Coevolution in the Classroom
© The Author(s) 2010
Published: 23 January 2010
In this special issue of Evolution: Education and Outreach, many different authors argue for the importance and utility of coevolution as a topic for classroom exploration—and for good reason. Coevolved relationships are likely to impress and interest students: acacias that produce thorny homes for the ants that protect them from voracious insects (Janzen 1966), flowers that exchange nectar for the sexual assistance of a pollinator (e.g., see Nilsson 1998), and newts that have evolved to be so toxic that they can easily kill most any predator—except the resistant snakes that normally feed on them (Brodie and Brodie 1990). Coevolutionary adaptations are often extreme and sometimes weird and wonderful.
In addition, coevolution offers a perfect way to weave evolution throughout instruction in ecology. Many policymakers and educators have advocated integrating evolution throughout biology instruction so that evolution is not relegated to a discrete unit at the beginning or end of the course, but is accurately portrayed as woven throughout scientific thinking in all areas of biology (e.g., National Academy of Sciences 1998; Alles 2001). Coevolutionary processes and phenomena clearly illustrate the deep ties between evolution and ecology. Using coevolutionary examples, students can understand how ecological relationships result from evolutionary processes and how an understanding of evolution informs ecological research.
In an article in this issue, Thompson (2010) reviews many different forms of coevolution. Here, we will delve into just a few of the processes he addresses (those most likely to come up in classrooms and textbooks), provide summaries of the basic mechanisms involved, give additional examples, and of course, provide relevant teaching resources.
The term coevolution describes a process in which two or more different species reciprocally affect each other’s evolution. This may take the form of a tight-knit relationship, in which one species evolves a trait in response to a pressure or opportunity from a second species (e.g., a plant evolving a flower color that attracts a particular bird pollinator), and the second species evolves in response to that change (e.g., the pollinator evolving a beak shape that allows it to better access the nectar of that plant). Coevolutionary relationships may also be more diffuse, involving a web of interactions between many different species (e.g., a plant species evolving a flower color that attracts a whole class of pollinators, which affects the evolution of each of those pollinator species in a slightly different way, which may, in turn, affect other species with which the pollinators interact). We can observe many relationships in the natural world that seem to have coevolved, but working out the details of the evolutionary processes that led up to a particular ecological relationship can take a lot of investigation. Biologists generally look for evidence that each species involved in the hypothesized coevolutionary process has evolved in response to the other(s).
Thompson lists three basic types of ecological interaction that can set the stage for coevolution (see Table 2 in Thompson 2010): trophic antagonism (i.e., predator–prey or parasite–host relationships), competition, and mutualism. We will examine each of these in turn.
Predator vs. Prey
Predator–prey relationships can lead to different sorts of coevolutionary phenomena, but one of the most interesting (and readily graspable by students) is an evolutionary arms race. This is exactly what it sounds like: two parties one-upping each other in terms of defense and counterdefense or attack and counterattack. It works like this: Imagine an insect that feeds on a particular plant species. Any individual plant that happens to carry a mutation coding for, for example, a slightly stronger insect-repellent chemical will be favored by natural selection, and we would expect the mutation to increase in frequency in the population. But, of course, as the mutant gene becomes more common, any insect that happens to have a mutation that provides a slightly higher tolerance for the defensive chemical will be favored, and over many generations, this gene will become more common in the insect population. This sets up another situation in which stronger defensive chemicals are favored in the plants—and if this trait evolves, it sets up another situation favoring stronger tolerance in the insects... and so on. The levels of repellence and tolerance may continue to escalate without either species “winning.”
Do arms races continue escalating forever then? No, and the explanation offers instructors a chance to introduce students to another important evolutionary concept: evolutionary trade-offs. Many different traits contribute to an organism’s overall fitness, and optimizing one trait often means downgrading another. For example, for our plants and insects, producing stronger chemical defenses and tolerances may take a lot of energy, decreasing the amount of metabolic energy available for reproduction. Eventually, the benefit of producing stronger defenses and tolerances will be outweighed by the detriment of decreased reproduction. At that point, escalating the arms race will no longer be favored by natural selection, and the evolutionary one-upping will stop.
