Re: Philosophies for Character Ordering

[David Marjanovic <david.marjanovic@gmx.at>]

----- Original Message -----
From: "Mickey Mortimer" <Mickey_Mortimer111@msn.com>
Sent: Thursday, May 20, 2004 11:03 AM

> Wilson, 1999- "Ordering these implies a developmental model in which
> vertebrae and phalanges are added or lost incrementally.  Embryological
data
> from living organisms, however, do not support this transformational
model.
> The axial column and digits begin as condensations of a certain length
that
> are later divided into any number of segments (Burke et al., 1995).

The length of the digit does seem to determine the number of phalanges,
though. In other words, the number of phalanges reflects the (continuous)
amount of expression of some genes, like that for fibroblast growth factor
4:

Neda Nikbakht & John C. McLachlan: Restoring avian wing digits, Proc. R.
Soc. Lond B 266, 1101 -- 1104 (7 July 1999, IIRC)

Adding enough FGF-4 to the lateral hand edge of chicken embryos can even
make their... I think... 5th metacarpals come back.

> Thus,
> vertebrae or phalanges can change in number without requiring intermediate
> stages.  A model of character evolution that allows one-step
transformations
> between character states (i.e., an unordered multistate character) best
fits
> available developmental information."

Perhaps, but I'm not sure, see above... the amount of gene expression looks
more like an ordered character (small, intermediate, large...).

> I have a few problems with this.  First, leaving it unordered would make
> intermediate states their own derived state.  So seven-sacraled
> confuciusornithids and Protopteryx would be inclined to make a clade
> separate from eight-sacraled ornithothoracines.  It doesn't make much
sense
> in my mind.

Well said.

> It would make more sense as a way to explain the presence of new
> vertebrae, such as in the neck of mamenchisaurs.

Indeed.

> But if one can't tell whether vertebrae are new or merely
> switched areas (dorsal to sacral, etc.), what is one to do?

I'd say keep it ordered -- under the assumption that a change in the total
number of vertebrae (except at the tail tip!) is less probable than a change
in identity (fusion with the sacrum...).

> Another consequence of this is that grouping more than
> one amount of vertebrae under one state would be inappropriate.  For
> theropod tails, you couldn't have "more than 44 vertebrae (0); less than
45
> vertebrae (1)" because it would be just as easy to evolve 50 vertebrae
from
> 49 as it would be to do it from 20.

I would say a slight lenghening of the tail is easier than a dramatic one...
hm... perhaps we should introduce something like gap-opening and
gap-extension costs...

> NDE only allows three states per polymorphism.

Then add the rest manually in PAUP*.

> For changes in a structure's size, Wilson recommends a particular ordering
> strategy called "easy loss".

For some characters this is definitely a good idea -- but it probably has to
be decided on a case-to-case basis what is easily lost and what not.

> Wilson would say, the pneumatic
> diverticulum that occupies the fenestra could halt its growth during
> development, so it should be treated as an easy loss character.  But
> structures usually grow in concert with another structure shrinking.  When
> the diverticulum enlarges, the surrounding bone shrinks.  So why not treat
> the bone growth as easy loss instead of the diverticulum growth?

Because the bone is actively dissolved by the diverticulum.