[Special thanks to Matt Taylor for his contributions to this blog post.]
Judging from the number of comments, the “Reptilobird” post is by far the most popular one on this blog. And no wonder. It is a simple, fun activity that combines the stages of meiosis with patterns of inheritance. Along the way, it beautifully illustrates how sexual reproduction produces an astonishing range of sibling variation.
Wouldn’t it be great if we could incorporate natural selection into this lab as well, to show the connection between meiosis, patterns of inheritance, variation, and evolution? I am now happy to report an idea for this as well! And, much like the reptilobird activity, it is not my idea. I’m just the messenger.
Like so many other great ideas, this one came from the annual workshop for the Association for Biology Laboratory Education. (It was wonderful, as usual. You will never find a friendlier bunch of colleagues, and they are devoted sharing what they know about high-quality education.)
One of the workshops, by Boston University’s Dale Pasino, was devoted to a clever invention that he calls Pazmo bugs (you can find the workshop abstract here). He has created imaginary insects with one trait for each of 12 chromosomes. Each student begins the “zero generation” with one bug that is heterozygous for all 12 traits. But then they start reproducing; each bug randomly creates two gametes, one for itself and one to give to a mate—that is, another bug in the same lab group. Students document the alleles passed onto their generation 1 bug. These bugs find a new mate, also within the lab group (though not with a sibling!), producing generation 2. These bugs mate as well, and so forth. Because each student receives one of the two bugs produced in each mating, the population size remains constant over the generations.
Unlike the reptilobird activity, the stages of meiosis are not the focus. Rather, the point is to depict the genotypes of five (or more) generations. From there, the possibilities are endless. If you teach Hardy-Weinberg equilibrium, you could have your students calculate p and q for each gene in each generation. You could show how genetic drift eliminates alleles from small populations. Or you could show how other mechanisms of evolution disrupt Hardy-Weinberg equilibrium.
The part that intrigued me most, however, was the way that the activity incorporated natural selection. Students are given a list of environmental changes and, without looking at their own Pazmo bugs, they select (with justification) the three traits that would contribute most to reproductive success under each condition. Then, students use a formula to estimate each bug’s likelihood of reproductive success based on its phenotype. They tally which bugs fall into each category, then repeat the process for one or more additional environmental conditions. (In Pasimo’s version, the students use their five generations’ worth of bugs as their “population,” even though these bugs do not exist simultaneously.)
It quickly becomes obvious that a bug that is extremely likely to survive in one condition might not have the same luck when the environment changes. Moreover, some populations might not have any bugs that are likely to reproduce at all. I love this idea because it is a powerful way to counteract the misconceptions that (a) evolution happens to individuals and (b) natural selection occurs “in order to save the species.”
How does this idea apply to the reptilobirds? When we complete the reptilobird activity in our lab, students tape their birds to the board to illustrate the huge range of genotypic variation among siblings. It would be easy to label each bird with unique number and have students judge which would be most likely to reproduce in different conditions. Listed below are a few sample scenarios, each ending with a note suggesting which reptilobird trait is most likely to be affected (see the original blog post for more information):
- Flowers with curved shapes become more common in the environment. At the bottom of each flower is sugary nectar that is a good snack for animals that can reach it. [beak shape]
- Changes in climate patterns have made the environment much windier. Some birds can soar on the wind while looking for food. [wing length]
- Predatory cats that are visual hunters migrate into the area. The cats have trouble seeing green prey against the grassy background. However, they can see showy tails with many feathers more easily than they can see simple tails. [tail color and number of tail feathers]
- After years of heavy rains, a stream in the area becomes larger and has a swifter current; it also has many small fish that are a good food source. Sturdier birds can more easily stand in the stream. [leg thickness]
- A virus moves through the reptilobird population. One of the symptoms is a condition called walleye, in which part of the iris becomes bluish-white. Reptilobirds begin to avoid any potential mate with white irises. [eye pigment deposition]
If you have questions or want to share more ideas for environmental scenarios, please add a comment to this post!