Sonntag, 26. Juni 2011

Article of the Week - Taxonomy, Systematic and Cladistic

When you read the articles of my blog, you maybe have noticed, that Botany use many Latin names to describe species, families or genera.

So, in this “Article of the Week”, I want to talk about the basic principles of Systematic and Taxonomy. As former student, I know how difficult and confusing this topic can be, because it requires a degree of abstraction. So, let us begin with one of the basic terms: Taxon

I. The definition of the word Taxon

If you read something about systematic work, you'll certainly have heard this word before. But what is a Taxon exactly? Simply put, this word describes a position within a hierarchical system. The most important thing you have to know is, that the term of Taxon is complete free of any rank. Everything can be a Taxon, regardless of where it stands in a pedigree. So the species Sorbus aucuparia L. is a Taxon within the Rosacae, but the Rosaceae are a Taxon within the Mangnoliopsida. An elephant is a Taxon of the mammals, while the mammals are a Taxon of the vertebrates.

After the term “Taxon” is now clarified, we continue with Pedigrees.

II. Pedigree and Cladistic

A Taxon is always part of a pedigree. A pedigree is a simply method to display affinities. In the most cases, the length of its branches presents the degree of relatedness. The longer the branches, the lower is usually the relationship. Pedigrees can be generated in many different ways. In past, the main method was the comparison of morphological characters, this method was later replaced by molecular genetic methods like PCR (polymerase chain reaction) and DNA-Sequencing. But molecular genetics isn't free of making mistakes, so today a combination of classic and modern science is applied. The following picture shows a schematic pedigree.

In this simple pedigree, Taxon “C” is the least related Taxon to “D”, while B and D are the most closely related species. The Taxa “A”, “B” and “D” form a subgroup, which differs by one or more characteristics from Tacon C. Such a subgroup is also called a clade. Within cladistics (the science of pedigrees and relatives), there are different kinds of relationships. The most important ones are the following.

a) Monophyly

At the Monophyly, all Taxa originate from one common ancestor and share its features. In our example, Taxa A, B and D are a monophyletic, because all have a common stem form (arrow). Taxon C doesn't belong to this clade, because it doesn't originate from this stem form. However, if we would add a new, more basal Taxon E in our pedigree, Taxon C could be a part of this clade in relation to E. It all depends on the angle, you are looking at the pedigree.

b) Paraphyly

The paraphyly is similar to the Monophyly, but in this case, the clade includes not all Taxa. In our example, the Taxa B and D are polyphyletic towards Taxon A, because they have the same ancestor but they have grown a part. Paraphyly can be confusing, but there is a good example from Wikipedia to remember the different: The Birds and today's reptiles descend both from the dinosaurs (they are monophyletic) but today, bird are a calde by their own and the reptiles are now paraphyletic towards them.

c) Polyphyly

In a polyphletic group the single Taxa has no common ancestor but they were summarized, because they share a common character or morphological feature. In our example Taxon D and C are no related, but maybe both are able to fly, so they are summarized to a paraphyletic clade. An example for a Paraphylum are the grasses: Cyperaceae, Juncaceae and Poaceae are complete different Taxa, but they are all called grasses.

The characters, which makes a clade to a Monophylum, are called Synapomorphy, while original features are called Plesiomorphy. If a character is unique to only one Taxon, it's called Autapomorphy.

In our graphic, the Plesiomorphy is a feature, which is common to all Taxa from A to C. Maybe all of this Taxa have four legs or a spine. Now, the clade of Taxa A, D and B has milk glands, which are the Synapomorphy of this Monophylum. Finally, the Taxon B is the only Taxon with Wings, which are its unique Autapomorphy, which differs Taxon B from Taxon D.

III. Systematic

So, after I give you a overview about pedigrees and cladisitc, it's now time to take a look on the hierarchical system of Systematic. Depending on which level the Taxon stands currently, it has a different ending syllable. As an example, we consider the systematic of Sorbus aucuparia L. agg. aucuparia

Kingdom (-tae): Plantae (plants)
-Division (-phyta): Tracheophyta (vascular plants
--Subdivision (-phytina): Spermatophytina
---Class: (-opsida at plants): Mangoliopsida (Angiosperms)
---(-phyceae at alga)
---(-mycetes at fungi)
---(-lichenes at lichens)
---- Subclass (-idae): Rosidae (Rosids)
------ Order (- ales): Rosales
--------Family (-ceae): Rosaceae (Roses)
------- Subfamily (-oideae): Spiraeoideae
-------- Tribus: (-eae): Pyrae
---------- Genus: Sorbus
----------- Species: Sorbus aucparia L
------------ Sub-species: Sorbus aucuparia L. agg. aucuparia

(don't forget, that the term “Taxon” is free from any hierarchy, so Rosales is also a Taxon)

This are not all ranks, but the most important ones. Other ranks are Sub-genus, Sub-tribe or Variety. The zoological systematic follows nearby the same scheme, but has other syllables. From the stage of Genus on, all Names are written inverse (don't ask me why ;-)). In botany, the shortcut of the first-determiner is always Part of the species name. In the most cases, this Shortcut is L. and refers to the Swedish botanist Carl Linnaeus, who created this hierarchical system and the basics of Taxonomy.

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