What is a 'tree of life'
The fundamental objective of a tree of life is to schematically represent the relationships between different species, and groups of species. Every species of organism, both extinct and extant, is derived through evolution from the simplest organisms that first evolved hundreds of millions of years ago. Therefore, it is theoretically possible to discern how all species are related to eachother, from bacteria to humans. In practise however, this rapidly becomes a complicated and unrealistic goal.
In the same way that a family tree provides an understanding into the relationships between generations of a family, a phylogenetic tree gives an idea of how organisms are related to one another, by reconstructing evolutionary events. Although such data could be presented in a table, they become far more readily accessible and far easier to understand and interpret when converted into a pictorial format in which all distances are summarised in a single-branching, tree-like diagram (Majerus et al 1996).
There are fundamentally two major processes used to translate the morphological or molecular data obtained for species into phylogenetic trees. These are phenetics and cladistics (click for more information). Although both are very different, they strive to achieve the same goal. There have been heated arguments over the relative merits of phenetics and cladistics, with both approaches contributing to the advancement of phylogenetics. One outcome of the competitive use of both methods is that they have rescued comparative morphology as a process of phylogenetics, from dormancy.