Friday, April 20, 2012

Evolution : FACTS and FUN



Evolution is a  serious  game in progress, of course at an impercetably slow pace. For us mortals, with short  life spans,   nothing really changes.   Mother Nature,  with her assistant  Natural selection, has been   playing the same game of speciation and divergence for close to 4 billion years. 

            One of the best fossils to study  various aspects of evolution   is the  simple tooth. (1).    In fact many extinct animals have left  behind only their fossilized teeth for us to know and  weave tales about them; how big  they might have been and what they would have eaten.  There are indications that though  jaws and teeth evolved independently, somewhere along the  evolutionary time lines, their pathways merged with the result that several things were firmly established once and for all.  The location of the teeth  on the jaws,  their function and  enabling dental features.  The frontal incisors and canines  help in  grasping and tearing, premolars and molars are  predominantly grinders, though they can also cut.   Cusps, the conical projections    on the surface of the molars  are   the enabling features to aid grinding and cutting process.    From the single cuspid reptilian to multi cuspid mammalian, there  are large variations in the dental features between species, but  relatively less within a  given species.   

            Harjunma et al(2) were fascinated by the developmental biology of  molars in laboratory mice.  Mice teeth are limited to   incisors and molars. In laboratory bred  mutant and transgenic mice very often cusps  are totally absent.  Except aesthetics,  these flat teeth may not have much functional use. Poor mice, flat teeth would have  robbed them off the whole  enjoyment of eating.  But why flat teeth, what leads to such a strange situation?.  Scientists have identified at least 29 gene mutations which can affect teeth formation in multiple ways, absence of cusps included. Mutation (altered  message) is one thing and signalling, (sending the message across to  intended recipients) is another thing.  Signals are clues to  turn on or turn off  or slow down  specific biochemical processes.   Tinkering with three such signalling pathways simultaneously   the team could ultimately increase the number of cusps even beyond the normal.   Harjunma team suggest that  there could be  multiple genes  at work in imparting cusp pattern to molars.  Adjusting  three genes they could make the cusps reappear, but then, in all possibility,  something else might have  gotten  impaired ?  No wonder evolution is a slow process, it is so intricate, so many messages need to be so finely balanced.......
 A friend of mine, who is working in the area of drug research,  spoke to me recently about   the  long years of study needed to zero in on a drug molecule.  A very promising  drug molecule  may prove to be causing some other ailment as a side effect. For example it might cure cancer but might lead to blindness. That is not acceptable.  Biological signals are never dedicated " For your eyes only"  type. One single signal  might be meant for several recipients and each  recipient  might interpret it in a different way. Hence the need for fine coordination. And that takes enormous amount of time.  
“Tailpiece
Playing the same game for close to 4 billion years  is not  a joke. Boredom is sure to  creep in at times and  that  is when Mother resorts to a few naughty tricks and treats. Her lazy assistant Natural Selection lets the players fight it out for survival, to decide   who wins and who gets wiped out.  Evolutionary biologists of course would  interpret these  as are necessary and well orchestrated  examples of  adaptive evolution, or survival tactics.   But just imagine wouldn’t it  be more fun to accept these as simple acts of pranks?  Take for example.  the rampant  practice of mimicry(3,4)the   Australian insect which resembles a twig,  the harmless hoverfly  which imitates the looks of the  sting bee,    the  venomous Texas coral snake and its innocuous twin, the  Mexican milk snake. 
More about mimicry  later 


1.Evolution and development of teeth McCollum et al   J. Anatomy. 2001 199, pp. 153-159
2. On the difficulty of increasing dental complexity: Harjunmaa et al Nature 2012, March 15, 483, 324-326.
3. Comparative analysis of the evolution of imperfect mimicry  Penney, H D et al, Nature 2012 March 22,   483, 461-464
4. A taste for mimicry ; Ruxton et al  Nature 2005 , January 20 433 205-206