Little Piggies of Urodeles
A recent set of exchanges between myself and a friend brought me upon
this Answers in Genesis article. In it, Jonathan Sarfati (an
acid-tongued spokesperson for AiG) claims that the digit development
of frogs is completely different from that of all other tetrapods. In
it, he claims, frogs grow their digits from developing buds. PZ Myers
responded to Sarfati's claims about bird/dinosaur digit homology and
amphibian digit development.
A minor correction is worth publishing. In the article Myers states:
We've got a pretty good handle on the outline of limb development
in multiple tetrapod lineages now, and they all use the same tools.
Contrary to Sarfati's implication, they all have apical ectodermal
ridges (with some rare exceptions in a few highly derived,
direct-developing frogs) and zones of polarizing activity, they all
use the same set of molecules, including FGF-4 and FGF-8 and the
same Hox genes and retinoic acid and BMPs. If there's one thing we
know, it's that limb development is dazzlingly well conserved.
Despite the fact that I think most of Myers's article is a wonderful
vulgarization of limb development, this last part is an overstatement.
It is true that many aspects of limb and digit development in
tetrapods is remarkably well conserved throughout tetrapods, but it is
not true that all tetrapods have an AER, it is also not true that they
all use apoptosis to create the interdigital spaces. All salamanders
that have so far been studied grow their digits from buds and lack an
AER, much like Sarfati explained for frogs (only Sarfati attributes it
to the wrong animal). Apoptosis has only been detected in the
developing digits of one salamander Desmognathus and its role in limb
development is not clear (Franssen et al. 2005). It is also the case
that salamanders begin digit development with digit 2, which is also
in stark contrast with the majority of other tetrapods which develop
digit 4 first. This is an interesting and perplexing problem for
developmental biologists.
So does this mean that Sarfati is right? Does this mean that the limbs
of salamanders cannot be homolgous to those of other tetrapods? The
problem comes from this statement by Sarfati which exemplifies a
rather deep ignorance of developmental biology (and I'm not even an
expert!):
If the birds evolved from dinosaurs, then one would expect common
genes. These in turn would code for a common development in the
embryo.
This is both overly simplistic and fallacious, since we already know
that most of the "genetic toolkit" for animal development is highly
conserved. The fact is, the same genes are used in nearly all
tetrapods. In fact, vast numbers of the same genes are involved in
similar processes in all animals. Some genes have been duplicated and
modified to produce different functions -- which is precisely how
animal form evolves!
As Myers points out beautifully, and what is the most important point
here, is that the patterning mechanisms are conserved. We know that
developing tetrapod limbs use a special hierarchical pattern of Hox
genes that lay out the "construction zones" in a developing embryo. We
see remarkable conservation in both the Hox genes and the placement of
special region known as the zone of polarizing activity (ZPA) which
plays a role in setting up anterior-posterior polarity of the
developing limb.
The emerging picture of salamander limb development is that there are
a striking number of "mechanical" differences in the way the
construction takes place. The order of digit development and many of
the processes that make the digit protrusions are different. However,
what doesn't appear to have changed, is precisely what we would expect
if the digits are homologous: the genes that control the identity of
the digits.
Animal development doesn't work in the same way as humans construct
buildings. It's not as logical as we would think it is. In fact, it's
rather perplexing and enigmatic and often proceeds along seemingly
very illogical courses. The process of making a digit (as in, a
protrusion of the distal part of a limb) is under different controls
from the process that says what those digits are (the thumb vs.
forefinger, etc.). Sarfati's oversimplification is academically
irresponsible. It is flat-out wrong making the entire thesis of his
argument baseless.
_________________________________________________________________
Franssen, R.A., Marks, S., Wake, D. and Shubin, N. 2005. Limb
chondrogenesis of the seepage salamander (Desmognathus aenus(Amphibia:
Plethodontidae). Journal of Morphology 265:87-101
posted by Martin Brazeau at 3:57 PM 0 comments links to this post
Museums respond to creationists
The New York Times has an excellent article on what museums are now
doing to combat rabid creationists. It starts with an appropriate
anecdote that points out how most creationists that one is likely to
encounter are not willing to learn. They ask questions hoping you
won't have an answer, they're not looking for an answer:
ITHACA, N.Y. - Lenore Durkee, a retired biology professor, was
volunteering as a docent at the Museum of the Earth here when she
was confronted by a group of seven or eight people, creationists
eager to challenge the museum exhibitions on evolution ... their
queries coming so thick and fast that she found it hard to reply.
That being said, Dr. Warren Allmon of the Museum of the Earth, made
this important distinction:
There is more than one type of creationist, he said: "thinking
creationists who want to know answers, and they are willing to
listen, even if they go away unconvinced" and "people who for
whatever reason are here to bother you, to trap you, to bludgeon
you."
One must remember, that the public face of creationism comes from
people like Kent Hovind, Ken Ham, Duane Gish, and that rabid troll in
the internet forum that just read a couple of pamphelets by these
savants of Science and Scripture. However, many folks out there want
to know and want to understand. The best we can do is provide the best
possible resources for people who want to learn.
Keep on bloggin'!
posted by Martin Brazeau at 11:13 AM 0 comments links to this post
Welcome to The Lancelet!
Add one more biology blog to the list, here comes The Lancelet! Here
you'll find the random musings and deluded ramblings of a
palaeontology graduate student.
Why "The Lancelet"? This is the common name for an animal sometimes
known as Amphioxus (Branchiostoma) - a primitive chordate that has
long functioned as a living archetype for the ancestor of all
vertebrates... maybe that will be a post for later this week: the
history of The Lancelet... but I'm sure there are a number of those on
the web.
Books I'm reading:
The Vertebrate Body by A.S. Romer and T. Parsons. Because I never
received any formal training in anatomy while I was an undergraduate,
I now have to spend my time teaching these important lessons to
myself.
From DNA to Diversity S. Carroll et al. Because, when one reads about
anatomy they want to know "how the hell did this evolve!?" Carroll and
co-authors have bring the complex answers to this question down to
lowly wannabe anatomist like me. I recommend it for anyone with a
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