Views from Understanding Evolution: Parasites and Pathogens Take the Leap

Even if you barely glance at the newspaper, it is hard not to feel that we have suddenly come under attack from a legion of pathogens. SARS, HIV, Ebola, West Nile Virus, and avian flu seem to have been quietly biding their time in other organisms before attacking human hosts en masse. But is that really the case? Are such emerging infectious diseases (EIDs) a new phenomenon? On DOI: 10.1007/s12052007-0022-7 of this issue, Brooks and Hoberg (2007) take an evolutionary perspective on these bugs and reveal that such invasions are far from rare—or recent. Although we first notice EIDs when they begin affecting human populations, these pathogens do not come out of nowhere. Generally, they are the longtime inhabitants of another animal (e.g., chimpanzees in the case of HIV, bats in the case of SARS) that have simply invaded a new host— who happens to be human. In this way, EIDs are just one example of a common evolutionary phenomenon: hostswitching. A sapsucking insect species that previously fed upon a single species of broom plant begins to feed upon a related broom species, a louse species that previously fed upon one species of gopher begins to feed on a related species, a liver fluke that previously parasitized sheep begins parasitizing cows, or a virus that previously infected chimpanzees begins infecting humans. Although different in their specifics, all of these examples rely on the same underlying evolutionary processes. As humans infiltrate Earth’s every nook and cranny, we transport with us many species that we host, and we come into contact with the native species that reside in each new environment. All of this “mixing” represents evolutionary opportunity. From a parasite’s or pathogen’s point of view, encountering a new body is a bit like standing at the edge of a vast and unexploited wilderness—perhaps it will turn out to be inhospitable and uninhabitable, but perhaps, just perhaps, one could survive there and gain relatively exclusive access to its riches (in this case, human cells and the energy and machinery they contain). When humans come into close contact with other species and their pathogens (often by living with them or invading their habitats), we are taking the chance that one of these attempted colonizations of our bodies will be successful. Brooks and Hoberg describe how such host switches can occur. Although we often think of natural selection in terms of survival of the fittest, survival of the fit enough is more like it. Natural selection requires time to act, and organisms are always saddled with the genetic baggage of their evolutionary history. Because of this, at any given moment, organisms are unlikely to be perfectly matched to their environments. Natural selection’s filter lets pass any organism that manages to get genes into the next generation, and that doesn’t necessarily require perfection. This all means that there is some “wiggle room” in which hosts a parasite can invade. A liver fluke species living in a sheep might not be perfectly adapted for living in a cow, but perhaps the fit is close enough to allow the colonization. In this case, the fluke’s sheep-adapted traits are coopted to serve the function of parasitizing a cow—an interaction that Brooks and Hoberg describe as “ecological Evo Edu Outreach (2008) 1:25–28 DOI 10.1007/s12052-007-0002-y