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Sonic Hedgehog & (Wings v Feet)



I was looking at the latest SciAm and ran across a article about how a single
gene makes remarkable changes in the structure of a chicks wing during the
development phase. The graphic which accompanies the article is what originaly
drew my attention. It shows the regular wing structure and the altered
structure which has many similarities to a foot or hand type structure. 

What I see happening with such info is something akin to the following:

Some animal is walking around a waterfront and having babies. One of these 
babies happens to have a genetic error in this gene that causes it to have
flippers instead of feet (or reverse the process for aquatic animal). If the
animal normaly lives on vegetation in shallow water and was a herding type
animal then it seems completely possible for it to survive. 

The change would be rapid and would not necessarily prohibit reproduction
of the error. If the error was linked to a environmental factor, say some
pollutant local to a single pond or tributary, then the rate of errors might
be high enough to produce a viable species.