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Bugs Fighting Back in Evolutionary War on Humans?

Bijal P. Trivedi
For National Geographic Today
September 7, 2001
 
Humans are accelerating the rate of evolution in species ranging from viruses and bacteria to agricultural pests. The consequences—such as drug-resistant pathogens, and insecticide- and herbicide-resistant insects and weeds—are costing at least U.S. $50 billion per year in the United States alone.

"What is missing among the general public is a perception that evolution impacts our daily life," says Stephen Palumbi of Harvard University in Cambridge, Massachusetts, and author of a new review article published in the current issue of the journal Science.

Many people think evolution is just a "dusty academic science" and "that the process only occurs over millions of years," he adds.


But evolution is a continuous process. It is a process that is particularly rapid among organisms that mutate and reproduce frequently. Bacteria and viruses have replication times that can vary from minutes to days, evolving new traits quickly. It is the reason that antibiotics, antiviral drugs, insecticides, and pesticides eventually become ineffective.

Presently humans are involved in a very expensive arms race, continuously trying to defeat organisms that have already developed resistance to the latest drugs. It is a game of catch-up that the researcher believes humans will never win.

But runaway evolution, Palumbi argues, must be slowed and the costs to society lowered.

Slowing evolution involves anticipating the evolution of undesirable species and developing prophylactic solutions that are incorporated into policy making.

Take for instance HIV, the virus that causes AIDS. Some have suggested that treatment with AZT—a cheap and powerful drug that prevents the virus from replicating—should be distributed in regions of the world where other combination therapies are unaffordable.

Palumbi disagrees.

The glass-half-full attitude, in which some therapy is better than none, results in some short-term benefits but in the end fosters the evolution of a large population of AZT-resistant HIV strains that are more expensive to treat, he says.

It's a battle that leads to a new class of diseases that are "medically manageable but economically incurable."

Palumbi supports a "drug-overkill" strategy that uses a cocktail of drugs that overwhelms the infectious agent. The strategy has been employed in HIV triple-drug therapy, in which each drug attacks the virus at a different point in its reproductive cycle.

The approach is successful because it is unlikely that a virus will have traits to protect it from all three forms of attack. The combination approach, at about U.S. $18,000 per year per HIV patient, is standard treatment in the United States.

"Using anything other than the full arsenal of weapons is very dangerous and ultimately very expensive," says Palumbi.

The overkill strategy is also relevant when dealing with agricultural pests. Rather than relying on chemicals alone to destroy unwanted plants or bugs, farmers should attempt to physically control the populations of these species. For weeds this means removing resistant individuals by hand.

Using a range of herbicides is also another way to slow the appearance of resistant species. A genetic variation that enables a weed to survive an herbicide during one season is unlikely to protect it the following year from a different chemical that kills through another mode of action.

To slow the evolution of resistant bugs also requires alternating insecticides. Additionally, Palumbi recommends planting some crop varieties that contain toxin-producing genes and some that do not. This crop diversification creates a varied environment in which both resistant and non-resistant insects can thrive; this prevents only resistant bugs from engulfing the population.

None of these approaches are anything but common sense, provided you accept that evolution is happening continuously, says Palumbi.

The bottom line: Evolutionary trajectories of unwanted species should be included when analyzing the use of new drugs and chemicals.

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