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: 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.
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First placed on web: 10/29/00 Last revised: 03/23/01