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Defending grades (Was: Re: Archaeopteryx (rant))



>Dinogeorge@aol.com wrote:
>
>>Just because a system has been used incorrectly doesn't invalidate the 
>>system
>>itself. We can simply learn how to use it correctly. For example, one class
>>(Reptilia) can give rise to another (Aves); why not? Descent doesn't mean 
>>you
> >must< include the descendant group within the ancestral group. And as far 
>>as
>>rank goes, it's purely arbitrary, so why not simply assign the groups their
>>ranks forevermore and be done with it?
>
>What a ghastly idea!
>
>Look, if you want to keep the Linnaean system around for the purpose of 
>classification - Linnaean stamp-collecting, if you will -  then go ahead.  
>But please - PLEASE! - don't incorporate it into any evolutionary or 
>phylogenetic discussions.  Statements like "one class (Reptilia) can give 
>rise to another (Aves)" are not helpful in communicating ideas about 
>evolution.  The term "class" is in itself a statement that a certain group 
>has attained a level of organization superior to that of its ancestral 
>"class".  These ranks (and what they imply of the natural process) have no 
>place in evolutionary discourse.

That one group of animals can give rise to another group that should not be 
included in the ancestral is a biological fact. On the lowest rank level one 
widely distributed species can give rise to several local specialized species 
that by virtue of reproductive isolation from the parent species cannot be 
included in it. The species rank is the one we cannot argue about.  Every rank 
from genus and above is open to discussion. And why not use the idea of new 
groups budding off from the ancestral stock for other ranks than species? Isn't 
that exactly the situation with the bird-deinonychosaur group and the 
theropods? And the genus Homo and Australopithecus? (But the "budding point" 
itself is difficult to handle, -as it should be. :-) )

IMHO statements like "one class (Reptilia) can give rise to another (Aves)" can 
in fact be helpful in communicating ideas about evolution, because it is about 
how new traits come to be. A purely phylogenetic classification system tells us 
basically nothing about the physical evolution of the critters in question. On 
the other hand, a system based on a combination og phylogeny and morphological 
evolution will tell us a lot.

-And, oh yes, before anyone shoots me: I am very familiar with the abyssal 
problems with the union of Linnean classification and cladistics, but dealing 
with evolution we could benefit from keeping two thoughts in our heads at the 
same time.

That a certain "class" should not be included in the parent "class" is not 
necessarily saying that the level of organization is superior in any way to the 
ancestral group. Think about parasites with redused body plans. -It only means 
that it is so different that it would require a serious redefinition of the 
ancestral "class" to include them. Or am I missing the point?

Cheers
Torfinn