MrDarwin

Let's take wings. What good is half a wing? Well, it depends very much on what you mean by "half a wing". The fossil record shows quite clearly that birds never had "half a wing"; rather, the early birds had an appendage--the reptilian forelimb--with a dual function: it was both a leg and a wing at the same time, and could be used for walking, running, grasping, and flapping. (In fact, the forelimbs of the earliest birds, except for their feathers, are almost indistinguishable from the forelimbs of small dinosaurs.) Over time this limb evolved to increase the efficiency of one function at the expense of the other, presumably because using it as a wing was far more advantageous than using it as a leg.

This seems to be a pattern in evolution: organs don't appear from thin air. Rather, they evolve from previously existing structures, which may have had a completely different function, but at some point had a dual function that changes over time. (Another example is the mammalian ear bones, which are highly modified jaw bones.)

In fact, the origin of wings is a pretty trivial problem at this point, since we can trace the wing back through the dinosaurian forelimb, through the early tetrapod forelimb, through the fish/amphibian transition, through the lobefin fish fin, right back to the earliest finned bony fish. At no point did the structure appear from nowhere; it was only modified along the way. The only non-trivial problem is where that earliest fin came from in the first place.

lpetrich

How side fins formed is certainly a riddle; one theory I've seen is that they are ectopic axial (top and bottom) fins.

And asking about "half a wing" is asking the wrong question, because as Mr. Darwin had pointed out, evolution does not work that way.

One good example of features being modification of earlier features is flower parts. Sepals look a lot like leaves, and petals like specially-colored leaves; they even have the same overall shape and venation pattern of the plant's "true" leaves.

[ August 12, 2002: Message edited by: lpetrich ]</p>

Oolon Colluphid

I read a while back of some experiments where a simple bit of gene tinkering produced flowers with leaves instead of petals. Didn’t note it down at the time ... anyone remember (Mr D)?

Oolon

MrDarwin

There's been loads of genetic and developmental studies of plant organogenesis. Try searching on "Arabidopsis thaliana", a little weed in the mustard family which is now the experimental subject of choice for these developmental studies.

nogods4me

Properly seasoned, one can make quite a tasty stew with half an appendage.

lpetrich

Here is a page on <a href="http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Flowering.html" target="_blank">the triggering and patterning of flowers</a>.

Flowers have four sets of parts, from inward to outward:

Carpels (the form the pistil)
Stamens
Petals
Sepals

At least in the plants that have been studied in detail, like Arabidopsis thaliana, the formation of these parts is controlled by three sets of transcription factors, A, B, and C.

Carpels: C
Stamens: B and C
Petals: A and B
Sepals: A

Mis-expression of A, B, and C genes can cause a flower part to develop as the "wrong" kind of part, in exact analogy to such famous fruit-fly mutations as Antennapedia.

However, the genes involved have very different structures, with the plant ones being "MADS box" genes instead of homeobox ones. Which is consistent with the separate evolution of this patterning.