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 Name
My House Number
My Street
My Town
My County / State
My Country
My Continent
The World
The Solar System
The Galaxy
The Universe

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.

One of 3,500 species of mosquito, what we do know
about this one is that it is female as they are the only
sex that feed on blood and then only when pregnant,
so she's expecting baby mosquitoes too

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".