An Online Introduction to the Biology of Animals and Plants





Key Concepts



Section 1

Chapter 2

Classifying Living Things 









Carl von Linne, a Swedish botanist (plant scientist) known as Carolus Linnaeus (Latin was the common language for European science, so writings and often names were Latinized), began work in 1735 on a system that would organize descriptive classification from the smallest of related groups up to the very largest.  The system he developed, with revisions, is the basic system still used today to systematically organize types of living things with their relatives.  The basic structure was similar to how human organizations of the time worked, with groups-contained-within-groups, be they feudal power structures or military structures.  Each particular type of living thing would be designated a species (from the same root word as "specific").  Closely-related species could be collected within a larger grouping, a genus; related genera are grouped into a family, families into an order, orders into a class, classes into a phylum, and phyla into a Kingdom, the biggest and most general group.  In Linnaeus time, there were just the Animal Kingdom and the Plant Kingdom, but later discoveries convinced biologists that some distinctly different types of organisms, such as Fungi and some tiny single-celled organisms, should be given their own separate Kingdoms.

Some subdisciplines of biology use a basic Linnaean type of taxonomy, but may change the basic names used for a few of the groups.  Commonly, for instance, plant and fungus taxonomy uses the term Division instead of Phylum.










KINGDOM:  Animalia
PHYLUM:  Chordata
SUBPHYLUM:  Vertebrata
SUPERCLASS:  Gnathostomata
CLASS:  Mammalia
ORDER:  Primata
SUBORDER:  Haplorhini
FAMILY:  Hominidae
GENUS:  Homo
SPECIES:  Homo sapiens

KINGDOM:  Plantae
PHYLUM:  Tracheophyta
SUBPHYLUM:  Pteropsida
CLASS:  Angiospermae
ORDER:  Dicotylodonae
FAMILY:  Brassicaceae
GENUS:  Brassica
SPECIES:  Brassica oleracea



KINGDOM:  Mycota
CLASS:  Dicsomycetes
ORDER:  Pezizales
FAMILY:  Morchellaceae 
GENUS:  Morchella
SPECIES:  Morchella esculenta

KINGDOM:  Protista
PHYLUM:  Sarcomastigophora
CLASS:  Sarcodina
ORDER:  Granulopodea
FAMILY:  Amoebidae
GENUS:  Amoeba
SPECIES:  Amoeba proteus





As you might see from the examples above, the system is a little more complicated than it sounded.  Sometimes, two or more groups are found to be more closely related than anyone thought;  they might be connected as supergroups ("super-" put on a group name).  And sometimes a group is not as unified as was thought, and is split into subgroups.  These added levels also allow new connections or splits to be added without a complete up-or-down shift of a column of groups.

Biology, like the sciences in general, is produced by human beings who often disagree with each other's ideas.  For the most part, it is perfectly allowable for someone to say that this or that group should be a subphylum rather than a phylum, or a family rather than an order (this will come up later in the discussion about Kingdoms).  However, what is not allowed is to on a whim change the accepted name of a particular group - you can't say, "I dont like the genus Ursus for bears, I'm going to use Yogi from now on in all of my papers."  Once a group is named and the name accepted, it may be tossed about on the "classification ladder," but one must gain a broad consensus and acceptance before a group's actual name is changed.  If one book places sponges in their own Kingdom and one puts them in a phylum, in both cases the group will have the name Porifera;  this limits confusion when doing background research on organisms.

Another set of rules, called binomial nomenclature (2-name naming), determines how species names are created.   Youll see in the examples that species names are two words:  a capitalized genus name and an uncapitalized specific.  The second word has no meaning by itself, and is never capitalized, not even if a proper noun is used as the source of the term.  Species names (and Genus names) are also treated as foreign words in English, meaning that they are italicized or underlined when printed or written.  The names of other taxonomic groups are often not italicized or underlined, but that usage seems to vary.

Typically, species names are abbreviated by making an initial of the first word and spelling out the second - you may be familiar with E. coli,  the abbreviated name of Escherichia coli, a common intestinal bacterium that can make you sick.









Deciding what living things should be classified in the same groups requires deciding what's related to what, and how close those relationships are.  Long ago, it was often done by lumping together analogous traits:  features that are used to do the same function.  This is why, in Biblical times, if they were streamlined and swam ("Beasts of the Water"), or had wings and flew ("Creatures of the Air"), they were classified in the same groups.  Certain simple forms were also used for grouping:  by this approach, snakes would be grouped with earthworms and eels.

