[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Digit Loss



(Steve Brusatte)
>>The frill and horns of ceratopsians were new structures, yet they evolved.  
>>Horny beaks evolved in some ornithopods.  The same is true for the plates of 
>>stegosaurs.  These were new structures in many ways, and they evolved.<<
Plates and horns are _far_ simpler structures than digits.  Horns are 
undifferentiated tissue, whereas a digit needs muscle, bone, skin, scales, and 
nerves in the right places.  The mutations necessary for each of these separate 
factors all grown together are quite complex, and the chances of them occurring 
by chance are slimmer than the chances of digit-deletion.

>> The earliest tetrapods, like Ichthyostega (sp?) had something like eight 
>> fingers.  Digits were created in this genus. <<
But Ichthyostega's fingers were modified (thickened) fin-struts, they didn't 
spring out of nothing.  Like the panda's thumb and the pterosaur's pteroid 
bone, those digits formed by conscription of another organ into a new function.

Someone is probably going to find counterexamples, but I'll stick my neck out a 
little more and say that evolution is notoriously bad at planning ahead.  Each 
of the selected mutations that drive evolution is useful only in the short 
term.  Each intermediate step has to confer either positive or neutral effect 
upon the organism. For instance, why haven't any vertebrates evolved another 
pair of limbs (or legless animals re-evolve them)?  I can imagine all sorts of 
advantages inherent in having two extra hands, or hooves, or claws. The reason 
is that the sequence of mutations necessary to grow another set of limbs is 
extremely unlikely to spring up by chance.  A single mutation in that sequence 
may be likely, but that single mutation may not be useful all by itself.

So what if step #1 may eventually lead to step #143 and a fully functional 
third pair of limbs (or extra digit), what use is step #1 to me, right now?  As 
the genome of an organism with a job to do, I can't be bothered to hope for a 
return on my investment who-knows-how-many generations in the future.  So, no 
way.

(Tracy Ford)
>>But they don't. The fingers have good claws, strong fore-limbs.
Tyrannosaurus rex has larger arms in relation to it's body than many earlier
tyrannosaurids.<<

So the tyrannosaur lineage stopped using their forelimbs, which atrophied, then 
found some use for the two-fingered remnant, which began to hypertrophy.

>>Estemmenosuchids and wart
hogs, Long skulled labyrinthodonts, phytosaurs and gavials, and numerous
others. How can all this be just an act of mutation? Question open to group
discussion.<<
If the original forms are similar enough in shape (proto-gavials and 
proto-phytosaurs), then mutations that result in similar adaptations will turn 
out to be useful if the two organisms' ecological niches are similar (our word 
for today is "convergent evolution").  However, convergence doesn't always 
happen.  Deinonychosaurs and cats likely occupy(ied) similar niches, but differ 
substantially in outward appearance (likewise, gazelles and kangaroos are 
dissimilar) because the original forms upon which the mutations acted were 
dissimilar.  There are no gene sequences hidden in our genomes that say "I make 
a long, toothy mouth and powerful flippers; turn me on to be a fish eater".  
Rather, there are certain forms that, due to the laws of physics, work better 
than others.  Forms that work may be arrived at independently, especially if 
those forms can be produced by a simple series of mutations.

>>>>You can tinker with the car in your garage, and maybe improve its
performance.  But you'll be darn careful to be sure that you don't damage
any important parts.  <<

Not if you knew how to work on cars and could improve. Even I did that with
my Firebird and I'm not a mechanic, now working on my truck...<<
True, but the human tinkering on the car is (presumably) an intelligent force, 
capable of creative thought, research, and planning ahead.  Alter the analogy, 
so that instead of a human mechanic, you have a robot that tinkers with your 
car completely at random.  The robot makes one random adjustment, and you 
test-drive the car.  If you notice no difference or an improvement, the robot's 
"mutation" is kept.  If the car no longer works as well, however, you get rid 
of the latest change.  This system (given a great deal of time), may make 
significant improvements, but only in an inefficient, jury-rigged manner, not 
in the linear, logical, and Lamarkian system of actual car-development.

Dan