of organisms in a clade to figure out who had to
descend from whom, depending on the dates of the separation of
various landmasses. We'll talk about "continental drift" later
in the semester. For now, just remember that biogeographical
distributions can be used as part of cladistic analysis.
4. Basically, just remember that taxonomy is a hot field now, with a
great deal of debate and controversy going on, and that cladistic
taxonomists are trying to come up with a binary approach to
classification that creates unambiguous taxa (plural for taxon)
that are demonstrably clades (all descendants of some basal group
ancestor).
You need to know some of the different ways that scientists in any field,
whether in the natural sciences or the social sciences, approach
classification (genetic approach, structural approach, and functional
approach). Be familiar with the Linnaean binomial nomenclature, including the
"etiquette" of using the Latin names properly, the traditional seven taxonomic
levels in the hierarchy (KPCOFGS), and how you can use the traditional levels
to determine which organisms are the most closely related. Be aware that the
Linnaean system needs expansion into many more levels to be able to handle the
affinities of most organisms. There is also a binary approach to
classification under development now, called cladistics, which has turned
taxonomy into a very hot field in a very short time.
Document and © maintained by Dr.
Rodrigue
First placed on web: 10/29/00
Last revised: 03/23/01