Evolution is a central principle of biology. However, key aspects of evolution can be very difficult to model in a classroom setting. Two such key principles are (1) how the accumulation of small, relatively minor modifications can lead to the diversity of life over extended periods of time; and (2) how closely related genomes between different organisms can yield such dramatic differences in patterning and phenotype. The second aspect is explained largely by evolutionary developmental biology (“evo-devo”), whereby tiny modifications in when, where, and how much the same genes are used during development can lead to dramatic differences in the adult. Developmental construction, using similar building tools, can lead to the formation of a large variety of forms. Here, I describe fun, inexpensive, and simple methods using drawings and Legos by which students can actively generate understanding of descent by modification and evo-devo. Specifically, students learn how the same genes, used in different ways, can lead to “endless forms most beautiful” as originally phrased by Darwin himself. With the accompanying guided discussion, these activities also illustrate other key principles of evo-devo such as combinatorial and modular enhancers, Von Baer's principles, and genetic and morphological homology.
Evolution takes millions of years, and thus it is neither an easily observable phenomenon nor a readily tangible concept. Even rapid selective events, such as those occurring in viruses or prokaryotic cells, are not easily accessible to students. The mechanism of evolution relies on small, random mutations in the genome, which lead to gradual phenotypic changes that, if advantageous within the environmental context, may become more prominent in the population (Davidson & Erwin, 2006). The divergence of individuals within populations, let alone speciation from these incremental changes, can be difficult to model. Here, I describe a simple drawing activity that allows students to participate in generating small incremental changes, ultimately seeing how these can lead to divergence of phenotypes from a common ancestor while retaining evidence of underlying homology. These concepts of descent with modification and underlying homology are difficult for students to visualize, and especially difficult to connect. By witnessing divergence of images that all begin from a simple structure like a heart drawing, students are better able to conceptualize and accept this as a key aspect of evolution. This activity, with appropriate follow-up and discussion, can also be used to demonstrate (1) homology, (2) co-option (these small iterations are built by using the same preexisting genes in different ways, with the preexisting genes repurposed for a new function by altering the way those genes are used; Werner et al., 2010; Jiggins et al., 2017; Van Belleghem et al., 2017), and (3) Von Baer's principles of development occurring from general toward more specific features (Abzhanov, 2013).
Another key aspect of evolutionary mechanisms is the ability of similar genomes to result in dramatically different phenotypes. Why do organisms that look so different have such similar genomes? How can mutations of key genes lead to changes without major negative effects on phenotype? The answer to both of these questions lies within the field of evolutionary developmental biology (“evo-devo”; Akam, 1998; Carroll, 2005). While lagging behind its counterparts of physiology and ecology with regard to understanding biology and evolution, evo-devo has emerged as a field that is critical for understanding the mechanism of evolution. Advances in our understanding of genetics, particularly the non-coding regions of the genome, have revealed much of the mechanism by which changes in gene expression, rather than changes in the genes themselves, can lead to dramatic phenotypic changes without necessarily altering the function of the key proteins, especially if changes happen early in the organism's embryonic developmental cascade (Rebeiz & Tsiantis, 2017). In early development, there are many key genes that organize the body structure. These developmental control genes are often referred to as “tool kit genes” in that they are the tools that can be used to build an organism (Akam, 1998; Carroll, 2005). These are highly conserved throughout evolution, largely because they play such a critical role in development of an organism. The fact that these genes are so important means that mutations to their coding regions, resulting in modified versions of the proteins, are often detrimental or even lethal to the developing organism. However, if the non-coding regions of these genes are altered, such as the modular enhancers that control when and where these genes are activated, this allows nonlethal changes to occur. Just as a hammer can be used in many different ways to help build a wide variety of objects from a chair to a table to a bicycle, a tool kit gene can be used in many different ways to construct different organismal structures. If you destroy the hammer, nothing can be built. However, use the hammer in a different way and something new emerges. In the same way, a mutation in a tool kit gene that causes a change in the way that gene is used, rather than destroying the gene product itself, can lead to emergence of new organisms. As Sean Carroll puts it in his wonderful book Endless Forms Most Beautiful, “it's not the genes that you have, it's how you use them” (Carroll, 2005). Small changes in where, when, and how much a tool kit gene is used, rather than changes in the protein coding region of the gene, can lead to diversity without lethality. This also explains why genomes of dramatically different organisms are so similar, especially within the coding region of genes. Just as two people can have the exact same set of construction tools in their tool box, but build completely different objects, two organisms with similar tool kit genes can build very different phenotypes based on how those tool kit genes are used during development (Carroll et al., 2008).
