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Re: tiny dinosaur
- To: firstname.lastname@example.org
- Subject: Re: tiny dinosaur
- From: Jonas Weselake-George <email@example.com>
- Date: Sat, 18 Jun 2011 10:40:12 -0400
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On Sat, 18 Jun 2011 17:04:00 +1000
Tim Williams <email@example.com> wrote:
> Umm...what? Surely flight evolved only once in theropods?
Well. true flight has evolved at least once in mammals and gliding
ability evolved at least twice independently (flying squirrels and
flying possums). A fourth time if you include heavier-than-air
Judging by what the local Grey Squirrels do when chased off of a branch
(stick out the four legs to the sides while falling), I'd say that a
third evolution is quite possible.
Of course, the interesting question here is: If two species in the same
genus independently evolve in a direction - are we talking about
separate occurrences? Similarly, we could ask if related species, in
similar niches, with similar bauplans evolve in the same direction - is
gliding evolving separately or once using the same evolutionary pathway?
I'd tend to suggest that flight is evolving separately if there is
reproductive isolation. In this case it is almost inevitable that
some degree of flight evolved several times among theropods and
probably re-evolved (eg. in the case of a marginally flying species
which starts becoming flightless only to later take a ground up
Of course, the evolutionary pathways would be similar and only one (or
two) branches would gain a global distribution, and eventually evolve
in such a way as to displace other competitors from the most
(evolutionarily) stable habitats and survive.
Meso-evolution is messier than micro-evolution or macro-evolution.
We just don't see it because it isn't easy to observe at either the
ecological or the geological timescales.