General Zoology Classification Page

In the 18th century, a Swedish botanist named Carolus Linneaus developed the basic principles of taxonomic classification still followed today for the classification of living organisms.  Taxonomy is a hierarchical system used to identify and classify organisms.  Although the actual classification of individual animals has changed drastically from the classification established by Linneaus, the modern system still uses:

A hierarchical system of classification.  Each organism is placed into descending categories that start broad and become more and more specific.  Each organism belongs to seven mandatory ranks (Kingdom, Phylum, Class, Order, Family, Genus, species), but some organisms also belong to more complex subdivisions (e.g. Subphyla or Superclasses). The named taxonomic unit at any level is called a taxon (plural, taxa). For example, mammalia, a taxon at the Class level, includes all the many Orders of mammals.

A naming system (binomial nomenclature), for identifying organisms.  Each organism belongs to a Genus and a species that comprises its scientific name.  The Genus is always capitalized, but the species is never capitalized, and both are underlined or italicized.  

A Familiar Example:

The US Postal Service classifies mail in much the same way that scientists classify animals.  If you want to mail a letter to your friend, you use an envelope containing identifying categories (i.e. the address) to which your friend belongs. These categories follow a hierarchical system, eventually ending in your friend’s name. Below the typical address label to reflect the principles used in taxonomic classification and identification:

KINGDOM                                                       UNITED STATES

     PHYLUM                                                          COLORADO

          CLASS                                                              DENVER

               ORDER                                                           1234 FRIEND

                    FAMILY                                                           SUITE 123

                         GENUS                                                             JOHN

                              SPECIES                                                          DOE

The scientific name of this organism would be written as John doe or John doe.

How will you remember these 7 Ranks??

There are many different pneumonics for remembering the classification hierarchy, for example, "King Phillip Came Over From Great Spain!!"  How well do you know your British history – what is this pneumonic referring to?

You as an example:

As humans, we all belong to the following classification scheme:

KINGDOM                                                       ANIMALIA

     PHYLUM                                                          CHORDATA

          CLASS                                                              MAMMALIA

               ORDER                                                           PRIMATE     

                    FAMILY                                                           HOMINIDAE

                         GENUS                                                             Homo

                              SPECIES                                                           sapiens**

Our scientific name is Homo sapiens or Homo sapiens.

The following table shows the classification of some other organisms:

Organism

Chimpanzee

Southern Leopard Frog

Katydid

Kingdom

Animalia

Animalia

Animalia

Phylum

Chordata

Chordata

Arthropoda

Subphylum

Vertebrata

Vertebrata

Uniramia

Class

Mammalia

Amphibia

Insecta

Order

Primates

Anura

Orthoptera

Family

Hominidae

Ranidae

Tettigoniidae

Genus

Pan

Rana

Scudderia

Species

troglodytes

pipiens

furcata

Scientific Name

Pan troglodytes

Rana pipiens

Scudderia furcata

This table was designed and created by Dawn M. Kitchen and Tim Susman, University of Minnesota, modified by Cynthia Church

Systematics: Connecting Classification to Phylogeny

How do we reconstruct evolutionary history to draw phylogenetic trees (family trees)?
The fossil record helps, of course. Assessing relationships between living species by comparing their anatomy also provides data. And we can compare two species at the molecular level by sequencing their DNA (or protein). But, how do systematists evaluate all of this info and construct a family tree? Today, most systematists use what is called cladistic analysis (or simply, cladistics). A phylogenetic tree based on cladistics is called a cladogram.


Cladogram

While systematics is the reconstruction and study of evolutionary relationships, classification refers to how we place species and higher groups in the taxonomic hierarchy. To understand why the two are not always congruent, we need to consider how species may be grouped based on their phylogenetic (evolutionary) relationships. A monophyletic group incudes the most recent common ancestor of the group and all of its decendants. In the figure below, Taxon 1 (consisting of the seven species B-H) is a monophyletic group. It is made up of an ancestral species (species B) and all of its descendant species. A paraphyletic group includes the most recent common ancestor of the group, but not all its descendants. Taxon 2 consists of an ancestor (A) and some, but not all, of that ancestor's descendants (Taxon 2 includes the descendants I and K, but excludes J and B-H, which are also descended from A). A polyphyletic group does not include the most recent common ancestor of all members of the group. Taxon 3 is polyphylectic, which means that it lacks the common ancestor (B) that would unite the species as monophyletic group. In other words, a polyphyletic group has members that can be traced to separate ancestors.

