Germs, Immunity, and Antibiotics

Of course, the most important function of the immune system is to prevent pathogenic agents from causing disease. Just as logically, the most important function of a pathogen is to circumvent immune defenses, invade body cells, and carry out its own metabolic and reproductive functions with impunity. In this case two rival sets of replicators must compete for their own survival, and competition naturally leads to evolutionary change. The rate of this process is normally very slow, but the introduction of antibiotics to supplement humans' natural defenses against pathogens has greatly increased the rate of evolution, making it quite visible to the modern observer.

Pathogens and Hosts: The "Arms Race" in Nature

As stated above, it is the purpose of the immune system to ward off invading pathogens; similarly, it is the purpose of pathogens to defeat the immune system and thus be able to survive and reproduce. This conflict of interests naturally, almost inexorably, leads to one of the most fascinating phenomena in evolutionary history: the "arms race" in nature.

When lineages of organisms participate in arms races, they essentially coevolve in an antagonistic manner, each side fighting for continued evolutionary success. It is vital to note that lineages, not individuals, are the participants in arms races; the lineages can be likened to nations, while the individual organisms are analogous to individual soldiers or government officers within the larger political entity.

In the case of the immune system vs. pathogenic organisms, the arms race is between the pathogens' ability to infect cells and the immune system's ability to stop them from doing so. Each time a mutation arises in the immune system that makes it better at combating pathogens, the human being bearing this mutation will survive disease better and will be more likely to pass his genes to the next generation. As a result, this mutated immunity will become more and more prevalent, and will be encountered by increasingly large numbers of pathogens, which will be defeated in greater numbers.

This sets up an evolutionary pressure on the pathogens to defend against the mutated immunity. Those pathogens that circumvent the immune system, obviously, will be those that survive and reproduce, and before long the characteristics that enabled them to survive the attack will become widespread throughout the population of pathogens.

The newly evolved pathogenic defense now renders the immune system's mutation less effective, and human populations will once again be more susceptible to disease and possible death due to the pathogen's infection. Those humans who survive the new plague will pass their resistant mutation on to their descendants, and the resistance will become widespread in the population once again.

For a second time, the community of pathogens will find itself decimated by the immune system's new defense. Those pathogens that survive will pass on their characteristics to their descendants, who will in turn spark new adaptations in the human population, and so on in a long-lasting cycle of population fluctuations. A population boom in humans would follow the spreading of a new adaptation, after which a population decline would follow the pathogens' adaptation to that modification, etc. This process generally does not lead to one species' gaining in efficiency against the other; rather, their efficiencies relative to each other remain about the same, on average. Instead, both species involved generally become more highly specialized - for example, a pathogen might begin infecting only humans, rather than a wide variety of mammals.

Running in Place

In many ways, the arms races of nature are a form of running in place - the majority of the time, no real ground is ever gained. If a pathogen fails to adapt to a new immune defense, it could become extinct. Conversely, if the immune system fails to adapt to a pathogen, the pathogen will continue to claim a large number of human lives and will spread virtually unchecked. By adapting as part of a natural arms race, species may gain a momentary advantage, but generally do not achieve major gains in ground. As such, arms races are one specific explanation for the evolution of extreme complexity, especially in environments that are successfully inhabited by simpler organisms. Participants in an arms race simply must adapt, or they will lose the race.

There are exceptions to this rule, however. Sometimes a species cannot adapt, for whatever reason, to a specific challenge offered by the opposing species. The un-adapted species could be forced out of the area, or retreat to other habitats or niches, or even become extinct. Also, sometimes these scenarios stabilize after a period of the fluctuating pattern described above, as the species settle into a pattern of coexistence (though this is not always peaceful).

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