Experimental Population Genetics in the Introductory Genetics Laboratory Using Drosophila as a Model Organism
Johnson, Ronald, Kennon, Tillman, Journal of College Science Teaching
Population genetics is the interface of ecology and evolution. Historical and recent events (e.g., gene flow, population declines or expansions, selection, speciation) can be inferred from present allele frequencies and genetic variation. Labs demonstrating evolutionary processes and principles often require both long periods of time and large amounts of space. Conversely, there are numerous computer simulations that have been developed to demonstrate these principles in the classroom (e.g., FlyLab, Pearson Higher Education; PopGene, Trinity Software; Magnetobacteria, Culp 1999; EVOLVE, Soderberg and Price 2003). Computer simulations provide the opportunity for rapid multigenerational analysis and the effortless analysis of large numbers of individuals. Nonetheless, there are advantages to the direct observation of these principles within living organisms (Eichinger, Nakhleh, and Auberry 2000).
We have developed a simplistic, inexpensive, and reliable hands-on approach using Drosophila to help students understand central theories of population genetics (natural selection and genetic drift) as they pertain to colonization and the establishment of populations. Drosophila are easy to culture in a classroom setting, can be maintained in high numbers, and have a short generational period. Additionally, the genetics of mutant variants are well understood and mutant flies are commercially obtainable.
The advantages of this laboratory exercise, which we use as the capstone experience for our introductory genetics class, are twofold. One is the development and testing of hypotheses by the science student. According to Chaplin and Manske (2005), "Biology too often becomes a recitation of facts that students must memorize to increase their knowledge base." In this exercise, students can begin to recognize science as truly "organized common sense" (Huxley 1909) and gain confidence with science as a practice rather than as a collection of facts. Many K-12 teachers utilize active learning, whereas most college science classes continue to be dominated by lecture and cookbook-style laboratories (Lancor and Schiebel 2008). This exercise is an example of active learning that is student centered and inductive in nature.
Second, this exercise has a strong writing component, which is lacking in many science curricula (Jerde and Taper 2004). Writing in science aids in the development of reasoning, serves to engage students' prior knowledge, and promotes investigation of new and alternative ideas (Hand 1999). Rather than writing up a canned laboratory exercise, students take an active role in the discussion of their own hypotheses and results.
Biogeography is the study of the distribution of organisms over space and time. Several ecological principles of island biogeography discussed by MacArthur and Wilson (1967) are demonstrated in this exercise. Islands are patches of suitable habitat, with the definition of "suitable habitat" characterized by the taxa of study. The islands of our exercise are containers of Drosophila media. Both physical and biological variables impact rates of island colonization. Island distance and size are important physical variables in determining rates of migration. Migration rates drop dramatically with increasing distance. Additionally, large islands provide greater potential for migrants to colonize, as well as greater resources for those colonizers.
Newly founded populations can differ genetically from parental populations by two important mechanisms: natural selection and genetic drift. Individuals of a population may have differing capacities of survival and reproduction under specific environmental conditions. The force exerted on certain individuals (and more fundamentally, their phenotypes) is natural selection. Those individuals surviving and having more offspring will exert a greater influence on the gene pool than individuals having fewer offspring. …