Here, I describe an activity using Legos that models how similar genomes can be used to make the diversity of life that we recognize on Earth. This activity can be modified in several ways to make it appropriate for students ranging from elementary to undergraduate. Again, with the appropriate follow-up discussion, this hands-on activity can be used to model key aspects of evo-devo. With the Legos acting as tool kit genes, students can use the exact same set of Legos to construct, or “develop,” a multitude of different structures similarly to how the same genes can be used to construct dramatically different organisms during development. The differences in phenotypes do not lie in differences in the genes, but rather in how those genes are used during the construction (development) of that organism. Teachers are encouraged to use and adapt the included PowerPoint discussion slides for their own classroom purposes. Together with the drawing activity, students actively learn how descent with modification, combined with changes in the gene expression of key tool-kit genes during embryonic development, can lead to what Darwin called “endless forms most beautiful.”
What You Need
For the drawing activity, you will need three sheets of regular-sized paper, each with the same simple shape drawn on the front. I used a heart, but any basic shape will work fine as long as they are all the same. For the Lego activity, students can work individually or in small groups, depending on the size of the class. You will need a small set of Legos for each of the students or groups. Each set must be adequate to build something small yet substantial, and each MUST contain very similar, if not the exact same, set of Lego pieces. I purchased four large buckets of Legos and then separated them evenly into 25 different sets, each containing the same number and shapes of blocks (I didn't worry about color; for a picture of a standard set of Legos, see Figure 2). These sets can be used for years as long as it is emphasized to the students that they are not allowed to swap Legos and that, at the end of the activity and discussion, the sets must be put back into their bags exactly as they got them.
I use this as a culminating lecture for my undergraduate Developmental Biology course after we have discussed enhancer regions, gene regulation, and how evo-devo explains key mechanisms of evolution whereby small changes in the genome (usually in the enhancers) can have profound effects on phenotype. Even for advanced students, this simple activity is highly effective in helping them tie together these key concepts in a more tangible and clear manner. However, with any appropriate level of pre-instruction, these simple activities can be used to help explain difficult concepts like how humans and apes (or even humans and more distant organisms) can have such similar genomes and vastly different phenotypes.
As part of my college course, I have the students read Endless Forms Most Beautiful by Sean Carroll (2005). Students then perform these activities (playing with Legos and doodle drawing) while watching the video “What Darwin Never Knew” in lab (PBS, 2011). I strongly encourage teachers to use some of the many great resources available, such as the NOVA interactive websites on evo-devo (PBS, 2009), gene switches (Patel, 2007), the “zoo of you” (Shubin, 2009), and “guess the embryo” (VanCott, 2009) to help supplement teaching and to build understanding of these key topics. The activities described here can then be used as an active-learning, concrete visual of how these concepts emerge to help explain evolution and are effective as an introduction to the concepts of evo-devo, or as a culminating experiential activity to help solidify those same concepts. “What Darwin Never Knew” is very good at explaining key concepts of evo-devo and can be appropriate for high school or undergraduate students.
