How-To-Do-It

Exploring Genetic Drift & Natural Selecton Through a Simulation Activ TimothyJ. Maret Steven W. Rissing

During the 1960s and early 1970s, diversity, potentially reducing a spe- exercisesdealing with genetic drift that the theory of as a cies' capabilityfor adaptive change in were published during the 1970s had Downloaded from http://online.ucpress.edu/abt/article-pdf/60/9/681/10396/4450580.pdf by guest on 30 September 2021 mechanism for was at its current or future environments. the same general flavor as the beans heyday. Other mechanisms of evolu- Genetic drift is a concept that an edu- in a bag exercises [Western(1976) used tion, such as genetic drift, were eatedpttbicTust understandin order beads, and Kramm(1977) used colored regardedas being of minor importance to make informed decisions on issues index cards].We decided that an inter- relative to naturalselection. Textbooks of preservation and biodiver- esting laboratoryexercise demonstra- of the time echoed that sentiment,and sity. The disclaimermade in old intro- ting drift in laboratorypopulations of genetic drift received little more than ductory textbooks (and fruit flies (Benner 1987) was a little a passing mention that it could con- courses)that genetic drift is not impor- more complicatedand time consuming ceivably be a factor in evolution in tant because it only occurs in small than what we were looking for in a populations of extremely small size. populations is no longer appropriate. nonmajors'course. An exercise devel- In an attempt to update the labora- oped by Hammersmith and Mertens ... majorevolutionary changes would tory exercises for our process-oriented (1990)that simulated random changes be unlikely to occur by chancealone nonmajors'biology course (see Law- in frequencieswith the aid of a in any except very small populations son et al. 1990 for a descriptionof the random number table demonstrated (Keeton 1967). course), we searched for a laboratory the randomness of genetic drift, but It is certainlyfairto say, however,that exercise to introduce students to the we felt students who were not science in the evolutionof averageanimal and conceptof genetic drift.While textbook majors would have a difficult time plant populations,genetic drift is usu- authors have responded to the connecting this concept to evolution- ally a negligible factor (Baker & increased theoretical importance of ary change. Allen 1971). genetic drift by including drift in dis- Because we were not satisfied with cussions of the mechanisms of evolu- what we found, we set about develop- Since that time, the potential role of tion, laboratorymanuals look like they ing our own exercise that would allow genetic drift in evolution has received were written during the early 1970s. students to explore the concept of considerabletheoretical attention. The We surveyed 11 laboratorymanuals genetic drift. Students in the nonma- growing acceptanceof Wright's (1931) written for introductory biology jors' course at ArizonaState University shifting balance theory and Mayr's courses that had a copyright of 1988 already use a simulation exercise to (1954,1963) theory of peripatricspecia- or later. Seven of these manuals did study natural selection in an artificial tion provided genetic drift a major not mention the concept of genetic population. This exercise is very simi- role, along with natural selection, in drift at all. The other four contained lar to the one described by Stebbins evolution and .Eldridge and a simple exercise in which an equal and Allen (1975), in which students Gould's (1972) theory of punctuated number of two colors of beans (or "prey" upon different colored paper equilibrium relies heavily on genetic beads) are placed in a bag, and stu- dots on backgroundsof various colors drift in isolated populations. In dents randomlysampled a small num- and patterns. We modified this exer- Kimura's(1983) neutral theory, genetic ber of the objects. Students are sup- cise to include the concept of genetic driftis the principalfactor in the evolu- posed to see that in small samples, drift coincident with natural selec- tion and divergence of species. the observed distributionoften varies tion-just like in the real world. Incor- The implications of genetic drift are from the expected distribution.While porating selection and drift into the also of increasingconcern to conserva- this concept of is same simulation provides students tion biologists (Meffe & Carroll 1994). important, we doubt that students with the opportunity to see the effect As populations of many species pulling beans out of a bag really grasp of both processes within the