Brett Aubrey  wrote in
news:batirr$qfe$1{at}darwin.ediacara.org: 

> "r norman"  wrote...
>> On Sat, 24 May 2003 17:24:44 +0000 (UTC), Brett Aubrey
>>  wrote:
>> > wrote:
>> >> I contend that due to my model, no species 'evolves'
>> >> unless they are forced to adapt to harsh changes - thus
>> >> As environmental adaptation increases:
>> >>    directional and diversifying selection decreases
>> >>    AND stabilizing selection increases.
>> >>     (and vice versa)
>> >> So I contend that the brain development in hominids
>> >> was due to the harsh environmental weather at the time:
>> >> 
>> >By way of example, I might point to (my understanding of) the
>> >giraffe's neck, tongue, etc. to permit better access to fairly
>> >poorly exploited Acacia trees;    
>> Be careful of the examples you use for evolution!  There is evidence
>> that the giraffe's neck is NOT an adaptation for feeding high
>> above the ground, but rather is the result of sexual selection.  See
>>   R.E. Simmons and L. Scheepers,
>>   "Winning by a Neck: Sexual Selection in the
>>     Evolution of the Giraffe"
>>   American Naturalist 148:771-786 (1996)
> 
> Thanks muchly for this, "r norman" - I'll certainly remember it in the
> future.  After perusing the Web for a while, I see this area is widely
> misunderstood as well as being somewhat controversial.  Your statement
> starts with "There is evidence that... " and many of the sources I
> looked at varied significantly, so I still have one question:
> Is there any support for the high, unexploited tree "reason" among the
> professionals (biologists and such), or is sexual selection really the
> only main reason cited these days (i.e. there could be more than one
> underlying reason, at least in theory).


Certainly there could be more than one reason. The sexual selection 
theory is interesting and supported by some behavioral evidence, but it 
is also possible that the long neck was first selected for by feeding 
high above the ground and then became involved in sexual competition. 
Long necks have appeared many times in evolution - perhaps because they 
can be created by a relatively simple change in developmental genes. But 
the reason may also be that they provide access to a modified food niche 
while maintaining most of the rest of a suite of adaptations to an 
existing ecosystem. If you want to eat leaves high up on trees, you might 
be better off in becoming a small animal adept at climbing. If you are 
already a large grazer with hoofs, there is no way to get there from 
here. Growing a long neck is an available option - but it may still take 
the spur from sexual selection to drive a bunch of happy low tree grazers 
into developing the enhanced cardiovascular pumping system that allows 
giraffes to browze the treetops without fainting.


Yours,

Bill Morse
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