For those that are still interested let's now look at some of the confusion in Taxonomy.
I've shown the Linnaean System though now in the last few years we now have evolutionary taxonomy, numerical taxonomy and cladism.
Evolutionary Taxonomy was used up thru the 1970's working with fossil material. They first used morpological resemblance the extent to which animals resembled each other. Second they used phylogenic relationships the way animals related to each other in the terms of a recent common ancestor.
The problem with this method is the limitations of fossils and the subjectivity of classification by observations.
Numerical Taxonomy uses quantified observations of animals in an attempt to decide natural groupings. If enough characteristics are measured, quantified and computed they can determine what are called cluster scatters which are used to measure distances between them showing the measure of their differences. However this still relies on subjectivity in the way to analyze the measurements taken and possibly give more weight to more important characteristics.
Cladism is a school of taxonomy founded about 1966 and tends to be the most effective way of determining phylogeny. It relies on polygenetic criteria alone by showing how features shared by organisms display a hierarchical pattern in nature which is evident in the distribution of characteristics shared amongst organisms. Cladism assumes that the recency of common origin can be shown by shared derived characteristics which in closely related groups would distinguish that group from other.
The central concept of cladism is that any characteristic is either derived or primitive. An example being: all vertebrates have backbones so the possesion of a backbone is primitive to all vetebrates and is not an indication of any relationship between any groups or individual vetebrates.
A primitive characteristic of vetebrates is however a derived characteristic compared to invertebrates. The sharing of both derived and primitive characteristics establish the relative status of particular groups of characteristics within organisms.
Below is what is called a cladogram.
A
|
X-----------B
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|
Y-----------C
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Z-----------D
This is a method to determine the recency of common origin in related taxa based on derived and primitive characteristics.
Taxa A and B share a unique common ancestor so are termed sister groups. They share an evolutionary novelty not possesed by taxon C. C however is a sister group of the combination of taxa A and B. D is also a sister group of the combination of the taxa A, B and C.
In the construction of cladistic analysis the taxonomist assumes that dichotomous branching has occured in each linneage and compiles an unweighted character database
There are 3 kinds of cladistic groupings :
Monophyletic groups: containing a common ancestor and all its descendents (A B C D and X Y Z ) or ( A B and X).
Paraphyletic groups: descending from a common ancestor ( probably extinct which is called a stem group. ( A B but not the extinct X)
Polyphyletic groups: results of convergent evolution. These are descended from different ancestors though they may look similar and are in effect an artificial grouping as far as a shared common ancestor with shared characteristics.
(end of third post)