same decrease in size, genetic drift may the role of genetic drift in evolution, experimental "populations" and pro- cause a significant loss of genetic much less conservationbiology. vides the pedagogical rigor of consid- We were searching for an exercise ering two viable hypotheses to explain that would do more than just demon- observed phenomena simultaneously. TimothyJ. Maretis AssistantProfessor strate sampling error, so we decided In this exercise, students simulate of Biologyat ShippensburgUniversity, a mus- Shippensburg,PA 17257; e-mail:.tjmar- to searchjoumals-onscience teaching changes within population of [email protected] W. Rissingis to see if we could find a more appro- sels inhabitinga rocky shoreline.There Professorof Biologyat ArizonaState priate laboratoryactivity. We did not are two sources of mortality-oyster- University,Tempe, AZ 85287-1501. fare much better than we had with catchers (large shorebirds) that hunt laboratory manuals. Two laboratory for mussels visually and drifting logs

EXPLORINGGENETIC DRIFT & NATURALSELECTION 681 that slam into the rocks and crush visually, but rather should remove 75 groups? Why do all of the oyster- mussels randomly. Since oystercatch- paper dots in a random fashion. We catcher graphs look similar and the ers are visual predators, the color of have found that an effective way to log graphs look so different? What a mussel will affect its survival. Color do this is to wrap maskingtape around other factors might affect the outcome will have no effect on whether or not a pencil sticky side out, and drop the of the simulations? What might hap- a mussel is crushed by a drifting log. pencil onto the habitatat randomuntil pen if color was not heritable (i.e. By monitoring changes in the popula- all but 25 dots are stuck to the pencil. offspring did not resemble parents)? tion over several generations,students An altemative method is for students How would and mor- can explore the simultaneous effects to wrap the tape (sticky side out) tality rate affect the outcome for the of natural selection (due to mortality around the tips of their fingers and oystercatchers?For the logs? from oystercatchers)and genetic drift randomly touch their fingers to the Now you are ready to introducethe (due to random mortality, at least in . terms natural selection for the process terms of color, from drifting logs) on The 75 mussels that were removed seen with the oystercatchers, and their population. We present the labo- from the habitat are dead; they can genetic drift for the process seen with ratory exercise using a learning cycle be returned to the appropriatevials. the logs. Natural selection results in approach, with exploration, discus- The 25 mussels that are left on the , whereas genetic drift sion/term introduction, and concept habitat survived the first selection epi- does not. application phases. The exercise has sode. They should be removed by gen- been tried with several laboratorysec- tly shaking the habitat. The next step tions, and we are quite satisfied with is to have each survivor reproduceby Concept Application Downloaded from http://online.ucpress.edu/abt/article-pdf/60/9/681/10396/4450580.pdf by guest on 30 September 2021 the results. Students gain an increased adding 3 paper dots of the same color The above exercise usually fits well understanding of the roles of genetic as the survivor. The new population into a two-hour laboratoryperiod. If drift and natural selection in affecting of 100 mussels will consist of the 25 time permits,allow students to modify change in a population and come to survivors and 75 offspring. Randomly the basic procedureto explore the role see that both hypotheses (change scatter this new group of mussels on of various factors in affecting changes through natural selection versus the habitat and repeat the selection in the population. What happens if change through drift) can be valid. and reproductionprocesses two more the population size is larger (or times (for a total of three rounds).Each smaller), or if mortality is higher (or group should have 100 living mussels Exploration lower)? Can sexual be at the conclusion of the simulation. simulated?What if both selection and The exercise appears to work best The next step is to have the students drift are operating in a population? when students work in small groups tally up the number of survivors of Can you predict ahead of time which of two to four. Each group will need each color and display their results colors will survive on a habitat with a piece of fabric (about 3 by 3 feet). using a bar graph, with "number of a new color pattern?What if mussels The