The introduction of the brown tree snake to Guam was an accident: A snake hitchhiker crossed the ocean on a human tanker—unbeknownst to the crew—and landed in a veritable bird buffet. But humans also directly affect the evolution of a population through artificial selection. Artificial selection works the same way as natural selection but the difference is that the selective pressure is us. For many animal species, from pets to farm animals and plant species, humans choose who breeds with whom in an attempt to produce new individuals with the traits they desire. By doing this over many generations, people have accentuated certain plant and animal traits, sometimes to extremes. For instance, artificial selection created domestic dogs from their wolf ancestors. INFOGRAPHIC 11.7

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But evolution is ever at work. Pesticide-and antibiotic-resistant populations can emerge as an inadvertent human-influenced selection. When we apply a chemical that kills a pest or pathogen, some individuals survive because of their natural genetic resistance; that is, the individuals were already resistant even though they had never encountered the chemical. These survivors are then the only individuals who reproduce, producing the next generation that is also pesticide resistant, ultimately changing the frequency of resistant genes in the population (see Chapter 17).

Evolution can be a contentious subject for some people who feel it conflicts with other views they hold. Whether you are convinced by the physical, scientific evidence that evolution has occurred or that natural selection is a mechanism by which it occurs, it is important that you base your criticisms of the theory on sound science and not on misconceptions of evolutionary theory. TABLE 11.1

Ultimately, by changing the environment rapidly, humans apply a number of new selective pressures on populations. Our changes have the capacity to be so great that natural selection simply cannot keep up: A new needed trait is not present in the population, or it cannot spread quickly enough to prevent a population collapse. The accidental introduction of the predatory brown tree snake is one such example; it introduced a major change, basically overnight, that was able to eat its way through the vertebrate populations before those populations could adapt.

Now, however, things are looking up for the birds of Guam. Thanks to efforts by the Guam Department of Agriculture and the USDA Wildlife Services, brown tree snakes are being controlled to allow the remaining bird populations to recover.

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“We are using traps in northern Guam to control snakes in areas where we would like to reintroduce native species,” explains Diane Vice, a wildlife biologist with the Guam Department of Agriculture. Her organization is also working to prevent the snakes from infiltrating Cocos Island, the atoll off of southern Guam, and an important haven for nesting sea birds, Micronesian starlings, and sea turtles. Scientists are also close to perfecting the mouse-drop strategy.

A follow-up study done by the Agriculture Department indicates that in earlier tests, the snakes did devour “dead neonatal mice adulterated with 80 mg acetaminophen” and that “baiting” with laced mice was effective in killing the snakes. The study also suggests that the impact on other species will be minimal; for one thing, the snakes have already wiped out the birds that might have also fallen prey to the dead mice. “One concern was that crows may eat mice with the toxicant,” says William Pitt, of the U.S. Wildlife Research Center’s Hawaii Field Station. “However there are no longer wild crows on Guam.”

Vice says the goal is not to eradicate the snakes (which probably isn’t possible) but to control and contain them so that they don’t find their way to other islands, as they found their way to Guam. “The hope is to create safe habitat,” Vice says, so that these beautiful native species can once again thrive.

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Select References:

Clark, L., et al. (2012). Efficacy, effort, and cost comparisons of trapping and acetaminophen-baiting for control of brown treesnakes on Guam. Human–Wildlife Interactions, 6(2): 222–236. http://www.aphis.usda.gov/wildlife_damage/nwrc/publications/12pubs/clark122.pdf.

He, F., & S. P. Hubbell. (2011). Species-area relationships always overestimate extinction rates from habitat loss. Nature, 473(1747): 368–371.

Hoekstra, H. E., J. G. Krenz, & M. W. Nachman. (2005). Local adaptation in the rock pocket mouse (Chaetopidus intermedius): Natural selection and phylogenetic history of populations. Heredity, 94(2): 217–228.

Owens, I. P. F, & P. Bennett. (2000). Ecological basis of extinction risk in birds: Habitat loss versus human persecution and introduced predators. Proceedings of the National Academy of Sciences, 97(22): 12144–12148.

Pimm, S., et al. (1995). The future of biodiversity. Science, 269(5222): 347–350.

Pimm, S., et al. (2014). The biodiversity of species and their rates of extinction, distribution, and protection. Science, 344(6187):1246752.

Savidge, J. A. (1987). Extinction of an island forest avifauna by an introduced snake. Ecology, 68(3): 660–668.

Vice, D. S., et al. (2005). A comparison of three trap designs for capturing brown treesnakes on Guam. USDA National Wildlife Research Center, Staff Publications, Paper 43, http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1041&context=icwdm_usdanwrc.

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