Replying to Invisible but some comments on what Darren kicked off about 
fingers.

On 8/01/2011 5:05 AM, Darren New wrote:
 >
 > I remember reading somewhere that there's a gene that controls how many
 > fingers you have *and* something about the reproductive system, so any
 > mutation in that gene tends to keep you from reproducing for entirely
 > unrelated reasons. Hence the reason why everything from fish to bats to
 > birds to people have five finger bones.
 >

On 11/01/2011 8:54 PM, Invisible wrote:
>
> I suspect what it boils down so is that there's no specific reason why
> some number other than 5 would be an advantage, so it hasn't changed.
> (This of course doesn't rule out random "neutral" changes I suppose...)
>

Given the enormous range of limb variations it is surprising that the 
number 5 is so common.  Sure some of the digits might be fused or 
reduced to almost nothing.

However I don't think it is true that *all* tetrapods have 5 digits.

Snakes are tetrapods.  While some snakes have vestigial hind legs, I 
don't know that they actually have digits.  Most snakes have no 
remaining legs to speak of - maybe a bump where the legs might have been 
but no identifiable limbs and certainly no digits.

Similar with whale hind limbs I think.  Some species have vestigial 
pelvis and hind limbs.  Usually within their bodies.  Occasionally some 
individuals have some protrusion.  Maybe digits are present during 
development of the foetus but as far as I know there are not actual 
digits by the time they are born.  Could be wrong on that but in any 
case they are well on the path to losing them altogether.

The limb changes here might start out as 'the limb genes are still there 
but the gene(s) controlling their size, placement, function etc cause 
them to get smaller, relocated or stop working'.  Given enough time 
though, genes that generate something that is irrelevant tend to 
accumulate changes and either become 'broken' or can be co-opted for 
something else.

Then going the other way there are animals with more than 5 digits on 
one or more limbs.  Some people for instance - see 'polydactylism'. 
Some of the pictures out there are fake but it is a real condition. 
Most common is 6 fingers or toes instead of 5.  It may be a quite 
harmless single gene mutation or part of a more severe complex.  As far 
as I know the milder form doesn't render the individual  prematurely 
dead or infertile.

There is a breed or at least a population of 'polydactyl cats' that have 
more than the usual number of toes on their front legs, back legs or 
both.  They even apparently can be more dextrous than normal so that 
might confer an advantage in some ways but there may be some 
disadvantages as well.  Again, I don't think that the genetic change 
that causes this is fatal.  It certainly does not prevent development 
from egg to adult because these cats are real.

The point that some genes are deeply interconnected with others or are 
so important to development or metabolism etc so tend to be highly 
conserved is true.  But as with most things in biology there are more 
things Horatio...

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