time. If susceptible hosts are not abundant, however, then the parasite that causes less harm to the host (i.e., is less virulent) may be favored since that would allow the host to live longer, thereby providing more time for the production of transmission stages. The hypothesis that virulence is always favored when hosts are plentiful and is reduced when there are fewer hosts neglects the fact that a feedback exists in the host-parasite interaction: a change in parasite virulence impacts the density of the host population, and this in turn alters the pressures of natural selection on the parasite population, and so on. Thus, although parasite virulence generally tends to decline over evolutionary time, it never becomes entirely benign, and in the process the parasite population becomes more efficient in regulating the size of the susceptible host population.
The view that parasites evolve toward becoming benign suggests that parasites are inefficient if they reproduce so extensively that they leave behind millions of progeny in an ill or dead host. Indeed, some biologists have contended that enhanced virulence is the mark of an ill-adapted parasite or of one recently acquired by the host. This is not true. The number of parasite progeny lost is not of evolutionary significance; rather, it is the number of offspring that pass on their genes to succeeding generations that determines evolutionary success. Natural selection does not favor the best outcome for the greatest number of individuals over the greatest amount of time, but instead favors those characteristics that increase the passing-on of a specific set of genes. Consider a particular species of weed that is growing in your garden. The production of 1,000 seeds that yields only 100 new weed plants might be considered wasteful in terms of seed death and the amount of energy the weed put into seed production, but if the surviving seeds ultimately yield more weed plants in succeeding generations, then that weed species is more efficient in terms of evolutionary success. Parasites are like weeds. They have a high biotic potential, and those that leave the greatest number of offspring in succeeding generations are the winners, evolutionarily speaking. Evolutionary fitness, be it for a parasite, human, bird, or bee, is a measure of the success of the individual in passing on its genes into future generations through survival and reproduction. When the fitness of the host is reduced by a parasite, there is harm, illness, and an increased tendency toward death. Host resistance is the counterbalance to virulence or the degree of harm imposed on the host by the presence of the parasite. If host resistance is lowered, a disease may be more pathogenic although the parasite’s inherent virulence may be unchanged. How negatively a host will be affected, i.e., how severe or how pathogenic is the disease, is thus determined by two components: virulence and host resistance. In addition, virulence is not so much a matter of a particular mutation but rather how that mutation is filtered through the process of natural selection; it is through natural selection that the final outcome may be a lethal outbreak or a mild disease, and, of course, when a new pathogen emerges, R0 must be a number >1.
Since parasite survival requires reaching and infecting new hosts, effective dispersal mechanisms may require that the host become sick: sneezing, coughing, and diarrhea may assist in parasite transmission. The conventional wisdom is that it takes a prolonged period of time for virulence to evolve; the evolution of parasite virulence, however, may be quite rapid (on the order of months) and need not take years, as was the case with the myxoma virus. The basis for this is that a parasite may go through hundreds of generations during the single lifetime of its host. Then, too, because of competition between different parasites living in a single host, it might be advantageous for one kind of parasite to multiply as rapidly as it can before the host dies from the other infectious species. Succinctly, the victorious parasite is the one that most ruthlessly exploits the pool of resources (food) provided by the host and produces more offspring, thus increasing its chances to reach and infect new hosts.
If parasite dispersal depends on the mobility of the host as well as host survival, then severe damage inflicted on the host by enhanced virulence could endanger the life of the parasite.
Consider, for example, the common cold. It would be very much in the interest of the cold virus to avoid making you very sick, since the sicker you become, the more likely you are to stay at home and in bed; this would reduce the number of contacts you would have with other potential hosts, thereby reducing the possibilities for virus transmission by direct contact. Similarly, the development of diarrhea in a person with the disease cholera or Salmonella infection (which causes “food poisoning”) facilitates the dispersal of these intestinal microbes via fecally contaminated water and food, and in the absence of diarrhea parasite transmission would be reduced.
AIDS is a consequence of an increase in the virulence of HIV. The enhancement in HIV virulence is believed to have resulted from accelerated transmission rates due to changes in human sexual behavior: the increased numbers of sexual partners was so effective in spreading the virus that human survival became less important than survival of the parasite. As the various kinds of plagues are considered in greater detail in subsequent chapters, recognition of the evolutionary basis for virulence may suggest strategies for public health programs. Clean water may thus favor a reduction in the virulence of waterborne intestinal parasites (such as cholera), and clean needle exchange and condom use would both reduce transmission and lessen HIV virulence. But some contend that this indirect mechanism may be too weak and too slow to reduce virulence substantially, and that a better approach could be direct selection by targeting the virulence factor itself. For example, immunization that produces immunity against the toxin produced by the diphtheria microbe also results in a decline in virulence. Future efforts will determine which strategy is the better means for effective “germ” control to improve the public’s health.
1 *See: Cells and Their Structure in the Appendix
2
Plagues, the Price of Being Sedentary
Figure 2.1 Hollywood’s view of Australopithecus as seen in the movie 2001: A Space Odyssey. Turner Entertainment Co. Licensed by Warner Bros Entertainment Inc. All rights reserved, Alamy Stock Photo
In Stanley Kubrick’s classic film 2001: A Space Odyssey, Richard Strauss’s music (“Thus Spake Zarathustra”) provides a haunting and frightening background to the sequence of scenes that represent the dawn of humanity. The sun rises on a barren African savannah. A band of squat, hairy ape-men appear; they eat grass. Though herds of tapirs graze close by, the ape-men ignore them, since the means and the tools necessary to attack or kill the tapirs have not yet been developed. These ape-men are vegetarians who forage for roots and edible plants. On the dawn of the second day, the ape-men are seen huddled around a water hole; the landscape is littered with bones. One, the leader of the group, picks up a bone, smashes the skeleton of an antelope, and then the bone is used to kill a tapir (Fig. 2.1). Shortly thereafter, the raw pieces of tapir flesh are eaten and shared by other hairy apelike creatures, members of the clan. At the dawn of the third day, the meat-eating, tool-using man-apes drive off a neighboring band of apelike creatures. Bone tools used for killing animal prey are now used to threaten and drive off rival tribes. In slow motion, accompanied by the slowly building tones of Strauss’s music, the leader of the man-apes flings his weapon, a fragmented piece of bone, into the air. It spins upward, twisting and turning, end over end. There is a jump cut of 4 million years into the future, and the bone dissolves into a white, orbiting space satellite. Kubrick’s science fiction film has been described as a countdown to tomorrow, a visual masterpiece and a compelling drama of human evolution. Absent from the film is an examination of how the enlightened roving bands of early apelike humans settled down and become increasingly disease-ridden. Here is that part of the story.
