Nature is another name for the Phenomenal Realm (saṃsāra). Nature "selects" that which survives on a population level of reference. What survives? In general, Nature itself; in particular, a population that produces offspring. What produces offspring? That which has a potential to produce offspring and realizes the potential.

i have never drunk beer, yet i can metabolize beer. i am able to tolerate a particular stress factor (beer) even before an exposure to that factor (beer). Does my capability to tolerate beer even before an exposure to beer affect my potential to produce offspring after exposure to beer? Yes, definitely.

Thank you ma'am. But I haven't got it clearly.

For example, a population of bacteria are exposed to heat. Among this population of bacterial cells, a specific clone survive a high temperature while the other cells die. And these cells which survived heat stress are also equipped to resist the action of antibiotics. The theory of natural selection explains the initial selection of bacterial cells which survived heat stress. Does it also explain the ability of bacterial cells to tolerate antibiotics? Do you mean to say that the ability to resist heat stress and the ability to resist antibiotics are inherently present in a same cell?

Stress factors are many. Bacteria are small and must keep things simple, neat, concise. If the bacterial cell were to create a specific response to each stress factor, it would have to give up its unicellularity and would lose its ecological niche. Many did and these have become multicellular organisms. Many did not and these have remained bacteria. So, how do bacteria cope with (a possibility) of an exposure to multiple stress factors if not by cellular differentiation? By a common response: the searching for a solution by "re-programming" itself (which is facilitated by a transient increase in the rate of DNA mutagenesis). Bacterial cells within the population that have the ability to re-program themselves will have better chances to survive (by dividing); not only will such a population survive but the proportion of cells with the re-programming ability will increase . . .

Thank you ma'am!

Am I right if I summarise both of your responses in this way?

Bacteria even before exposure to a particular stress are inherently programmed to tolerate a specific type of stress. When they get exposed to that particular stress factor, they tolerate it and concurrently undergo genotypic genotyping and phenotypic re-programming. And this common pathway of stress response also enables them to tolerate other stresses.

Bacteria (some more than others) are inherently programmed to cope with rapidly changing environmental conditions, including having an inherent (acquired/evolved) ability to re-program themselves. Hence, the basis for the phenomenon of resistance to multiple stresses is genetic, and the success of the "specific stress, common response" strategy is continually tested by the population's ability to survive and reproduce---and this is the very act of natural selection.

(By the way, i am a male---so being called ma'am sounds strange to me, though i cannot deny the fact that it resonates with my inherent (albeit small) female element.)