To best understand the science of classification, it will help to first examine a few basic terms:
- classification - the systematic grouping and naming of organisms based on shared structural similiarites, fuctional similarities, or evolutionary history
- taxonomy - the science of classifying organisms (describing, naming, and categorizing organisms)
- systematics - the study of the diversity of live and the relationships between organisms
Types of Classification Systems
With an understanding of classification, taxonomy, and systematics, we can now examine the different types of classifications systems that are available. For instance, you can classify organisms according to their structure, placing organisms that look similar in the same group. Alternatively, you can classify organisms according to their evolutionary history, placing organisms that have a shared ancestory in the same group. These two approaches are referred to as phenetics and cladistics and are defined as follows:
- phenetics - a method of classifying organisms that is based on their overall similarity in physical characteristics or other observable traits (it does not take phylogeny into account)
- cladistics - a method of analysis (genetic analysis, biochemical analysis, morphological analysis) that determines relationships between organisms that are based solely on their evolutionary history
In general, Linnaean taxonomy uses phenetics to classify organisms. This means it relies on physical characteristics or other observable traits to classify organisms and does consider the evolutionary history of those organisms. But keep in mind that similar physical characteristics are often the product of shared evolutionary history, so Linnaean taxonomy (or phenetics) sometimes reflects the evolutionary background of a group of organisms.
Cladistics (also called phylogenetics or phylogenetic systematics) looks to the evolutionary history of organisms to form the underlying framework for their classification. Cladistics therefore differs from phenetics in that it is based on phylogeny (the evolutionary history of a group or lineage), not on the observation of physical similarites.
Cladograms
When characterizing the evolutionary history of a group of organisms, scientists develop tree-like diagrams called cladograms. These diagrams consist of a series of branches and leaves that represent the evolution of groups of organisms through time. When a group splits into two groups, the cladogram displays a node, after which the branch then proceeds in different directions. Organisms are located as leaves (at the ends of the branches). To view an example of a cladogram, see Cladistics.
