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Examples to Use When Teaching About Selection and Evolution: Fox Domestication and Poison Resistance in Rodents
Evolution and the role that natural selection plays in evolution can be difficult to teach. Evolution occurs over many generations and generally over long periods of time, increasing the abstraction of the concept for many students. Using concrete examples of changes that have occurred in organisms over a shorter period may help students understand how selection can alter the genotype and phenotype of a population. This article discusses 2 examples of the effect of selection on the characteristics of a population. The first example is a study of the domestication of foxes that was conducted to gain understanding of how the domestic dog evolved from the wolf. The second involves the spread among rodents of resistance to anticoagulant poisons in response to the use of these chemicals to control their populations.
The ancestral origin of the dog has been traced back to gray wolves (Canis lupus), but exactly how the wolf evolved to become the domestic dog (Canis familiaris) is subject to debate. Estimates of when dogs were domesticated range from 8,000 to 135,000 years ago. Studies have shown that the difference between the 2 species is not simply physical appearance and socialization. Wolf pups raised like domestic dogs do not behave like domestic dogs. The more we study how dogs interact with humans, the specifics of how dogs have evolved to work well with humans become clear. Dogs will follow a human’s gaze toward something and will follow human gestures to find a hidden toy or food. Wolves generally do not respond to human gazing or gestures. In addition, studies have shown that when a dog and its owner sustain eye contact, the level of oxytocin in each increases. Oxytocin is a hormone associated with the formation of social bonds between individuals. The hormone is involved in the formation of the bond between a human mother and child. The rise in oxytocin seen in a dog and its owner in response to sustained eye contact does not occur during interaction between a person and a wolf, even if the person raised the wolf.
In discussing the evolution of the dog with your students, you may want to point out that the evolution of the dog may have involved both natural and artificial selection. The early stages of dog evolution may have been a case of natural selection. Some wolves, perhaps those that were less afraid of humans, hung around human camps to scavenge any scraps and as a result were better able to thrive. Their offspring may have carried the genes that enabled them to tolerate humans without fear and thus maintained their proximity to human camps. The later stages, during which breeds with specific traits were developed, are clearly a result of artificial selection, in which humans actively selected to breed animals for specific traits.
Russian scientist Dmitry Belyaev hypothesized that the differences between dogs and wolves resulted from strong selection for friendliness and non-aggression towards humans. To prove his hypothesis, in 1959 he began an experiment with Vulpus vulpus, the silver fox, an animal in the same family (Canidae) as wolves, but not yet domesticated. In the experiment, he selected for foxes that showed the lowest defensive reaction and the least aggression towards humans. The experiment used foxes from a commercial fur farm, so some selection for animals that could tolerate captivity had already occurred. Early in the experiment, the foxes were divided into 3 categories, from least friendly to most friendly to humans. Only the most human-friendly animals were selected to be parents of the next generation. Through this selection, within 2 to 3 generations the researchers eliminated foxes from the population that reacted fearfully and aggressively to humans. In the sixth generation of selection, the experimenters had to add a new category for describing the animals because the animals displayed a level of friendliness not yet seen. They were actively seeking out humans and vying with the other pups for human attention. Over time, pups that were very friendly to humans came to dominate the population.
Physical and physiological changes were associated with these changes in behavior. Early in selection, researchers observed changes in some foxes’ coat colors, such as a loss of pigmentation in some parts of the body and yellowing or browning in the animals’ normally silver/black coats. Later in selection, they observed floppy ears and rolled tails (tails curled over bodies). In addition, the tame foxes’ heads were higher and wider and the snouts shorter as compared to the control farm foxes. Researchers also noticed changes in the domesticated foxes’ reproductive pattern. Normally, the females go into estrus once a year. This began to change. Some of the new physical traits they observed, such as floppy ears, are characteristic of juvenile animals.
Unselected juvenile foxes are, in general, more curious and less fearful of unfamiliar situations than unselected adults. However, by the age of about 45 days unselected foxes develop a fear response to unfamiliar situations. In contrast, foxes of the same age that were selected for friendliness to humans generally do not exhibit such a response. This is in keeping with the human-friendly foxes having more juvenile traits. In addition, in animals not selected for friendliness to humans there is an increase in glucocorticoid levels when the animals are confronted with an unfamiliar environment. This increase in glucocorticoids is absent in selected animals of the same age placed in an unfamiliar environment. Additional tests demonstrated that the function of the pituitary-adrenal system as well as the level of some neurotransmitters (which may play a role in the fear response to unfamiliar environments) were different between the selected animals and the controls.
Early in Dmitry Belyaev’s experiment, his researchers performed additional experiments to determine how much of the behavior change in the foxes was genetically determined. They crossbred foxes of different behavior, had foxes with one behavior type fostered by a fox of a different type, and cross-transplanted embryos between animals exhibiting different behaviors in order to demonstrate that genetics played a role in the differences in behavior between the different groups.
What is striking and illuminating from an evolutionary viewpoint is how strong selection for a single behavior trait can so rapidly alter so many genetically programmed traits. Dmitry Belyaev died in 1985, but at the time of this writing, others are continuing to work with these foxes to study the details of the changes in their behavior, morphology, and physiology.
Rodents’ development of resistance to anticoagulant poisons is another good example of how natural selection leads to genetic and phenotype changes. This example also demonstrates how such changes then allow a population to live in a previously hostile environment.
Anticoagulants, including warfarin and other related compounds, have been used in rodent control since the 1950s. Resistance in brown rats to some of the anticoagulants was first observed in 1958 and was found shortly after in house mice. In response, more potent anticoagulants were developed, but rodents also developed resistance to these compounds shortly after their introduction. Rodent resistance to anticoagulants has now been documented throughout the world.
Warfarin and warfarin-related compounds work by inhibiting the vitamin K reductase reaction, a reaction that recycles vitamin K from its 2,3 epoxide form to its hydroquinone form. Vitamin K hydroquinone is used for the necessary modification of several clotting factors; in its absence blood will not clot. The reaction that recycles vitamin K to its hydroquinone form is catalyzed by the vitamin K-epoxide reductase (VKOR) enzyme complex. This complex includes a protein called VKORC1. In the VKORC1 gene of both brown rats and house mice, studies have found multiple mutations associated with resistance to warfarin. The mutations allow the VKORC1 protein to function even in the presence of warfarin.
Have your students perform their own hands-on exploration of how strong selective pressure can change the genotype of a population, using Carolina’s Evolution in Real Time: Bacteria and Antibiotic Resistance Kit.