Evolution from a virus's view
Where's the evolution?
As an example, consider a unique ecological challenge faced by many pathogens: appropriate habitats can be few and alarmingly far between. Put yourself in the position of a virus in its natural habitat a human host. You've infected some cells and managed to reproduce, but the host's immune system is onto you now and is turning up the heat. This environment is no longer so hospitable. How can you get your descendents to a friendlier habitat (i.e., a new, unexploited human body)? Without legs, wings, fins, or any of the usual means of locomotion, your descendents' prospects for reaching a new host under their own power are nil. However, natural selection has provided pathogens with a number of sneaky strategies for making the leap to a new host, including:
Pathogen lineages that fail to meet this challenge and never infect a new host are doomed. They will go extinct when their human host dies or when the immune system destroys the infection.
Since transmission is a matter of life or death for pathogen lineages, some evolutionary biologists have focused on this as the key to understanding why some have evolved into killers and others cause no worse than the sniffles. The idea is that there may be an evolutionary trade-off between virulence and transmission. Consider a virus that exploits its human host more than most and so produces more offspring than most. This virus does a lot of damage to the host in other words, is highly virulent. From the virus's perspective, this would, at first, seem like a good thing; extra resources mean extra offspring, which generally means high evolutionary fitness. However, if the viral reproduction completely incapacitates the host, the whole strategy could backfire: the illness might prevent the host from going out and coming into contact with new hosts that the virus could jump to. A victim of its own success, the viral lineage could go extinct and become an evolutionary dead end. This level of virulence is clearly not a good thing from the virus's perspective.
On the other hand, diseases like cholera (which causes extreme diarrhea) are, in many situations, free to evolve to a high level of virulence. Cholera victims are soon immobilized by the disease, but they are tended by others who carry away their waste, clean their soiled clothes, and, in the process, transmit the bacterium to a water supply where it can be ingested by new hosts. In this way, even virulent cholera strains that strike down a host immediately can easily be transmitted to a new host. Accordingly, cholera has evolved a high level of virulence and may kill its host just a few hours after symptoms begin.
Though transmission mode is far from the only factor that affects how virulence evolves the immunity level of the host population, the distribution of the hosts, and whether the host has other infections, for example, matter as well this key piece of the pathogen's ecology does help illuminate why some diseases are killers. More importantly, it suggests how we might sway pathogen evolution towards less virulent strains. In situations where high virulence is tied to high transmission rates (e.g., cholera), reducing transmission rates (e.g., by providing better water sanitation) may favor less virulent forms. The idea is to create a situation in which hyper-virulent strains that soon kill or immobilize their hosts never get a chance to infect new hosts and are turned into evolutionary dead ends. In fact, biologists have observed this phenomenon in South America: when cholera invaded countries with poor water sanitation, the strains evolved to be more virulent, while lineages that invaded areas with better sanitation evolved to be less harmful.
And that brings us back to Adenovirus-14. Adenoviruses are transmitted through the air or via contact. We might expect this sort of transmission to require a fairly healthy host (one who gets out and comes into contact with others) and, hence, to select against virulent strains. Indeed, adenoviruses are rarely killers, but in close quarters for example, in the military barracks where Adenovirus-14 has been a particular problem barriers to transmission may be lowered. This could open the door for the evolution of more virulent strains. Military personnel, however, are in the process of pushing this door shut again. At Lackland Air Force Base, which has seen the most serious outbreak of Adenovirus-14, wider testing, more hand-washing stations, increased attention to sanitization, and isolation of patients is helping to reduce the transmission of the disease and, in the process, may favor the evolution of less virulent strains of the virus.
Discussion and extension questions
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Adenovirus micrograph provided by CDC/ Dr. G. William Gary, Jr.
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California