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 
        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 ]

              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 
           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 

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