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Re: flapping from gliding



On Mon, 30 Sep 1996, Stan Friesen wrote:

>  > EXCEPT - if gliding has evolved so frequently ... why did powered
>  > flight evolve only
>  > three times in tetrapods?  If this were indeed the pathway I would have
>  > expected a lot more flyers.  The fact that flight has evolved so rarely
>  > suggests to me that it developed by an infrequently, not a frequently,
>  > "chosen" pathway.
> 
> Hmm, good point.
> 
> However, is it no possible that the "difficult" step is the transition
> from gliding to powered flight?

        That was my thought. Say you have to climb some mountain, and it's
really difficult. Perhaps a hundred people reach the base, and one or two
of them actually ever make it up. You still have to start at the base to
get up, but it's just not very likely you'll actually get up there. I hate
to use metaphors, so maybe I'll try an example:
        This isn't exactly "ground up" but it sure as heck isn't
"trees-down": aquatic fliers. There are a number of animals, which I'm not
sure whether we profit by bringing into the discussion, that leap into the
air from the water and glide. There are two and four-wing flying fish (are
these related?) the flying gurnard (I think...) and, unless Iheard
wrong, some kinds of squid. I'd guess they evolved leaping to evade
predators, started using the pectorals to extend the leap, then enlarged
them. And there is one powered flier, the hatchetfish, which can beat its
pectorals to get quite a bit of distance. I don't think that makes it
unlikely that the hatchetfish evolved along these same lines, from
leaping to gliding and finally to powered flight. Maybe it does, perhaps
it used the pectorals to accelerate from the water (I don't know enough
about them to say) and evolved straight to powered flight. I'd assume at
least some enlargement for gliding would be necessary before they'd be
large enough to make decent flapping wings, however.
        The big mystery is insects. There aren't many insects today
that give a clue to how flight may have evolved- perhaps this is because
the overwhelming majority of them today already are winged, so they don't
really need to evolve winglike structures. 
        Nick Longrich