The Classification of Living Things
The need for a system of categorizing
living organisms and how it is done
Biological specimens collected by Charles Darwin
on his voyage on HMS Beagle 1831-1836 A system of classification is necessary because of the overwhelming abundance of the variety of life on earth. There are currently around 1.5 million species that have been described, i.e. they have been given scientific names and can be identified if they are found. In 2011 it was estimated that there are altogether about 8.7 million species on earth (give or take 1.3 million), of these around 86% on land and 91% in the seas have yet to be discovered! Though to be fair many of them are probably quite small and lots will be similar to ones already found or to each other.
Organisms are classified together because they are similar, and they are similar because they stem from a common ancestor, as pointed out by Charles Darwin.
The main purpose of modern systematics, the science of classification is to arrange biological organisms so that it reflects the evolutionary history of those organisms.
In order to make sense of the overwhelming variety of life on earth, there are a number of groups at different levels called taxa to which an organism will belong.
Can you remember when you were little and wrote your address as:
My House Number
My County / State
The Solar System
It works a bit like that, but the different levels, called taxa (singular taxon) have specific names that are equally important in the way that your street is still your street whether it goes from 1-10 or 1-10,000 and your county or state is still your county or state whether it's Rutland, England at 382 sq km or Western Australia at 2,645,615 sq km.
On this scale you are:Eukarya - Domain
Animalia - Kingdom
Chordata - Phylum
Mammalia - Class
Primates - Order
Hominidae - Family
Homo - Genus
sapiens - Species
Note that all taxa start with upper case letters apart from the species which is the only one with lower case.
Hence we are Homo sapiens or H. sapiens
You'll notice that Vertebrata doesn't come in any where, this is because there can be additional in between groups such as sub, super, infra and others. In the case of vertebrates, the Vertebrata is a Subphylum of the Phylum Chordata.
Generally you can get by without worrying about the details. The important things are what the major groups are, the kinds of plants or animals they contain, and then right down at the bottom of the list, the genus and species that gives you the unique name of the organism you are looking at.
There is only one Homo sapiens (genus - Homo (man), species - sapiens (wise), just like there is only one Felis catus, the domestic cat. When a new species is discovered it is given a unique name in this format, if it is very like another already recognised species, it may have the same genus, if it is not much like anything else, it may be give a new genus of its own, or even family, order or higher grouping if it is very different.
If you didn't know already, all of these group names are derived from Latin and are referred to as the Latin or Latinized name of a species, or more correctly the binomial name of a species, because there are two parts.
Some other binomial names:
Bufo bufo - the common
Pathera leo - lion
Panthera tigris - tiger
Hippopotamus amphibius - the hippopotamus, one of the few times where the Latin name is the same as the common name
Panthera pardus - leopard
Giraffa camelopardalis - giraffe
Pan troglodytes - chimpanzee
Chrysiridia madagascariensis - Madagascan sunset moth
Quercus alba - white oak
All living organisms that are know to science have such a name. As well as showing useful information about other organisms that they are related to, these Latin names make it easier for scientists (and other people) to know what they are talking about as the same species can have different names in different parts of the world or even the same country, and sometimes the same name can be used for different organisms altogether.
For instance, Puma concolor is alternatively known as: cougar, puma, mountain lion, ghost cat, catamount amongst others.
At the other extreme, there are about 3,500 species of fly that could be called Mosquitoes. It can be very important to know whether you are talking about Anopheles fundus, Anopheles gambiae or Anopheles funestus especially when planning ways of preventing the spread of malaria, while at the same time you don't want to get confused with Mansonia titillans, all four of which are amongst those 3,500 "mosquitoes".
A Historical Perspective
In days gone by organisms were classified by simple means.
"Lives in the sea, must be a fish" - easy
Anything that lived in water was a fish.
As well as fish being fish, crabs, lobsters, jellyfish, marine worms etc. etc. were all also considered to be fish.
Even barnacle geese were considered
to be fish. They were thought to grow on trees somewhere
"foreign" then hatch and fall to the water when they
became geese - so the Catholic Church considered them
fish so they could be eaten on Fridays (quite handy
if you were Catholic and didn't like fish much).
As time went on, people started to look more carefully at animals and plants, their body plans and internal features as well as external features. Lifestyles and habitats were realised to be of little or no value in determining fundamental similarities.
Initially anthropocentrism was quite significant (the giving of importance to animals that are similar to ourselves), but slowly over centuries and increasingly so in the last 50 years or so we are arriving at a position where we are understanding the variety of animal life and the relative size and influence of the various groups.
The study of the classification of organisms is called systematics.
Europeans didn't have a monopoly on being odd. The following is taken from the Chinese Encyclopedia:
"Celestial Emporium of Benevolent Knowledge"*
which categorizes animals as follows:
* Most probably actually made up and not Chinese at all.