Talmudic Question #81

 

 
Given that most mutations are deleterious, why does the mutation rate not evolve to zero?

 

 

 

 

 

Comments

[Varun C N]

I have a thought experiment. Say, I have a bacteria that produces a mutation once in every 1 generation (practically mutation rate is calculated per base incorporation. This is just for making calculations easy). That means when the cell divides there is a mutant and a normal. And lets say the mutant is not fit for survival. The mutation is deleterious. The unmutated cell survives and replicates. Lets say this happens a million times. The cell has actually lost a million cells and by chance one mutation is good, which leads to appearance of a variation. The variation being a good one can compete with the normal type when there is a selective pressure. If there is no selective pressure, the mutation being non lethal still survives and replicates, the number being less than the normal.

Overtime (after billions of replication rounds, I have a heterogenous population. There is more than 99% of normal type, and 1% of different variants and during the process lost a billion cells in trying to be different. To this if I add an antibiotic, the mutant (which maybe less than 0.01%) survives which can now proliferate. If I had a zero mutation rate, everything would have been killed in one boom. Eventhough in the process I lost billions of cells (deleterious mutation), the remaning 0.01% of cells will carry the gene pool. Without mutation whole gene pool is lost. Given the fact that cells are exposed to all kinds of challenges, mutation provides a means to escape total anhilation even though there is significant loss (This is in contrast with total loss by trying to save more cells). This is clearly advantageous.