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Discuss: The Cladistic Method of Taxonomy
10 years ago
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Blog: The Cladistic Method of Taxonomy
(2 comments) — the past, various methods of classifying the organisms of Earth have been tried. First, there were only two kingdoms, Animals and Plants. Then a third kingdom, the Protists...  more

10 years ago

hi dale...great share thanks...yes the priciples of evolution and creation are opening their dynamics further and further to us...our sciences established on practice is re-aligning constantly with actuality...the chain of physical events is an amazing flow of links...each unique and within a structured purpose but reliant also on chaos to cultivate itself unrestricted from its origional origin...earths air...

all on and in this space of earth has cultivated from the biggest and most present invisible force...the air...what is transparent holds a greater magnitude than the apparent infront of us...we are what we eat/consume and we are physicallywhat we come from...this is a circle of evolution and creation that has its own unique and individual philosophies and facts...

thank you again dale for this share

x tracy

10 years ago
Cladististics(?) is a t the very least, a great tool.  There are some objections to some of the stronger claims, but I'll have to brush up before I can contribute much.  I know Gould said he wasn't a cladist.
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10 years ago
10 years ago

Checking Gould's Structure of Evolutionary Theory, I find that he does state that he isn't a cladist, but that he seems to use, or at least cite cladistic arguments.  His disagreement seems to be NOT with the use of cladism in taxonomy, which he uses himself, but that "cladism" whatever it means, neglects rates of evolutionary change which Gould is (was) strongly committed to exploring and understanding.  This seems a subtle point, because he implies that Niles Eldredge might be a cladist, by denying he, Gould, is a cladist but not denying Elderdge is. From Wikipedia:   (  )

"Transitional forms and cladistics

Before the general acceptance cladistics in paleontology, evolutionary trees were often drawn as the emerging of one group from another. The transitional forms were placed at the borders of these. With the establishment of cladistic methods, relationships are now strictly expressed in so-called cladograms, illustrating the branching of the evolutionary lineages.

The different so-called 'natural' or 'monophyletis' groups form nested units that do not overlap. Within cladistics there is thus no longer a transition between groups, but a differentiation within groups. In this context, transitionals can be defined as the different branches of a cladogram between one particular branch and the crown-group, i.e. the group that is placed at the end of a lineage."

So it appears Gould's issue was not with how cladistics works, but with what it ignores or minimizes, namely evolutionary rates and the transitional process.  I will report more if I find anything more that is useful in Gould's monumental, and imperfectly organized, Structure of Evolutionary Theory.

Support for the cladistic method of taxonomy
9 years ago

Using trees for classification

Clearly, evolutionary trees convey a lot of information about a group's evolutionary history. Biologists are taking advantage of this by using a system of phylogenetic classification, which conveys the same sort of information that is conveyed by trees. In contrast to the traditional Linnaean system of classification, phylogenetic classification names only clades. For example, a strictly Linnaean system of classification might place the birds and the non-Avian dinosaurs into two separate groups. However, the phylogeny of these organisms reveals that the bird lineage actually branches off of the dinosaur lineage, and so, in phylogenetic classification, the birds should be considered a part of the group Dinosauria.

The Dinosauria, including birds

Advantages of phylogenetic classification
Phylogenetic classification has two main advantages over the Linnaean system. First, phylogenetic classification tells you something important about the organism: its evolutionary history. Second, phylogenetic classification does not attempt to "rank" organisms. Linnaean classification "ranks" groups of organisms artificially into kingdoms, phyla, orders, etc. This can be misleading as it seems to suggest that different groupings with the same rank are equivalent. For example, the cats (Felidae) and the orchids (Orchidaceae) are both family level groups in Linnaean classification. However, the two groups are not comparable:

  • One has a longer history than the other. The first representatives of the cat family Felidae probably lived about 30 million years ago, while the first orchids may have lived more than 100 million years ago.

  • The have different levels of diversity. There are about 35 cat species and 20,000 orchid species.

  • They have different degrees of biological differentiation. Many orchids belonging to different genera are able to hybridize. But the same is not true of cats — house cats (belonging to the genus Felis) and lions (belonging to the genus Panthera) cannot form hybrids.

There is just no reason to think that any two identically ranked groups are comparable and by suggesting that they are, the Linnaean system is misleading. So it seems that there are many good reasons to switch to phylogenetic classification. However, organisms have been named using the Linnaean system for many hundreds of years. How are biologists making the transition to phylogenetic classification?

Switching to phylogenetic classification

Biologists deal with phylogenetic classification by de-emphasizing ranks and by reassigning names so that they are only applied to clades. This means that your use of biological names doesn't have to change very much. In many cases, the Linnaean names are perfectly good in the phylogenetic system. For example, Aves, which is the class of birds in the Linnaean system, is also used as a phylogenetic name, since birds form a clade (right).

Most of the specific names that you are accustomed to using (e.g., Homo sapiens, Drosophila melanogaster) have not changed at all with the rise of phylogenetic classification. However, there are some names from Linnaean classification that do NOT work in a phylogenetic classification. For example, the reptiles do not form a clade (and cannot be a named group in the phylogenetic system) — unless you count birds as members of Reptilia too.

Aves is a valid group in both classifications


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