Wings and intermediate phenotypes

Ben Jones jonesbb at BELOIT.EDU
Tue Jan 5 00:20:31 EST 1993

Nice to be back from vacation.  Happy New Year, everyone.  Sorry it took so
long to respond to this.

Thomas Boyce writes:

>The recent discussion by Ben Jones of the value of so-called
>less than perfect phenotypes uses the evolution of flight as
>an example where incremental changes towards a flight mechanism
>can be incrementally more fit.  
>While this may be true for big, furry animals, its probably not
>true for insects.

[Do I detect a note of sarcasm?  Hey, some of my biggest ANCESTORS were
big, furry animals!  My ancestors could EAT insects for BREAKFAST!  It's
the breakfast of CHIMPANZEES!  :-) ]

Sorry, I couldn't resist.  Now to be serious, mostly:

I thought the flying squirrel example was to the point and easily
understood.  However, let's talk about insects:

>The work of Joel Kingsolver and Mimi Cole has
>shown in an elegant fashion that the incremental addition of 
>length to pronotal lobes in proto-flying insects actually has
>no beneficial areodynamic effects.  Thus, the insect would have 
>no incremental gain in fitness via flight by incremental lengthening of
>its proto-wings until they had become large enough to actually
>affect its gliding ability. 
> What could have happened, is selection for larger
>and larger surfaces for thermodynamic control of body temperature
>(large surfaces for basking) which were then coopted for flight
>once they were large enough.

This does raise some questions, which perhaps someone out there in netland
can answer.  Very small insects remain very close to the temperature of the
ambient air.  How big does an insect have to be before has enough mass to
benefit from thermoregulating structures?  Are there non-flying arthropods
that use any proto-winglike structure for heat regulation?  If so, how big
are they?

I prefer the hypothesis that primordial wings in insects served the same
function as the wings in many plant seeds:  to increase dispersal distances
when using the wind.  To obtain the maximum distance, the seed should stay
in the air as long as possible, and therefore should have as much
aerodynamic drag as possible.  Showing that a lengthened pronotal lobe has
no aerodynamic benefits is not the same as showing it has no benefit as a
parachute.  In fact an aerodynamically streamlined shape would be a
definite liability if the goal is to stay airborne longer, unless the shape
is capable of gliding.  However, a body part that had lots of aerodynamic
drag might easily evolve into a wing, to generate lift.  First it would be
used for gliding, but later if the insect started flapping it, it could be
used for active flying.

This example still has the primordial structure being used for the same
sort of function that it later evolved into:  flying.  However, it would be
equally valid if the original use and the evolved use were in a completely
different sphere.  Spider silk may be an example.

Consider young spiders, some of which use a strand of silk to catch the
wind and parachute them to exotic lands never imagined by their
fore-arachnids, a process called ballooning.  In contrast to hypothetical
insect body parts which evolve to increase drag, a strand of silk does not
pre-adapt a spider to winged flight the way a modified body part might, and
perhaps that is why spiders did not evolve winged flight.  Note that
creating a strand of silk can be used for ballooning without any of the
other traits that make it useful for capturing prey.  So I suggest that
silk was used first to aid dispersal via parachute, and then only later was
used for webs, their primary means of support [pun not unintended :-) ]. 
Insects, in contrast, used modified body parts to aid dispersal via
parachute first, and later the body parts evolved into wings, THEIR primary
means of support.

[If you don't like spiders, imagine how much worse it would be if they
could fly.  :-Q ]

>The flying-squirrel hypothesis for the evolution of insect flight
>does not appear possible. 

If you think of both as reducing the rate of descent during a fall, even
though for different reasons, it appears possible to me.

>The lesson from this is that small, incremental changes in phenotype
>toward an optimum may be the most obvious path, but might 
>not be biomechanically or physiologicaly possible and may
>have arisen through various
>mechanisms at various times in its evolutionary development.

I agree with this completely.  I hope I did not come across as claiming
that evolution always works by small, incremental changes.  I had rather
intended to dispute Colin Rowat's original postulate.  He said:

>>  I have a question regarding the evolution of any species with
>>specialized members that would require many generations to serve a
>>useful purpose ... 

In my original post I was reacting (perhaps over-reacting) to the wording
of the question.  He ASSUMES the existence of specialized members that
would have required many generations to serve a useful purpose.  If this
assumption were ever proven to have actually occurred, it would be taken as
proof of the Creationist view of the origin of life.  However, it was
probably just an unfortunate wording of a quite valid question.

There is one exception which has not been mentioned yet:  Sexual selection.
 The word "useful" becomes a bit fuzzy when it is applied to traits which
are useful only in attracting members of the opposite sex.  Female
preference can be orders of magnitude more powerful than selection for
things like survival or foraging ability which we normally associate with
natural selection.  And it can result in wildly fanciful traits which could
be downright liabilities in survival, but could also give rise to strange
new adaptations.  However, I know of no examples of this happening. 
Perhaps someone out in netland?

Ben Jones                  BioQUEST / Department of Biology
jonesbb at         Beloit College, Beloit, Wisconsin

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