Competition and Coevolution
Ecological conflicts can arise, not just between the eater and the eaten, but between two species that play the same role in an ecosystem (e.g., eater vs. eater). This occurs when two species compete for food, space, or other limited resources. Thompson explains that this competition can result in a coevolutionary phenomenon called character displacement: when two species compete for the same set of resources, natural selection may favor traits in each species that allow them to specialize, subdividing the resources, or accessing slightly different resources. For an example, imagine two species of bird that wind up on an island together after a hurricane. The species have similarly sized beaks and feed on similarly sized seeds—and so must compete for the same limited resource. One population happens to have a few members with mutations that increase beak size, allowing them to eat slightly larger seeds more efficiently. In that population, these large-beaked birds are likely to reproduce more and spread their genes, since they won’t have to compete with as many other birds and will likely be able to get more food. As one species evolves slightly larger beak sizes, the other species is likely to experience selection favoring birds with smaller beak sizes, which allows them to access a resource with less competition. Over many generations, the character (beak size) is likely to be displaced (i.e., likely to diverge) in the two species. In fact, this is almost exactly what biologists think has occurred with two species of Galapagos finch, Geospiza fuliginosa, which has evolved a smaller beak and body size, and Geospiza fortis, which has evolved the larger beak and body (Schluter et al. 1985). For another example of character displacement involving stickleback fish, see the article of Dolph Schluter in this issue.
Scientists gather many different lines of evidence to determine whether character displacement has occurred. First, they may study how the character varies over the different locations in which the species are found. If character displacement has occurred, we would expect the trait to be divergent in places where the two species both live, but to be less extreme in places where only one of the species lives. For example, on the Galapagos island where G. fuliginosa lives alone (i.e., in the absence of G. fortis), the population has a larger beak than it does on the islands where the two species both live. And the reverse is true for populations of G. fortis (Lack 1983; Schluter et al. 1985). This is exactly what we would expect to observe if coevolutionary character displacement took place on the islands where the two live together. Scientists may also look for direct evidence that natural selection is operating on the character. For example, biologists were able to observe directly how exploiting small seeds (normally the food of G. fuliginosa) increases the fitness of G. fortis individuals on the island where G. fortis lives alone (Schluter et al. 1985). These observations strongly suggest that having to share resources on the islands where the species both occur depresses fitness and sets up a situation in which we would expect natural selection to act on the species.
Trophic antagonism and competition suit an image of “Nature, red in tooth and claw” (from In Memoriam A. H. H., Tennyson 2007), but coevolution also has a warm, fuzzy side. Coevolution occurs not just as a result of conflict between species, but also as a result of cooperation. In mutualistic relationships, each species involved gets some benefit (i.e., a boost in fitness) as a result of the interaction. A classic example of a mutualism is pollination: the pollinator gets a food source (nectar or pollen), and the plant gets its gametes distributed to other members of the same species. As discussed above in “The Basics” section, coevolution as a result of mutualisms may be diffuse, involving whole groups of species, but Thompson also describes coevolutionary, mutualistic relationships that evolve to be so specific that the participants become completely codependent and cannot live without one another.
Such tight-knit mutualisms often occur when one species actually lives inside another, a situation known as endosymbiosis. This might seem like a rare phenomenon, but in fact, such extreme associations of distantly related organisms are found across the tree of life: Giant clams harness solar energy through their endosymbionts, photosynthetic zooxanthellae (e.g., see Lee et al. 2005). Tubeworms living near deep sea vents have a special organ that harbors their endosymbionts—bacteria that can convert the sulfurous compounds released by the vent into usable energy (Cavanaugh et al. 1981). And like Russian nesting dolls, some termites’ guts are inhabited by wood-digesting flagellates, which are themselves inhabited by bacteria (e.g., see Ikeda-Ohtsubo et al. 2007). Such examples demonstrate that each species involved in such a close relationship must have adapted to the opportunities presented by the other—especially in cases where neither species can live on its own.
