Teaching about time: A biologist's perspective

Erica Crespi, School of Biological Sciences, Washington State University

My research primarily focuses on investigating how the neuroendocrine stress responses vary with environments, across the life of an animal, as well as across different groups of vertebrates. The time scales I deal with range from milliseconds (the time it takes to propagate an action potential down the axon of a neuron to secrete a neurotransmitter) to minutes (cellular responses to hormones), to days (circadian rhythms), to many years (within the life span of the animal), to hundreds of years (generation times), to thousands-millions of years (e.g., evolutionary time scales), to billions of years (to the origins of life). In biology, we mark time using molecular clocks, metabolic clocks, seasonal and circadian rhythms, and divergence times. On occasion, I've discussed all of these metrics of time and their biological significance within the same course (e.g., Comparative Anatomy and Physiology). How odd that I never explicitly considered the importance of effective teaching methods that convey conceptions of time. Upon some introspection about the importance of making sure students understand concepts of time in biology, I came to the conclusion that time is one of the most important physical properties that constrain biological processes, and it is a common thread throughout biological disciplines although the time scales vary.

Most relevant to the topic of the Teaching about Time Workshop, I have had to teach about geological time in several courses (Introductory Biology, Vertebrate Zoology, Invertebrate Zoology, Comparative Vertebrate Anatomy) in order to explain evolutionary processes and phylogenetics. I discuss the fossil record as evidence for large changes in biological diversity over deep time scales, and I explain how molecular clocks are vital to the study of phylogenetics and how biologists more commonly use these clocks to estimate divergence times of lineages. In both cases, I explain that we rely on techniques such as carbon dating and estimations of when plate tectonic movements and climate shifts took place to "calibrate" the biological clocks we use. Because I'm focusing on the biology in my courses, I only give cursory explanations about how these geology-based timepieces work and I barely mention that there is error associated with the time estimates generated by these clocks. While this gives me more time to spend explaining molecular clocks and evolutionary processes, I suspect that an expanded discussion of geological clocks is critical in students' understanding of deep time and organic evolution.

In my teaching experience, I have spent the most time on developing pedagogy to teach time in the context of phylogenetics, in which I introduce the concept of a molecular clock, different kinds of molecular clocks, how molecular clocks are derived and used in phylogenetics, and the assumptions and caveats associated with their use. I assign a reading that covers this topic (the content in most Evolution text books is minimal) followed by lecture period devoted to the introduction to the topic (which typically spills over to a second lecture). I use analogies to convey the concept of mutation rates (e..g, "like a gieger counter") or evolutionary distance by showing family trees, and I explain how mutation rates are estimated from empirical data, e.g., graphs of numbers of base pair substitutions over time, different rates of substitutions in different parts of a gene and genome and for different molecules such as immunological factors. In the laboratory portion of the course, students calculate evolutionary distances based on the numbers of similarities and differences in gene sequences or morphological traits that they empirically observe between different kinds of animals. These evolutionary distances are used to generate phylogenetic trees, whose branch lengths and numbers of bifurcations between groups are proportional to the numbers of differences between groups. With our handy molecular clocks and/or estimates of the timing of geological or climatic events, we can estimate the divergence times among the groups. I assess student learning at the end of the unit by asking students 1) a short answer question to explain the concept of a molecular clock 2) to critique a phylogenetic hypothesis (tree) generated in a published paper to asses their ability to recall caveats and assumptions about how biologists estimate divergence times, and 3) to generate a phylogenetic tree from a second dataset and report evolutionary distances and estimated divergence times. This pedagogy was designed within the Integrated Inquiry-Based Instructional Unit paradigm, in which students are given the opportunity to develop a concept of deep time through the process of actively answering a biological question (i.e. what is the evolutionary history of a group of animals presented to them in the lab).

While I have spent considerable effort in designing a pedagogical strategy for teaching about molecular clocks and estimation of evolutionary divergence times in the context of phylogenetics, there are many challenges I have yet to address. Specifically, I do not assess whether students understand the concept of deep time. I attempted to teach this by showing slides and giving out handouts that depict the different geological eras in proportionally sized boxes with dominant species and land mass associations associated with each, combined with series of maps that illustrate land mass configurations, climate zones, or patterns of proposed animal/vegetation distribution maps associated with the different eras. But these snapshots do not convey any dimension of time, and at best, rely on student memorization. In addition, my pedagogy for teaching evolutionary time was designed for a very specific phylogenetics context with lab; however, discussion of deep time is more often raised during lectures when covering biological diversity in lower- and intermediate-level organismal-level courses. Incorporating effective pedagogy to teach concepts of deep time in these contexts would be very desirable for the biology instructor. Finally, I think teaching about deep time in a biological context, particularly when students apply what they learn about evolution to humans, poses an additional challenge as many students experience cognitive dissonance between the concepts of time they learn in the classroom vs. the concepts of time they learn through their religious teachings and belief systems. Given the national debate that pits evolution science against religion, this problem is especially an issue when trying to teach deep time within the biology curriculum.