Bryn Mawr College

Alumnae Bulletin November 2009

On Course - EvoDevo

By Dorothy Lehman Hoerr

What do a human being and a butterfly have in common? You might expect them to share some genes. But only recently are we discovering common links in the development of various organisms, often in unexpected ways. For example, one of the genes used to make appendages in many organisms - such as the legs of both humans and insects - is also involved in the development of eyespots on a butterfly's wings.

Discoveries like this fall within the purview of evolutionary developmental biology, known as EvoDevo, a field of science that has developed rapidly in the past 30 years. Although the relationship between evolution and development has attracted interest ever since Darwin, the more recent resurgence in EvoDevo has been enabled by the advent of molecular biology as well as a desire to provide a more complete picture of how evolution works.

Assistant Professor Gregory Davis uses the example of the acquisition of bipedalism in the lineage leading to humans from a more ape-like ancestor."To have a complete understanding of that evolutionary transition, you would like to know not only what genetic changes were responsible, but exactly how those genetic changes altered development to produce all the skeletal and musculature changes associated with walking upright. This is the type of information we'd like to know to obtain a more complete picture."

Over the last 30 years,Davis says, the search for that complete picture has branched out in multiple directions, some rather different from the field's origins.One of the major goals of the senior seminar in EvoDevo is to introduce students to what Davis calls in the syllabus the many "flavors of evolutionary developmental biology."

EvoDevo has even gone beyond its initial boundaries of filling the gap between evolution and development."The more radical version is that there are ways in which the developmental process of an organism actually influences the direction that organisms can evolve," Davis explains."So once you've got this complicated developmental process, you can only tinker with it in some ways. It's this idea of developmental constraint,which is really different. It's not just saying that to have a complete picture, you need development. But it's saying that development actually controls or influences the direction, the evolutionary possibilities, that are out there. In amphibian species that have evolved loss of digits, for example, the digits that are lost tend to be those formed last during limb development in species that possess all of their digits."

For the first half of the course, students in the class select articles for discussion in areas that might include "classic" EvoDevo (the broader comparisons of the original field),micro or genetical EvoDevo (the study of small changes in more closely related species),molecular developmental evolution, polyphenism, or other miscellaneous topics such as environmental sex determination or evolutionary loss."Because the field has really matured and broken and splintered into all these flavors,"Davis says,"I try to provide a sort of a la carte menu of different case studies and topics and let students choose which ones they want to pursue."

In each class period, two students present articles on their topics and solicit feedback from the group to help them identify a problem on which to focus. In a recent seminar, Jean Chung chose to study the symbiotic relationship of the Hawaiian bobtail squid with the bioluminescent bacterium V. fisheri. Her question was how the squid, as it develops, is able to select that particular bacterium. Caroline Feldman's thesis focused on the sources of morphological variation in vertebrates. She used dogs as the basis of her study because, due to selective breeding, dog morphology is highly variable and yet the genetic changes that cause these variations are relatively easy to pinpoint. Similarly, Benjamin Pyenson chose to study turtles in which sex determination is the result of environmental factors like temperature and acidity."You could really tease apart these bi-partite influences," he explained in his presentation.

Another major goal of the course,Davis says, is for students to "effectively communicate a scientific problem and how they're going to approach it." To this end, students give two presentations of their thesis.One is a 10-minute talk to the entire biology department; the other is an hour-long defense before Davis and another faculty member who has also read the student's paper.

In the second half of the course, students choose their own topic to be the focus of a 20 to 25-page thesis.While the seminar students don't do any actual research in the lab, the papers are written in the style of a grant proposal, identifying a problem in the chosen field and designing an experiment to address it. The goal,Davis says, is for students "to become more adept at weighing evidence and identifying holes in arguments, and then designing experiments to fill those holes, even if they don't actually pursue the experiments."

This is one of the ways in which students grow most in the course, Davis says.He describes how, in early conversations with students about their topic, they may be reluctant to share their ideas, but over the term, he watches their confidence and ownership of the project grow."By the time they get to their defense, there's less need to have qualifiers on the ends of their sentences. Based on the intonation, there's less hesitancy."One of the greatest rewards, he says, is when the other faculty member questions some aspect of the student's work and the student is able to respond to the criticism confidently and convincingly.

Davis stresses to students that the defense is a unique opportunity to discuss their project with two other experts in the field. But a lot of preparation is necessary to take full advantage of that opportunity. Over the term, the students' theses go through numerous revisions, receiving feedback from Davis and a student partner. Students also practice their presentations in front of the class and work together to improve them.

Nicolette Belletier '09 found the presentations the most rewarding part of the course."The most challenging aspect of the presentation," she says,"was to explain my project in a nutshell while providing enough detail for everyone in the audience to understand the topic and appreciate why it is interesting." She feels the assignment gave her valuable experience in making a formal presentation.

Belletier says she was nervous before the thesis defense but quickly became comfortable.My defense was with Professor Davis and another professor I felt comfortable with and we just talked through what I had written." In the end, she says,"The whole experience was kind of fun and made me less intimidated about receiving criticism of my writing."

A sample of course readings:

For a reasonably good survey of different topics in the field:

• From DNA to Diversity by Sean Carroll, Jenn Grenier and Scott Weatherbee. Blackwell Publishing, 2005 (2nd edition).

For recent discussions on the role of developmental plasticity in evolution and medicine:

• Developmental Plasticity and Evolution by Mary Jane West- Eberhard. Oxford University Press, 2003.
• Ecological Developmental Biology: Integrating Epigenetics, Medicine, and Evolution by Scott Gilbert and David Epel. Sinauer Associates, 2009.

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