Coevolution should be a key component of evolution instruction because the process has been so important in shaping the history of life. As Thompson points out, we (and all other complex organisms) are dependent on the coevolved relationships that form the basis of our ecological interactions and even our own metabolism. Just as importantly, coevolution offers many compelling examples for students to sink their teeth into. Coevolution encourages students to think one step beyond an evolutionary scenario, to consider the likely ramifications that one species’ evolution is likely to have for other species it interacts with, and in so doing, helps students appreciate the blooming, buzzing complexity that characterizes the natural world.
Give Me an Example of That
A case study of coevolution: squirrels, birds, and the pinecones they love. In an article in this issue, Benkman (2010) describes the coevolution of pine trees with the squirrels and crossbill birds that eat the trees’ seeds. The case study below explains the basic biology of the three players and highlights some of the evidence that has convinced biologists that this interaction represents a case of coevolution. It is written at a level appropriate for high school students. Read it at: http://evolution.berkeley.edu/evolibrary/article/evo_34
It takes teamwork: how endosymbiosis changed life on Earth. To learn even more about the merging of bacterial lineages through endosymbiosis and coevolution, check out a case study on the topic. Written at a high school level, this resource answers basic questions, such as: What is endosymbiosis? What role did endosymbiosis play in the evolution of eukaryotes? And how did endosymbiosis change our view of the branching pattern on the tree of life? Read it at: http://evolution.berkeley.edu/evolibrary/article/endosymbiosis_01
Evolutionary medicine (reprinted on the Understanding Evolution website with the permission of Roberts and Company Publishers, Inc.) http://evolution.berkeley.edu/evolibrary/images/evol_medicine.pdf
Studying living organisms: viruses and host evolution. http://www.accessexcellence.org/AE/AEPC/WWC/1995/viruses.php
Natural selection: the basics. Darwin’s most famous idea, natural selection, explains much of the diversity of life. Learn how it works, explore examples, and find out how to avoid misconceptions. http://evolution.berkeley.edu/evolibrary/article/evo_25
Misconceptions about natural selection and adaptation. Natural selection is often misconstrued as a process that perfects organisms and provides them with exactly what they need. Find out the truth. http://evolution.berkeley.edu/evolibrary/article/misconcep_01
In the Classroom
Clipbirds. In this lesson for grades 6–12, students learn about variation, reproductive isolation, natural selection, and adaptation through a version of the bird beak activity. http://www.ucmp.berkeley.edu/education/lessons/clipbirds/
Breeding bunnies. In this lesson for grades 9–12 from WGBH, students simulate breeding bunnies to show the impact that genetics can have on the evolution of a population of organisms. http://www.pbs.org/wgbh/evolution/educators/lessons/lesson4/act1notes.html
Toxic newts. This five minute clip from Evolution: Evolutionary Arms Race tells the story of a species of newt and its garter snake predator. http://www.pbs.org/wgbh/evolution/library/01/3/l_013_07.html
Biological warfare and the coevolutionary arms race. This case study for high school and college students explains how an evolutionary arms race has pushed a mild-mannered newt to the extremes of toxicity, and how evolutionary biologists have unraveled its fascinating story. http://evolution.berkeley.edu/evolibrary/article/biowarfare_01
Got lactase? This news brief for high school and college students explains that the ability to digest milk is a recent evolutionary innovation that has spread through some human populations. Recent research reveals how evolution has allowed different human populations to take advantage of the nutritional possibilities of dairying. http://evolution.berkeley.edu/evolibrary/news/070401_lactose
The author wishes to thank Judy Scotchmoor for helpful comments on earlier drafts, as well as David Smith for help developing images.
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