quote:
Originally posted by Tranquility Base:
The point is that the LCA of flies and chicks did not fly!
But the LCA and the intermediate forms all had LIMBS, and contrary to your peculiar interpretation of this article, these genes are, indeed, useful in the expression of ALL limbs (and antennae, too). Not surprisingly, such useful genes are highly conserved. Therefore, their continued function as limb expression, orientation, and position regulators is equally unsurprising.
Read some more from Shubin to clear up you confusion...
"
Hox genes play a critical role in the development of the vertebrate axis and limbs, and previous studies have implicated them in the specification of positional identity, the control of growth, and the timing of differentiation.
These Hox genes [including sonic hedgehog]
were initially involved in specifiying region identity along the primary body axis, particularly in caudal segments. This function is similar to the role of Hox genes in Drosophila development. "
to whit...
"In summary, hedgehog is a necessary element in the establishment of polarity during segmentation of the fly, and during the development of appendages"
In other words, the same gene that regulates the anteroposterior polarity of Drosophilia parasegments that later develop into head, thorax and abdomen (and thus prevents feet sticking out the fruitflies' back) is also used to establish polarity in the digits of the autopod compartment of all tetrapods (so you don't get thumbs where your pinkie should be).
In tetrapods,
sonic hedgehog performs this polarity control function during growth of the zeugopod and autopod compartments. Note that this occurs in ALL tetrapods, not just flying ones, and in both feet and wings (or arms).
In
Drosophila, hedgehog performs a similar task to control the polarity of the parasegments that will produce legs, wings, and antennae. Since arthropods lack any semblance of zeugopods and autopods, claiming that "
the same genes are specifically used, in the same way, to set up wing anatomy. " as you put it, is wildly wrong.
The only common function in these homologous genes is that they are both used to control polarity. The fact that they do it using the same biochemical pathway and the same expression pattern is evidence of their common ancestry, not design, since establishing polarity in body segments has been essential since the LCA of chicks and flies. Not surprising at all. What
is surprising is that you, trained in genetics and biology, would miss this obvious fact.
http://sdb.bio.purdue.edu/fly/segment/abdmlb2c.htm
http://sdb.bio.purdue.edu/fly/segment/hedghog1.htm
If you weren't so desperately eager to shoehorn any likely sounding data into your anti-evolutionary bias, you might have learned something about how useful genes like these are co-opted into new functions in different lineages long after their LCA.
The article you cited even told you this up front - "
new features are often cut and pasted on different groups at different times." ... in other words, polarity regulating genes were borrowed from their pre-existing function in body segment polarity specification to perform a new polarity specification in the last two compartments of the tetrapod limb.
If you step back a bit and look at the larger picture, you might be interested in learning about how the Hox gene duplication events in evolutionary history are reflected in the present homeotic gene sets for arthopods (one), jawless fishes (3), and jawed fished and tetrapods (4 or 5). Thus the duplicated Hox genes became available to regulate the new structures at each major stage of evolutionary development. You might even argue that the duplication event
caused the evolution of the new structures (head, jaw, tetrapod limbs).
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