I do not tell the students ahead of time why I am having them “play” with Legos or draw. Rather, I have them think that it is a treat while they watch the video. Having no anticipation of the purpose, the outcome where students truly grasp the concept, which I call the “lightbulb” or “aha” moment, is dramatic. For the drawing exercise, arbitrarily divide the students into three sections. I group them by rows of desks that are near each other so that the drawings can be easily passed from student to student in intuitive circles of around 10 students each. Students are told that they can only add or erase within a 1 cm2 section of the drawing at a time, at which point they must pass the picture on to the next person, who will alter another small section of the drawing before passing it further down the row. I usually include an example of the size of the limited area that they can modify during each iteration at the top of the drawing. Any alteration (additional drawing or erasing) must be connected to the preexisting image. Students cannot start a new drawing area out in empty space, but must connect their drawing to a preexisting part of the image, which will expand as the activity progresses. This is a key point in evo-devo, as new structures do not emerge out of nothing, but rather as modifications of preexisting structures. Note that some aspects of the image may end up looking like they are unconnected if the students choose to erase a connecting line later. However, as Williston's law points out, the loss of structures is just as key to evolution as the gain of structures (Carroll, 2005). Other than these two simple rules, there are no explicit instructions and so each change is “random,” although depending on the maturity level of the class, teachers may want to emphasize “appropriate” drawings only. Each drawing gets passed around within its specific group (no passing between groups) repeatedly over the course of a couple of hours. In doing so, the three drawings repeatedly get modified in small increments over the course of a reasonable amount of time. If you have relatively limited time, you may wish to expand the area to about one square inch per iteration so that adequate divergence between the drawings can emerge. I have the students do this drawing activity, along with the Lego activity described next, while watching the video. College students are able to multitask in this manner, and I have found that the majority of their focus on the Legos and drawing is during the first hour of the video, which is more of a review for them anyway. By the time the evo-devo-heavy portion of the video comes around, students’ focus on the video is strong. In some ways, I've found that having the students multitask in this manner actually helps keep them focused rather than dozing off. However, each teacher should plan the activity however works best for their class, bearing in mind students’ maturity and attention levels.
For the Lego activity, students are explicitly instructed to build using their own Lego set (no swapping). Remember that each set contains basically the same number and shapes of Legos. Other than that, there are no restrictions. Although students are able to use all of the Legos in their set, they do not have to use them all, just as not all genes are used in any given organism; there are vestigial genes or pseudogenes that are not used by certain organisms (Chandrasekaran & Betrán, 2008). Structures that are built without use of all the pieces or “genes” can also be used to discuss the classic experiment whereby a mouse mesenchyme is used to stimulate development of teeth in the chick oral epithelium. That experiment demonstrates that the chicken genome must have genes that encode tooth development, remnants from the last common ancestor with mice, even though those genes are no longer used or induced under the normal chick developmental program (Harris et al., 2006). Once students have built something they like, they should save it to share with the class at the end.
Outcome & Discussion
At the end of the activity, the three drawings will have become highly diverse (see Figure 1). These drawings can be used to demonstrate four main concepts: (1) that small, random, incremental changes over time can lead to diversification, especially between separated populations; (2) that underlying homology still remains – the heart can still be seen underlying all the details that have progressively emerged, just as the underlying homology of physiological structures like organismal limbs are apparent beneath the more detailed phenotypical differences that have evolved; (3) that these changes are based on relatively simple changes in how the same “genes” (in this case the pencil graphite and the eraser) are used rather than the de novo synthesis of new genes; and (4) the order of development from simple (general) structures to more specific and complex aspects of an organism's morphology.
The only rules that the students were given regarding the drawing were that they were to alter only a small region at a time (small incremental changes), that their changes must attach to a preexisting structure on the picture (modifications can only be made on preexisting structures), and that they could use only the provided pencils and erasers (limited to preexisting pseudogenes). During discussion, I emphasize that the small, random incremental changes that lead to diversification are normally selected for in the context of natural selection. While a strong understanding of natural selection is also a key aspect of understanding evolution and should not be addressed as a passing comment, I am assuming that the students have previously been taught natural selection, and therefore this discussion can be used to bridge the concepts of incremental change, natural selection, and descent with modification. A minor modification that can be used to add a natural selection component to the model activity is described below. In addition, the modification from a general image (heart) to much more diverse and specified structures demonstrates how development occurs. The more general parts (e.g., the homologous bone structure of limbs) develop first, and the evolutionary “modifications” develop later. This is why it is very difficult to discern the species of very different organisms from their early embryos; the early embryos look remarkably similar and the specific features appear later (VanCott, 2009). This is a result of developmental necessity in relation to common ancestry; descent with modification requires that the underlying general commonalities develop first, and very similarly between organisms (Abzhanov, 2013). Students often modify the additions made by other students by further drawing or erasing parts of the new structures, demonstrating how continued incremental changes can modify structures that have already evolved from the original form (Figure 1A).
