Modeling Natural Selection
Natural selection, the process by which heritable traits become more common in a population over generations, is a cornerstone of biology. This vitally important concept is often difficult for students to fully grasp, however, because it is abstract and cannot be observed in the laboratory. Consequently, students’ understanding of natural selection is often vocabulary-driven at best.
Models and simulations, though, can bring clarity and heightened comprehension to the process. Our seventh grade science students recently used a Flash simulation from http://www.techapps.net and a STELLA model adapted from The University of Michigan’s Global Change Curriculum to interactively explore how predation and pollution can drive natural selection in peppered moths.
Part A: Simulating Natural Selection
The evolution of the peppered moth over the last two hundred years has been studied in detail. In the 1950′s, H.B.D. Kettlewell ran a series of experiments and field observations to find out if natural selection had actually caused the rise of the dark peppered moth. The animated Peppered Moths: Natural Selection in Black and White reviews Kettlewell’s work and includes a “Birds Eye View” simulation in which students act as birds of prey in light and dark colored forests. In addition to providing a solid overview of moths and industrial melanism, the simulation makes clear the idea of selective pressure and serves as a nice introduction to STELLA modeling.
Part B: Modeling Natural Selection
To paraphrase George Box, all models are flawed but some are useful. STELLA models offer a practical way to dynamically visualize and communicate how complex processes, such as natural selection, really work and enable students to explore “what if” questions. Our model, based on Kettlewell’s research, monitored the phenotype and genotype frequencies of a moth population subjected to predation and various levels of industrial pollution. By manipulating the amount of pollution and charting the number of homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa) moths present over a time frame of 200 years, students were able to predict and then see the relations between pollution, selective pressure, genetics, and ultimately evolution. While the model is flawed (e.g. moth survival rates do not account for other genetic variations), it does make the concept of natural selection experiential.
Model and simulations can and should play an important role in the classroom, particularly in the sciences. Online simulations abound (the University of Colorado at Boulder has a very nice collection) and the STELLA website includes several sample models and the free STELLA player. If you would like to try our Natural Selection model, download the model, the player, the guiding questions, and get started; it’s just that easy!