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>But is it reasonable to expect to see
>two distinctly different derived characters in ONE animal?  Besides, you're
>not talking about flight, you're talking about incubation--or brooding, or
>whatever.  If incubation and/or brooding was the primary selective agent, why
>not one solution?  If you insist on two, why not flaps of skin on the right
>side of the body and feathers on the left?  Or feathers on the leading edge
>and flaps on skin on the trailing edge?
It is of course possible that seperate selective pressures were working on both brooding and flight characters SIMULTANEOUSLY. I am afraid nature isn't as tidy as paleontology would like it to be. Selectional pressures can and do operate at the same time, EVEN IF THEY ARE CONFLICTING.  In artificial selection programs this is known as tandem breeding (however breeders will try to avoid conflicting selectional pressures). Now one selectional pressure (character) obviously came first, but not necessarily for all animals in a population. Please note I stated Not Necessarily. But nothing prevents both traits from being selected for at the same time.
    |    select trait 1
    |                     ____________ hold trait 1 (hopefully)
    |                   /                     /
    |                 /                      /
    |                /                      /
    |              /                       /    select trait 2
    |            /                       /
    |          /                       /
    |          ------------------------
    |_____hold trait 2_________________________
                                                                                              x axis             Time---->
                                                                                              y axis              Benefit---->