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>The problem is that flight evolution among vertebrates is not _that_ rare.

The situation may very well be that flight, once evolved, acts as such a
powerful ecological releasing agent that the flying lineage rapidly evolves
to fill a wide range of niches for which flight is advantageous, effectively
closing these niches to other lineages that might have developed flight
later.  Thus it has been speculated that birds and bats coexist because they
have, by and large, partitioned nocturnal and diurnal niches between them
(yes, yes, I know there are nocturnal birds), and that the rise of birds may
have had something to do with the extinction of pterosaurs which, if I am
not mistaken, declined sharply in known diversity well before the K/T.
Similarly flight has evolved in only one line of invertebrates.  This may
explain why, though there is a tremendous range of variation within flying
lineages, the number of lineages themselves is relatively small.

By contrast, specialized gliding does not seem to permit much further
radiation.  As  a result there are far fewer species overall of gliders, but
many more lineages - gliding has evolved at least four and probably more
times in living mammals (depending on how frequently it evolved in
marsupials), while flight has evolved, it seems, only once - yet bats easily
outnumber all the members of the gliding lineages put together.
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court                  Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net
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Toronto, Ontario Canada M5H 3P5