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Re: Dinosaur Revolution Review

 For example (parting ways now) I consider the ease with which
 breeders have grown feathers sticking out from scales in a few short
 decades (centuries?) to show it's really not challenging to produce
 an intermixed dermal type with feathers (or fuzz) and scales. Yes,
 it's very uncommon in extant birds, but I consider that a derived
 condition that saves on metabolic cost (birds have long since lacked
 a need for scales on most of the body, so why not make sure there's a
 gene to prevent them from being grown where they aren't needed).

 As an analogy consider avian beaks and teeth: Tooth placodes have
 been famously shown to still be viable in extant chickens, just not
 in their beaked mouths, where they are actively suppressed. Given
 the developmental genetics of extant birds one could conclude that
 it's therefore impossible for beaks and teeth to co-exist in the
 same animals mouth...after all, the presence of beaks turns off
 tooth development! But of course in this case we have an extensive
 fossil record that shows that teeth and beaks co-existed in many
 dinosaurs (including early "birds"). We therefore accept that there
 were several evolutionary "experiments" with various configurations
 of tooth placement and morphology that coincided with various
 configurations (and morphologies) of beaks.

Another example: In insects, expression of the Hox gene abdominal-A inhibits leg formation. In the other arthropods, it does no such thing, despite producing the rest of the abdomen phenotype. (In caterpillars, abd-A expression is secondarily turned off precisely in those tiny patches where the abdominal legs are to form.) A gain-of-function mutation has occurred.