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Author Topic:   The Creationist Challenge - Can You Identify Kinds?
Taq
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Joined: 03-06-2009
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Message 16 of 18 (622164)
07-01-2011 12:22 PM
Reply to: Message 13 by ZenMonkey
06-30-2011 10:22 PM


I don't know about that. I think that the current version of the Linnean taxonomy system is possibly arbitrary in an absolute sense.

Linnaean taxonomy arbitrarily puts species into genera, classes, etc. For example, whether or not chimps belong in the Homo or Pan genus is an arbitrary distinction. It was also an arbitrary decision to put all mammals into one class instead of multiple classes. There is only one non-arbitrary classification in biology which is at the species level. Cladistics solves these problems by rooting species by shared characteristics within a phylogeny.

This is made even more obvious if you think about the history of life. At one point in history, there was only one species of mammal. If we had to categorize life at that point in history with the Linnaean system then this one species of mammal would comprise a single genus within the class Reptilia. So how does a genus become a class within Linnaean taxonomy? Well, it can't. The arbitrary nature of Linnaean taxonomy has a tough time organizing evolving populations over time.

Start with the class Mammalia. Pick two orders within that class, say rodents and primates. Do all the families within one order (e.g. the mouse family and the mole rat family in the rodent order) show the same degree of difference as all the families within the other order (e.g. the great ape family and and the lesser ape, i.e. gibbon family in the primate order)? Would that imply an absolute classification system?

Why are rodents and primates in separate orders?


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 Message 18 by caffeine, posted 07-04-2011 4:57 AM Taq has not yet responded

  
caffeine
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Posts: 1600
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 2.4


Message 17 of 18 (622489)
07-04-2011 4:50 AM
Reply to: Message 13 by ZenMonkey
06-30-2011 10:22 PM


Start with the class Mammalia. Pick two orders within that class, say rodents and primates. Do all the families within one order (e.g. the mouse family and the mole rat family in the rodent order) show the same degree of difference as all the families within the other order (e.g. the great ape family and and the lesser ape, i.e. gibbon family in the primate order)? Would that imply an absolute classification system?

Sorry for my slow reply - been very busy in work. Others have mostly answered this, but a quick couple of thoughts.

If you look at the ranks assigned to different groups, it's clear that they're not consistent. There are amphibian genera that have been around since the Palaeocene, about 60 million years ago, while many mammal genera are less than 10 million years old - Gorilla, Pongo and Homo probably all diverged from each other within the last ten million years.

Now, it's possible of course that this is because these groups haven't undergone evolutionary change at the same rates. Maybe the great apes have had a burst of sudden diversity, while mudpuppies haven't changed much for the last 60 million years. If this were true, though, it creates a clear problem for the idea of objective ranks, though, if we're trying to fit it into a framework that shows relationships.

Imagine we have one genus, with four species: A, B, C and D. A and B are sister species; C is the sister to the clade A+B, and D is sister to the claide C+(A+B). Over a few million years, B experiences intensive selection pressure and undergoes rapid morphological change, while the remaining three species are relatively static.

At the end of this process, B is now sufficiently divergent from A to justify its own genus, in order to be consistent with how we classify genus. But, if classifications are supposed to represent relationships, C and D must also be given different genera to A, otherwise the genus becomes paraphyletic with respect to B. So, we wind up with four monspecific genera, three of which (A, C and D) are all more similar to each other than the objective standard of difference we've agreed upon to define the idea of 'genus'.


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 Message 13 by ZenMonkey, posted 06-30-2011 10:22 PM ZenMonkey has not yet responded

  
caffeine
Member
Posts: 1600
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 2.4


Message 18 of 18 (622490)
07-04-2011 4:57 AM
Reply to: Message 16 by Taq
07-01-2011 12:22 PM


There is only one non-arbitrary classification in biology which is at the species level.

Species is also pretty arbitrary, since there is no single definition of 'species'. The most commonly mentioned one, the Biological Species Definition, is inconsistently applied where it can be applied and, for the majority of species, cannot be applied at all. Most species reproduce asexually.

In a specific context, you can define 'species' and say you're using it non-arbitrarily. When we're making grand comparisons across the whole tree of life, though, species often doesn't mean the same thing and is not an objective classification.


This message is a reply to:
 Message 16 by Taq, posted 07-01-2011 12:22 PM Taq has not yet responded

  
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