Normally, when you look at a population of animals, they all look about the same—what biologists call the “wild type.” But every now and again, a mutant shows up, which may be radically different. Among birds, a good example is the albino barn swallows that have appeared in the radioactive area around Chernobyl.
Normally, mutants are an evolutionary dead end—easily picked off by predators or ostracized from mating by the larger population of wild types. Surviving as a mutant may take additional resources, so mutants may not do well in the competitive daily arena of life.
But, mutation is also the raw material of evolution. If environmental conditions change, the characteristics of a mutant may be just what it takes to survive. Those who hope to reestablish the American chestnut in the eastern U.S. depend on the successful breeding and survival of a few mutant trees that are resistant to the chestnut blight.
Mutants that are resistant to recently altered environmental conditions are all around us—especially amongst short-lived organisms that reproduce rapidly and which have been subject to massive changes in their environment. After World War II, when huge quantities of DDT were used to quell populations of mosquitoes, a few mutants, resistant to its effects, began to take over. Now, the evolution of resistance to drugs and pesticides plagues efforts to reduce malaria in Africa. Resistant mutant weeds are also showing up in agricultural fields that are treated each year with Roundup (glyphosate) herbicide.
Unfortunately too, mutants are showing up in bacterial populations in response to massive use of antibiotics in medicine and agriculture. Many hospitals harbor bacteria that are resistant to the common antibiotics that are used to prevent infections, especially after surgery, among the patients who stay there. Infections by resistant bacteria are estimated to kill 23,000 Americans each year.
Down on the farm, massive doses of antibiotics—perhaps 70% of annual sales in the U.S.—are used to prevent infections when pigs and poultry are grown in crowded conditions—what we affectionately call CAFOs, for concentrated animal feeding operations. And cattle are fed antibiotics because, for poorly understood reasons, these drugs stimulate weight gain. Most of these antibiotics are the same types used to treat human disease. What we have created in agriculture is a massive theater of natural selection for drug-resistant mutant bacteria, which can infect humans.
We need some clever ideas to address this situation. The Centers for Disease Control and Prevention (CDC) reports that more than 15 bacteria that cause serious diseases are now showing resistance to antibiotics. A new government initiative, to develop new, more powerful antibiotics that bacteria have not yet experienced, simply buys time before the next generation of drug-resistant bacteria appears. So does the reintroduction of older antibiotics, retired when resistance to them developed decades ago. Interestingly, at least one group of researchers has isolated an antibiotic from nature that attacks the fundamental basis of bacterial growth, so that even resistant mutants cannot survive.
Common sense would suggest that antibiotic use should be curtailed and restricted to situations critical to human health, to minimize the conditions in which resistant bacteria arise. Humans may be master of the evolutionary process, but none of us are immune to the consequences.
References
Day, T., V. Huijben, and A.F. Read. 2015. Is selection relevant in the evolutionary emergence of drug resistance? Trends in Microbiology 23: 126-133.
Ling, L.L., T. Schneider, A.J. Peoples, A.L. Spoering, I. Engels, B.P. Conlon, A. Mueller, T.F. Schaberle, D.E. Hughes, S. Epstein, M. Jones, L. Lazarides, V.A. Steadman, D.R. Cohen, C.R. Felix, K.A. Fetterman, W.P. Millett, A.G. Nitti, A.M. Zullo. C. Chen and K. Lewis. 2015. A new antibiotic kills pathogens without detectable resistance. Nature 517: 455-459.
Mueller, H.C. 1970. Predators select odd prey. American Zoologist 10: 475-
Palumbi, S.R. 2001. Humans as the world’s greatest evolutionary force. Science 293: 1786-1790.
Williams-Nguyen, J., J. Brett, Sallach, S. Bartlett-Hunt, A.B. Boxall, L.M. Durso, J.E. McLain, R.S. Singer, D.D. Snow and J.L. Zilles. 2016. Antibiotics and antibiotic resistance in agroecosystems: state of the science. Journal of Environmental Quality 45: 394-406.
