Theoretical courses

 

Folder Classification

What is a classification?

Classification is the ordering of similar entities (objects) in a hierarchical series of nested classes, in which each more inclusive higher-level class is subdivided comprehensively into less inclusive classes at the next lower leve

In biological classifications, the ordered grouping of organisms into classes accords to their similarities and is consistent with the evolutionary history of the organisms in the group. The latter is inferred from their taxonomic characters.

Why do we need classification

The complexity of the living world - be it at genetic, organism or ecosystem level - is so large that scientific explanations become possible only after the diversity is reduced to understandable, manageable groupings. This is achieved through the process of biological classification that has the following objectives:

 

  • it sorts similar and/or related taxa into groups and delimits these against other such groups; 
  • it serves as the summary of the information contained in the groups; 
  • it serves to retrieve information; 
  • it serves as the basis for comparative research. 

 

Biological classification is thus concerned with the hierarchical ordering of organisms into objective (principled) hierarchical groups.

How are classifications made?

Basically there exist two fundamentally different methods of biological classification: phenetics and cladistics. Prior to discussing the differences between both systematic approaches it is worthwhile to mention that both methods deal with as much characters (and their character states) as possible and thus are fundamentally different from the essentialistic/typological methods of the ancients that concentrated only on a limited number of key-characters.

Phenetics, also termed numerical taxonomy, departs from the philosophy that the phylogeny of organisms can never be known to satisfaction and that it thus cannot be used in biological classification. As such, pheneticists argue that grouping by overall similarity is the most objective option to achieve a natural classification. According to the phenetic approach such is possible by considering as much characters as possible and giving each the same weight (or importance). Multivariate statistics on the whole set of character states yields a hierarchical phenogram. It is the topology of that phenogram that dictates the hierarchical classification whereby arbitrary chosen similarity coefficients determine the ranks in the classification.

Cladistics departs from the fact that phylogenetic relationship is the only valid principle by which organisms can be classified. The three basic criterions of cladistics are: apomorphy, monophyly and parsimony. An apomorphy is a derived character state. Shared derived character states (=synapomorphies) define the taxa that are most closely related by common ancestry. Monophyly is the term used for a group of taxa that all share the same common ancestor, all the descendants of that ancestor must be included. Parsimony departs from the hypothesis that evolution is a rare phenomenon and that evolutionary pathway thus has to be as simple as possible, that is requiring the fewest number of steps. The branching diagrams produced with caldistic methods are called cladograms whereby each node represent a speciation event.

Course

(under construction)

References 

 

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