On 12/01/2011 9:06 AM, Warp wrote:
>
>    AFAIK snakes have vestigial limbs (usually quite obvious when looking
> at their skeletons).
>

Yes.  I said as much.  Vestigial.

Snakes are tetrapods and yet you cannot find 5 digits on the end of a 
limb in the vast majority of individuals.  Some individuals may have 
more evidence than the average.  Rarely a quite recognisable but non 
functional leg.  All may have some relic or indication of where limbs 
existed in ancestral forms.

The original assertion that 'fingers' always number 5 in tetrapods is 
not totally correct.

There is a strong connection between limb development and the overall 
foetal development path.  And genital development is a big part of that 
as well.  The 'Homeobox' or 'Hox' genes are what Darren had read about 
and the gist of what he said is true enough.  All tetrapods (as far as 
is known) have 5 'buds' or zones per limb during development that are 
eligible to become fingers / toes.

But there are variations in the Hox genes that alter limb development 
and are not fatal.  There can be other genes involved that modify the 
basic structures and even cause them to disappear as the foetus develops.

As shown by polydactylism in humans and cats there can be more than 5 
fully formed functional digits on each limb.  From what I've read, the 
mechanism leading to this is duplication of one of the existing digits 
rather than a completely new type of digit.  Having an exact copy of a 
digit alongside the original doesn't seem to have enough advantage to 
overcome the associated risks to the rest of the development process.

Evolutionary development can proceed from a duplicate gene.  So long as 
it isn't fatal the possibility exists that some other mutation or gene 
variant will modify the duplicate some time in the future to be actually 
useful.

As a conjecture, there is probably selection for mechanisms that provide 
genetic flexibility.  A species that has a certain degree of variablity 
is more likely to have individuals within it that can survive some 
change in the environment than one that doesn't.  So evolving the 
ability to have mutations and carry variations has its own advantage. 
On the flip side, really optimising for a niche may come at the cost of 
that variability.  Evolution of the mechanism of evolution!

Post a reply to this message