A Three-Step Method for Teaching the Principles of Evolution to Non-Biology Major Undergraduates

Replication simply refers to reproduction, the production of offspring generations from parent generations. While I point out that it occurs among asexual as well as sexually reproducing species, in this lesson I focus on sexually reproducing species because many (e.g., dogs and cats) are well-known to students. Here, I briefly introduce the DNA molecule, indicating that DNA is the instruction set for building an individual of any species; this allows me to introduce the concept of genotype and phenotype (Table 1). I reiterate that modern biology has decoded the DNA of many species (partially or completely) such that accurate predictions are made, every day, about how certain discrete parts of DNA molecules (genes) will guide the construction of certain proteins, and how those are involved in the building of the organism; I do not go into the regulation effects of genes, though I do mention that these can be significant and have driven the recent field of developmental evolutionary biology.

While I do introduce the difference between sexual and asexual replication here, I focus on the fact that replication does of course happen—in whatever way—and that it leads us to the next observable fact, that of variation.

In Fig. 1a, I represent replication of two individuals of a given sexually reproducing species by drawing two circles (representing a male and female) and a bracket indicating their reproductive union to produce offspring. I draw this simple diagram on the chalkboard or whiteboard, and give students time to copy it.

Variation simply refers to the fact that offspring are not normally clones. I remind students that while individual offspring may look alike at a glance, if one looks closely enough, one will find differences between both offspring and their parents, and offspring and their siblings. These differences, or “variations,” I indicate, are the result of a variety of processes in the replication of DNA and the construction of the phenotype from genotypic instruction (Table 1). While the sources of variation are, I mention, fascinating and many—I mention copying errors and recombination, for example, in the production of mutations—for the moment, these mechanisms are beside the point. What is most important at this time is that we can see variation every day, and I discuss and show images of variation in well-known species, for example in a litter of puppies. Depending on the composition of the class, I may or may not, at this point, introduce what kinds of variation may occur, and/or constraints on variation (see Table 1). With discussion and illustrations, then, students are reminded of the fact of variation; that variation is the rule of nature, rather than cloning. Why that is so important will become more evident in the next section, Selection.

In Fig. 1b, which I continue to draw on the board, I connect replication to variation by extending the bracket union of male and female parents to the variation panel, where I show variation by depicting a number of offspring similar to the parents, and to one another, but with some variations as well; in Fig. 1b, these variations are shown as differences in shading (in this case, two offspring—the colorless circles—are very similar to one another and parents, while there are others—the darker-shaded circles—that vary more from one another and from the parent generation).

Selection simply refers to the fact that, of the offspring in a given generation, those better suited to the selective environment (due to their variations) tend to live in better health, and ultimately do have a better chance of finding mates and therefore, passing the DNA that made them on to the next generation. This introduces the concept of fitness, which I refer to as an individual’s likelihood of having offspring, a sort of probability figure that, for reasons we will learn soon, can change from moment to moment.

I introduce the concepts of selective agents and selective environments, and ask students to consider these when they observe any species; what were the selective pressures that a given organism’s ancestors had to endure, resulting in the life form we see today?

I note that selective environments are extremely complex and difficult to characterize; they may change, for example, seasonally, and/or through the course of an individual’s pre-mating life, such that different pressures may “evaluate” individual fitness at different times.

It is important here to indicate that (except in the case of human beings and “artificial selection”) selective pressures are not conscious entities calculating how to shape a species; the temperature regime that shapes a species’ pelage over time, for example, is strongly conditioned by geography and physics, not a designing mind. I find that this is a significant intellectual barrier for many undergraduates to overcome. I strive to help students understand that while selective pressures may be thought of as essentially random, the effects of selection are certainly not random, and that they do generate apparent order by “tailoring” organisms “to” certain selective environments, over time. Depending on the constitution of the class, at this time I may or may not introduce the phrase and concept that evolution may be considered the “nonrandom differential survival of randomly varying replicators.”

In Fig. 1c, I continue to draw on the diagram by showing selection via copying the individuals depicted in the variation panel, but crossing out all but one individual. Those crossed out are individuals “selected against” (they do not pass their genes on to another generation). In this case, I propose that a particular new variation (medium-gray shading) makes some of the offspring slightly different from their parents as well as their siblings, and that (for whatever reason) this makes medium-gray shaded individuals better suited to their environment (more fit), such that they are more successful in finding mates and having offspring.

After walking the students through these three steps, using brief examples—but being careful to stay on track—I physically draw a line from the selection panel to a new replication panel, showing that the new variation now begins to spread because it was selected for (Fig. 1d). When this happens, I mention, the properties of the gene pool of that organism have changed; the essence of evolution. When this happens, I tell the class, “evolution”—the consequence of these processes—is happening, and it is observable.

It is important at this moment in the lesson to tell students that the implications of R, V, and S are that the characteristics of the population will change over time and that when this change is substantial, it leads to speciation; the “appearance” of a “new” form of life, the phenomenon of “speciation.” This allows me to introduce the species concept at large, though I do not spend much time on it at this point (a good review is found in Mallet 1995; a longer, popular-science discussion is found in Schilthuizen 2001).

I also reiterate that replication, variation, and selection occur every moment of every day in nature, and that we can see them. I ask students to consider, when they hear someone say that they “do not believe in evolution,” just which of the processes—replication, variation, or selection—does that person not accept to be occurring in the natural world.

I have had considerable success with this method. By the end of the lesson, students often laugh at the idea that evolution can be dismissed with the bumper-sticker summation that it is “just a theory.” Instead, they recognize that evolution is indeed an observable fact.

To follow up this lesson, I assign my students the task of researching and writing an essay describing replication, variation, and selection in any species they choose. This reinforces the lesson, gives the essay structure, and allows students to research any life form they find interesting. If the instructor wishes to include speciation in the lesson, some guidelines are found in Table 2.