After the students have finished building with their Legos, I have them briefly describe their creations. Each will be unique and often vastly different, despite starting with the exact same set of pieces (see Figure 2). Based on their construction (their choices of when and where to use each piece), the vast differences emerge. This is analogous to organisms that have very similar genomes yet very different phenotypes. Based on when, where, and how much those genes are used during development, or how those genes are used to “construct” the organism, “endless forms most beautiful” can emerge. To further strengthen the point and visualize the concept, I put up a slide of all the different animals built out of Legos at Legoland. In order to make a giraffe out of Legos, it is not necessary to develop a new “giraffe neck piece.” To make an elephant, an “elephant trunk piece” is not required. Instead, these animals are built using the same, standard Legos in a new way (see slides 6–8 in the PowerPoint discussion file). The same genes, used in different manners (timing and location) during development, can result in dramatically different phenotypes.
These concepts fit perfectly into what the video “What Darwin Never Knew” discusses. I then work to tie it all together with a short PowerPoint guided discussion about how enhancer regions and mutational alterations therein provide the framework for evo-devo. These slides can be accessed as Supplemental Material with this article (see Figure 3). Teachers are encouraged to download and modify the slides for their own discussion purposes.
By this time in my undergraduate course, we have extensively discussed the modular and combinatorial nature of enhancers, as well as co-option. The main concepts can be presented to students at all levels, with varying levels of detail as appropriate. The drawing and Lego activities should be accessible to all and will help explain (1) that small incremental changes can lead to diversity and (2) that the same genes (Legos) can result in many different phenotypes based on how they are used to construct the organism during development (when, where, and how much those genes are used).
Potential Modification of the Incremental Drawing Activity
I have also tried to include a “natural selection” component to the heart drawings by periodically removing sharp edges specifically in one drawing, specifically removing soft edges in another drawing, and leaving the third drawing untouched as they are continuously passed around the group. This can be done by having an instructor and teaching assistant (if available) sit within a group and erase hard vs. soft edges each time the drawing comes to them, thus modeling selective pressure against specific phenotypic changes. The third group would have no such selective pressure and thus the image would emerge completely as modified by students. If it is not possible to have the instructor or teaching assistant perform these modifications, the instructor can enlist the help of a trusted student within each group to be given that same job. This shows the importance of environmental selection of certain traits in determining the evolutionary phenotypic outcome, as the drawings can become quite distinct based on the selection of hard or soft edges. The concept can be exemplified well with this slight modification, but the results have varied with regard to the extent of relevant differences in the final drawings. The teacher should be prepared to adjust on the fly if the differences are not dramatic. Similarly, not all images change evenly, often depending on the level of excitement you've managed to generate in the class regarding the images (Figure 1C). In that case, the teacher can also emphasize that not all species evolve. Many are still very similar to the original common ancestor while other branches may have evolved quite extensively. I do try to make the drawing activity a “friendly competition” between the groups to help motivate them to be creative and take the drawing activity seriously. This can sometimes help to ensure its success.
Using this hands-on activity, paired with an associated video and PowerPoint slides (see Figure 3), can successfully help students understand key aspects of evolution, including descent with modification, and the key aspect of evo-devo whereby the same genes can be used to generate dramatically different phenotypes, depending on how those genes are used during development. Students come away from these activities having a better grasp of how species such as humans and apes, or even more distant animals, can have very similar genomes, yet with a few small modifications to the enhancer regions that control when, where, and how much a gene is expressed, “endless forms most beautiful” can emerge. Teachers are encouraged to use and adapt these activities and the associated discussion slides – which include human examples of polydactyly (Gilbert & Barresi, 2018) and eye color (White & Rabago-Smith, 2011) to demonstrate the roles of enhancers in causing phenotypic changes.