As more and more people studied Nature in detail, it became obvious that a butterfly's wings were very different structures than a bird's wings.  And sometimes, it could be seen that two structures used for very different functions - such as a human hand, a bat's wing and a whale's flipper - all contained the same basic internal architecture, with changes in parts producing the outward changes.  Traits with similar internal structure are called homologous traits, and it was eventually decided that these traits were a better measure of relatedness than analogous traits.  Keep in mind, however, that traits can be both analogous and homologous (like a monkey hand and a human hand), it isn't automatically an either / or situation.

There are, in modern taxonomy, two somewhat different approaches to putting together "family trees" of organisms.  In systematics, branches occur when one species splits into two - at the point of a common ancestor to the new branches.  Cladistics is similar, but the focus is on when certain "new," special traits arise - humans might split from chimps, for instance, at the point that our ancestors began walking upright.  Both are applying fairly rigid "branchings" to points that in real time probably were more spread out, and only sometimes do they seem at odds with each other.  Their representations also tend to differ - systematics, the older approach, usually uses fluid, naturalistic branching diagrams, while cladograms tends to branch at geometrical angles and use a lot of straight lines (it was developed when computer printers dealt well with straight but not so much with curvy lines).  There is a third approach that might be added in - molecular evolutionary taxonomy is concerned with when certain key genetic differences arise.  They tend to be an offshoot of cladistics, but that connection may get less clear over time.

Much basic taxonomy is still done anatomically, although the level of detail has gotten smaller through the use of microscopes and broader through the discoveries of genetics and biochemistry.  These will be covered later as they come up in the historical journey.









The original two Kingdoms were Plantae and Animalia (plus the minerals), which remained the only Kingdoms until the middle of the 20th Century.  During the last 40-50 years, those groups have been splintered a bit - Fungi was split off from the plants, Protista removed the single-celled eukaryotes (and the problems of their often-combined characteristics) and Monera was made for the prokaryotes.  Those five Kingdoms have been considered "the" Kingdoms in most basic biology books, even though the splintering has continued.  The latest basic books now recognize a sixth group, the Archaea, once thought to be odd bacteria but now considered a fundamentally different group. 






Prokaryotes (no nucleus);  always unicellular (single-celled).  Bacteria.  May have plant, fungus, or animal characteristics.


Prokaryotes;  always unicellular.  Adapted to unusual and/or extreme conditions, such as very hot, very salty, or no-oxygen environments.  Have several different cellular chemistries from Monera.


Eukaryotes (nucleus in cell);  mostly unicellular, or collections of very similar cells.  May have plant, fungus, or animal characteristics.


Eukaryotes;  multicellular;  capable of photosynthesis, production of complex molecules from simple molecules using light.


Eukaryotes;  multicellular;  must obtain complex food molecules from external source, broken down and absorbed internally.  Usually capable of movement.


Eukaryotes;  almost all multicellular;  must obtain complex food molecules from external source, absorbed through external surface.  Almost never capable of movement.

The modern definition of classification groups depends upon each species in the group evolving from a single ancestral type with the basic group characteristics - plants all share an ancestor with simple plant characteristics, but the ancestor they share with fungi is neither distinctly plant or fungus, so they have been designated into different Kingdoms.  Based upon this criteria, many zoologists think that the Animal Kingdom should be splintered into at least two Kingdoms.  The Protista and the Monera are often "made up" of multiple Kingdoms in advanced books on the subjects.

Keep in mind, like all aspects of classification, this fits into the convenience of human labeling, which doesn't always comfortably fit what the real organisms are doing.  And with that fresh in mind...





Labels require definitions, and species is a very particular label that has been defined in different ways through the past.  It first just meant a distinctly-describable type;  then it was distinct types that could not interbreed;  then it was distinct types that could breed and produce offspring that themselves could go on as adults to breed (some crosses between species can produce young, such as horses and donkeys producing mules, but they grow up to be sterile adults).  Today, the best, latest, nontechnical definition of species is...

Species:  A group that, in natural surroundings, breeds exclusively within the group.

In effect, we now let the organisms themselves determine what belongs to their species and what doesn't.  This still is not a great definition - it says nothing about asexual species.  And, like almost any biological definition, it still has exceptions, such as with coyotes, dogs, and wolves, which can and do interbreed (but not commonly) and are definitely considered separate species.  But it works fairly well.





Informational Links

A book from 1866 on the classification of animals.

Lists of odd species names. 


Click on term to go to it in the text.
Terms are in the order they appear.



Carolus Linnaeus
System Organization Levels
"Newer" Kingdoms
Super- and Sub- groupings
Rules for disputes
Binomial nomenclature
Analogous traits
Homologous traits  

Molecular Evolutionary Taxonomy  

Modern 5-Kingdom System
Determining Groups
Species definition








Online Introduction to the Biology of Animals and Plants.


Copyright 2001-2022, Michael McDarbyContact.

Reproduction and/or dissemination without permission is prohibited.

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