Stress and the Evolution

Stress, reduced to its simplest form, is the fact of being in an environment that does not suit us, confrontation with events as unpleasant as sugar in his coffee if you are a PhD student, or a sudden change of temperature below 0 ° C if you are a tropical species. The appearance of a stress in the life of an organism (population) will have different possible consequences, it is important to consider with the eye of an evolutionist. The duration of the stress will determine many things, including the response that is mobilized, and therefore the effects on the population. Besides, as stress is biotic (predators or pathogens) or not, the situation is quite different.

We can distinguish, with a spoon, two situations. In the first, we manage stress, and a simple adjustment of physiology enough to get by. If the temperature increases, we can always respond by producing halos (say, under the arms) as unsightly as they are physiologically justified. Even if the answer is not, a priori, selective, we do have a price in terms of fitness, the fitness that determines whether or not we pass our genes (try to land someone preceded by a good old smell of perspiration if you doubt the previous point). The second situation is more difficult: no one is doing more by responding with a slight adjustment of its physiology, it must now be genetically adapted to the new situation in order to survive: stress will become a selection factor, which will lead to the elimination of susceptible individuals in the population.

If the intensity of stress is a factor that determines whether action selection, the exposure rate is also crucial: even if a stress is very important, if not longer applied, or not often, the effect of selection may be minimal. And if the alternating stress (that is, if we oscillate around the optimum), it may take something quite interesting: the selection of different strategies. We saw the two main types of strategies that could be met: either we face stress (resistance), or you live with the consequences (tolerance).

One issue that arises when one is an organization that wants to survive stress and is of course "how we get out? ". By what devious trick we going to get out of this predicament? The answer is pretty obvious. If we want to resist stress, it is a busted adaptive mutation, and the best way to do this is to go as soon as possible to accumulate mutations. Particularly in bacteria, increasing the mutation rate has an important adaptive value in the context of stress (1). An interesting way to quickly acquire mutations is to divide more quickly, ie to increase its growth rate. We have the assurance of having left descendants, and the more you increase the chances of hitting a beneficial mutation.

You see that we must listen to those who tell you that there is no better anti-stress than sex!

Except that, as always, there are parasites. By definition, the parasite modifies the features of life of its host, reducing its fitness, and overall, making every effort to make his life impossible. In short, it's a stress like any other. Again, we can say that the more you breed, the more likely to fall on the mutation that makes us strong. And besides, in the presence of parasites, it was noted that those that mutate most do best (2). Except that if you want to increase their chances of mutation breeding in addition, it is a bit in the process of shooting itself in the foot. For one simple reason, too: the more you are, the more it gives the parasite the ability to reproduce, and therefore, the more likely to pick ourselves a parasite. Besides, also in the case of bacteria, it was shown that the more one is divided, the sooner we help the parasite to do the same (3).

To summarize, we have a strategy that seems great advantage in case the stress is not alive, but becomes incredibly dangerous when the same becomes biotic stress. This is an example that highlights how the interaction with the environment and with other related species, and organisms grow in directions often very different.

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