Now this is cool.

It seems like it is a corrollary of the theory of Natural Selection that the most highly conserved regions of the genome will be those which are necessary for existence -- if a region is not necessary then it will slowly mutate away and if it does not do this there must be some reason why mutations in that region are selected against. This is particularly interesting in the context of certain "ultraconserved" regions which do not code for proteins -- long regions that are conserved between fish, frogs, and men and which can be very distant from any coding region. For a while, it was supposed that these regions were necessary regulatory elements. However, conservation does not imply necessity, a recent study points out:

It is widely believed that the most evolutionarily conserved DNA sequences in the human genome have been preserved because of their functional importance and that their removal would thus have a devastating effect on the organism. To ascertain this we removed from the mouse genome four ultraconserved elements—sequences of 200 base pairs or longer that are 100% identical among human, mouse, and rat. To our surprise, we found that the mice lacking these elements are viable, fertile, and show no apparent abnormalities. This completely unexpected finding indicates that extreme levels of DNA sequence conservation are not necessarily indicative of an indispensable functional nature.

I find this very cool. I have no idea what it means, but it's very very cool.