Each evolutionary branch in a cladogram is called a clade (from the Greek clados, meaning "branch"). Notice that clades, like taxonomic levels, can be nested within larger clades. For example, the turtle-leopard clade represents a clade within the salamander-turtle-leopard clade. But not all groupings of organisms qualify as clades. A clade consists of an ancestral species and all of its descendants. Such a group of species, be it a genus, family, or some higher taxa, is said to be monophyletic. Paraphyletic and polyphyletic taxons are groupings of species that are unacceptable in the practice of cladistics.

The Compleat Cladist is a phylogenetic workbook that will give you more practice with cladistics.

Definitions
Classification – Organization of organisms into groups or taxa (singular – taxon)
Nomenclature – Assignment of a distinctive name to each species
Taxonomy – General art, craft, and science of classification
Systematics – Modern taxonomy based on evolutionary history
Taxon – A group of organisms that is given a name
Natural taxon – A group of organisms that exists in nature as a result of evolution
Characters – Heritable features or attributes of an organism that can be compared across organisms (e.g., physical characteristics, genetic sequences, behavioral traits)
Cladistics – A method of analyzing the evolutionary relationships between groups to construct their phylogenetic tree (family tree)
Clade – An evolutionary branch in a cladogram; a clade refers to all the descendants of a particular common ancestor. By definition, a clade is a monophyletic group; i.e., a natural taxon
Cladogram – A cladogram is a tree-like diagram showing the relationship between different organisms with respect to a common ancestor (always remember that a cladogram represents a hypothesis based on a particular data set that has been analyzed by a particular technique)
Phylogenetic tree – Branching tree diagrams that depict the phylogenetic history within a taxonomic group with respect to the evolutionary time
scale); a cladogram is one kind of phylogenetic tree, but with no relative time axis
Ingroup – Those species whose evolutionary relationships are being investigated by the researcher; the study group
Node
– Branch points in a cladogram; nodes represent the common ancestor of all taxa beyond the node
Outgroup
– An organism (or group of organisms) used in a cladistic analysis for comparative purposes. It is a related organism that is outside of the study group with a known evolutionary relationship to the group. (Another way to put this: the outgroup is a different organism–but not too different–that is not included in the study group); multiple outgroups are often used to increase the effectiveness of the comparisons.
Ancestral characters (plesiomorphic characters) – Attributes of a group that all members of the group possess; ancestral characters are not useful in analyzing evolutionary relationships within the group
Derived characters (apomorphic characters) – Characters that have arisen since ancestry with the outgroup; derived characters give clues to evolutionary relationships
Synapomorphy
– A derived character (apomorphy) that can be traced back to the most recent common ancestor; a synapomorphy unites two or more groups; the fact that we can move/bend our thumbs, while most animals cannot, is a synapomorphy we share with chimpanzees, monkeys, and other primates.
*It may be useful to think of the terms "original" and "changed" when comparing ancestral and derived characters

Using these definitions
The figure below is a hypothetical cladogram showing 5 taxa (1-5) and the characters (A-H) used in deriving the taxonomic relationships. The study group is taxa 1-4; taxon 5 is the outgroup for taxa 1-4 (remember that the outgroup is a reference group outside of the study group). The outgroup shares only character A with taxa 1-4. All other characters (B-H) are absent from the outgroup and must have arisen after the divergence of the study group from the outgroup. In other words, they are derived characters. Character B separates taxa 1-4 from the outgroup. This character cannot be used to describe relationships within the group because it is common to all members of the group. Closely related taxa 3 and 4 share derived character D. Character C is a synapomorphy for taxa 1 and 2, separating them from all other taxa. In other words, character C was passed down to both groups by the last common ancestor that they shared. Character.H is an apomorphy for taxa 1, separating it from all other taxa.

Humor in Taxonomy

Some whimsical taxonomic names exist…..  Agra vation is a tropical beetle that was apparently very difficult to collect.  Another insect, a true bug, is named Heerz lukenatcha.  Organisms are sometimes named after people.  A louse that lives on owls has been named Strigiphilus garylarsoni; a bacterium bears the name Salmonella mjordan, named by a microbiologist who is a basketball fan.  My advisor named a parasite after his ex-wife.  The names of some amphipod crustaceans exceed 40 letters in length, such as Polichinellobizrrocomic burlescomagicaraneus.

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