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



--- Torfinn_Ørmen <torf-o@online.no> wrote:
> 
> That one group of animals can give rise to another group that should not be
> included in the ancestral is a biological fact.

No, it's a human construct, and, I would argue, not a very useful one.

The only fact is that one individual organism's genotype can be copied
(partially or totally, perfectly or imperfectly) to another individual
organism's genotype. A clade (at least, in some usages) consists of an ancestor
plus all of those organisms which have, directly or indirectly inherited part
of its genotype. In this sense it can be said to be a natural entity -- take a
certain genotype, all copies of it, and include the organisms "built" from
them.

To draw a line at an arbitrary point and say, "This no longer counts as part of
the original lineage," is artificial.

> 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.

But avians are not a separate entity from reptilians -- they are modified
reptilians. And not very much modified, if you consider only the changes right
at the _Aves_ node. Most "avian" features were acquired before it (feathers,
wings, bird-like feet) or after it (pygostyles, toothlessness, carpometacarpi).

"Avians are a type of reptile," conveys the idea better, IMHO. It points out
that they have something setting them apart but are stil part of the lineage.
PT works perfectly with the idea of "descent with modification".

> A purely phylogenetic classification system
> tells us basically nothing about the physical evolution of the critters in
> question.

A classification tells us nothing physical, really. It's just names and
patterns.

> 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?

I don't see how "class" could be redefined when there isn't a solid definition
of it to begin with.

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