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

Torfinn Ørmen <torf-o@online.no> wrote:

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.

My objection is to the use of the word "class". It's typological baggage. Linnaean hierarchy (Kingdom, Phylum Class, Order) is not an evolutionary concept, but (as pointed out) a human construct, and entirely arbitrary. Linnaean taxonomy may have a use in simple bookkeeping - Linnaean stamp-collecting, as I said in a previous post - but not in evolutionary discussion.

For a nice take on this topic, check out http://www.cmnh.org/dinoarch/2002Jan/msg00992.html

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.

Uh-huh. Cladograms are based on anatomical characters (and sometimes molecular data too, for extant groups). You can have your cake and eat it too.

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?

No serious redefinition is required. Clades are typically not defined on the basis of anatomical characters of behaviors. For good reason...


Anatomical and/or behavioral traits are used to *diagnose * clades. But even here, not every member must exhibit this trait. The bird clade (Avialae) is diagnosed by the acquisition of powered flight, which is evident in the first members of the clade. But loss of the ability does not result in a species' expulsion from Aves. Kiwis, dodos, penguins and flightless rails are still members of Aves.

Your example of the "parasites with reduced body plans" conforms to the same theme. Thus, highly morphologically divergent taxa are still included in their "parent" clade.

For a nice analogy, read http://www.cmnh.org/dinoarch/2000Aug/msg00156.html (scroll to the bottom.)


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