This write up is not about relationships on a romantic or friendly level, but how being related is viewed form an evolutionary viewpoint. (I shall make the topic more simple by talking only about animals - not bacteria, plants etc.)

Are organisms related and if so, to what extent? This brings us to our first rule - All life is related i.e. all life originated in a single 'incident'. So the question now becomes to what extent is one organism related t another?

Firstly what are we talking about when we say biological organisms?
Usually an organism is defined as a living thing made up of organs (before you say it -yes this is not the only definition). So now we have our complex organism, we need to put them into groups. To do this we section them in to species, if two organisms are capable of reproducing to produce viable offspring they are of the same species. Usually you can tell this be common sense, but not always!

There are many sub groups which species can be placed e.g. vertebrates. The main reason for this is to try and match characteristics to ascertain relationship, a bit like giving you a family name and then positioning you within that family e.g. Wharf son of Mouge.

To measure a relationship between two organisms it is necessary to construct a family ‘tree’ made up of nodes. The result of this is a structure containing all present day organisms being looked at (external nodes) as well as past ancestors. The ancestors shown are always common to two or more organisms and are called internal nodes. These internal nodes are placed between two organisms in direct proportion to how related they are, the closer an organisms relation, the closer there ancestor node lies.

Key :
E = External node (present day organisms)
(I) = Internal node (ancestor common to 2 or more organisms)

E   E   E  E      E
 \    /     /    \      /
  \  /     /      \    /
   \/     /        \  /
  (I)   /          \/
     \  /          (I)
      \/           /
     (I)         /
        \        /
         \      /
          \    /
           \  /

As you might have figured out, these ancestors are inferred and these inferences are based on the rule – all organisms are related (to some extent).

A tree can now be constructed by looking at genetic similarity as opposed to classical methods. This makes it possible to show a quantifiable difference between organisms and a date of divergence given. The amount of divergence in relation to a suitable atomic clock, can then answer how related two organisms are.

An alternative (none quantifiable) method for determining relationship between organisms is RFLP analysis.

At this point you probably have more questions than when you started, if this is so then good! I have deliberately left out certain areas of which I have written about before. Research what you don't know, expand your horizons. I am currently writing a little more about this topic (some of which will be inane rambling) so keep posted. If you have any specific questions - please dont hesitate to ask, I will be happy to clarify anything which seems a little vauge)