Guns, Germs, & ‘Skeetos: Human Activity-Related Selection PressuresPublished 10 September, 2012
Antibiotics are, of course, wonderful things, and countless lives have been saved because of their widespread use. However, it has been known for some time that their use carries a potentially important long term cost for reasons that are likely to be familiar to most readers. If one imagines a population of pathogens in which there is heritable variation in the degree to which they are resistant to a particular antibiotic, then individuals that carry the relevant genes for resistance will be favored relative to non-resistant strains. Using the antibiotic, then, sets up a selection pressure for resistance to the antibiotic. A great deal of ink has been spilled about this ongoing important public health problem.
However, other sorts of selection pressures humans produce are not as obvious. A recent paper in the Proceedings of the Royal Society B by Ciuti et al. is a case in point. They report some data from a study of an elk population (Cervus elaphus) in Alberta, Canada. These elk risk being killed by hunters (the guns, in the title, see?), who number a couple of dozen per day during the weekends of hunting season. The researchers put GPS collars on 122 elk, allowing them (the researchers, not the elk) to investigate if differences among elk predicted the likelihood of their being killed by hunters. (The authors use the term “harvested” to refer to what happened to about a quarter of the elk in their study, as opposed to the 97 that survived.)
In particular, the authors discuss two sorts of anti-predator strategies that an elk might use: “shy hiding” versus “bold running.” They find that the bold running strategy tended to end poorly for the elk. Animals that moved faster and stayed in open areas were more likely to be harvested. As the authors put it:
Males that were harvested responded to hunters by moving faster than elk that survived, especially during weekends, close to roads and in flatter terrain. Flatter terrain is generally more accessible to hunters, while using sloped terrain gives an ungulate a better vantage point from which to watch for predators. Thus, males that were harvested had adopted exactly the movement strategy that would increase their detectability where and when the probability of being spotted by a hunter was higher (p. 6).
Given that humans have been hunting elk for some time, why haven’t elk populations already been pushed to the shy strategy? The authors speculate that this is due to new hunting technologies. Perhaps bold running wasn’t a bad strategy when weapons couldn’t kill accurately from such long distances, but now the best strategy is to avoid being seen as opposed to avoid being close. The systematically higher rate of death among bold elk, then, is not unlike the systematically higher rate of death among non-resistant strains of pathogens. In both cases, human activity is sculpting the traits of our fellow species.
In some ways, then, human-produced selection pressures are something of an uphill battle. It was cases like this that led my former graduate student Marc Egeth and me to think about trying to roll the ball downhill, using human-produced selection pressures for good rather than for ill. In a paper recently published in the online journal that hosts this blog, Evolutionary Psychology, we wondered aloud about such a case.
Millions of people are infected with Plasmodium, the parasite that causes malaria, which is transmitted by mosquitoes. Suppose the power of selection could be harnessed to give those mosquitoes who prefer not to bite humans an advantage over human-preferring mosquitoes.
The technical obstacles might be insurmountable, but we were interested in the general idea. Suppose that mosquito populations are limited by the amount of standing water available as opposed to the amount of blood female mosquitoes can acquire. If one could give mosquitoes blood from feeders (as opposed to from people, who take various measures to defend themselves and kill attacking mosquitoes), then you could give an advantage to feeder-preferring mosquitoes, potentially selecting for the preference. The key point is, of course, to harness the power of selection rather than fighting it, as we’re doing with antibiotics.
And, yes, we recognize that the scenario we’re suggesting has an odd resonance with the plot line of True Blood, in which vampires drink blood-substitute so they don’t need to feed on humans to survive and reproduce.
Anyway, are there other ways that we can establish human-produced selection pressures to bring about positive outcomes? Maybe. My guess is that there are many cases not unlike the elk example where human activity is already systematically affecting species in ways that we haven’t yet realized or, at least, measured. (Something I wondered about recently: could size restrictions on the fish that one must throw back when one is fishing be selecting for smaller fish or slower growth? Anyone?)
Ciuti, S., Muhly, T. B., Paton, D. G., McDevitt, A. D., Musiani, M., and Boyce, M. S. (2012). Human selection of elk behavioural traits in a landscape of Fear. Proceedings of the Royal Society B. doi: 10.1098/rspb.2012.1483
Egeth, M., & Kurzban, R. (2012). Artificial natural selection: Domesticating the mosquito. Evolutionary Psychology.