Oxygen 

Given the tendency to destruction, the rate of replication is profoundly important. If we assume a steady rate of destruction, then to ensure survival, the rate of replication must surpass the rate of destruction. The importance  of this relation was addressed by the chemist Leslie Orgel ... Orgel theorized about the likely behavior of populations of 'immortal' cells in culture, if subject to different levels of irradiation. In this sense, immortal refer to a population of cells that has the potential to continue dividing indefinitely without any appearance of senescent; it does not mean that individuals cells cannot die by accident or old age. Two cell types that behave in this way are bacteria and cancer cells. Both can be grown in cell culture without any appearance of senescence, but at any one time a proportion (perhaps 10 to 30 percent) are unable to divide. They are doomed to die. Their place is taken by the progeny of cells that do continue to divide. Orgel made the point that if these 'immortal' cell populations are irradiated, so that the probability of any daughter cells surviving is less than 50 percent (in other words, the rate of replication fails to surpass the rate of destruction), then the overall population will gradually decline and die out, ... 

Imagine that they are being irradiated at an intensity calculated to kill more than 50 percent of them. For a while. the population dwindles, as predicted by Orgel; but then it shows signs of recovery, even though it is still being irradiated at the same intensity. After a little longer, we may have a thriving population once more, which seems to be immune to radiation. ... What is going on?

This is natural selection at work. Several changes take place in the cells. First of all, some of the cells divide faster. These faster replicators are disproportionately represented among the survivors, because they are more likely to have replicated themselves before their DNA was destroyed. For the population as a whole, each population doubling now takes place in a shorter period. The survivors produce a new set of genes in a shorter time than that required for radiation to dismember a single set. The progeny now have a greater than 50 percent probability of surviving intact to the next generation (p. 218)

Comment: Try to make a formal modeling to describe this.