fabricshould have a complex pat- mussels" on the vertical axis and varied in size rather than color (you tern with a variety of colors (Note: "color of mussels" on the horizontal can even add in differences in repro- the choice of fabric is not critical- axis. To facilitate the comparison of duction, with larger mussels having any fabric with a pattern of several results among groups, have all groups higher numbers of offspring)?Differ- colors will suffice). All groups should list the color of mussels in the same ent laboratorysections can be provided have the same color pattern on their order along the horizontal axis. When with different color backgrounds and fabric.Each group will also need paper the graphs are complete, each group the oystercatcher results from those dots punched out of construction should attach its graph to the front sections can be compared. paper with a standardhole punch. We board grouped with the other results To reinforcethe concepts of natural provide each group of students with for oystercatchersor logs. selection, well known examples such 10 vials, each containing about 100 as speciation of Galapagos tortoises, dots of a certain color (we use black, iguanas or finches can be shown/ dis- white, blue, red, yellow, purple, green, Discussion/Term cussed. Generally,such species assem- orange, pink, and gray or brown). Introduction blages appear to have risen after an Other necessary materials are graph initial "founderpopulation" colonized paper and masking tape. Starting with the results for the an area. By what process might such To begin the exercise, have each oystercatcher groups, have students similar species, each differently group of students spread out a habitat discuss similarities and differences adapted to its habitat, arise? In addi- (piece of fabric)and scatter 10 mussels among the graphs. While no two tion to natural selection, how might (paper dots) of each of the 10 colors graphs will be identical, if all groups genetic drift, immigration,emigration randomly over the habitat (for a total used the same color and pattern of and have played a role? of 100 mussels). Assign each group to habitat fabric, the graphs should all Consider the problems conservation act either as an oystercatcheror a log. be similar. Mussels that matched the biologists face in the design and man- Students who act as oystercatchers habitat survived and reproduced; agement of naturepreserves. How will are to hunt visually for the mussels. those that didn't became extinct. Next, the processes of genetic drift, migra- Each group should catch 75 of the 100 discuss the graphs for the log groups. tion, and natural selection be influ- mussels. To do this, students should Many of the colors of mussels will enced by management choices, and look at the fabric habitat, pick up the have gone extinct,but each group will how, in turn, will these processesaffect first paper dot they see, remove the probablyhave differentcolors that sur- organisms in the preserve?Do - dot, look back at the habitat, pick up vived. If some occurred isms in a nature preserve evolve? Do the first paper dot they see, etc., until through this random process (logs), they do so adaptively? It is the ten- only 25 paper dots are left. might there be a random and dency for genetic drift to reduce Students who act as logs slamming "directed"component to the genetic variance,potentially increasing into the rocky shore should not hunt patterns observed in the oystercatcher the likelihood for small populations of

682 THEAMERICAN BIOLOGY TEACHER, VOLUME 60, NO. 9, NOVEMBER/DECEMBER1998 organisms (or those descended from Kimura, M. (1983). The Neutral Theory Huxley, A.C. Hardy & E.B. Ford small populations) to go extinct, that of . Cambridge, (Eds.),Evolution as a Process. London: concernsconservation biologists today. UK: Cambridge University Press. Allen and Unwin. Imagine if an economically important Kramm, K.R. (1977). Demonstration of Meffe, G.K.& Carroll,R.C. (1994). Prin- species were becoming rare and that population . The American ciples of ConservationBiology. Sunder- you were charged with a breeding Biology Teacher, 39, 558-559. land, MA: Sinauer Associates, Inc. program to guarantee its persistence. Lawson, A.E., Rissing, S.W. & Faeth, Stebbins, R.C. & Brockenbrough, A. The individuals you might preserve S.H. (1990). An inquiry approach to (1975). Simulating evolution. The in a natural sanctuaryor use to estab- nonmajors biology. Journal of College American Biology Teacher,37, 206-211. lish a population Science Teaching, 19, 340-346. Western, D.E. (1976). can drift: likely will not represent the full range Mayr, E. (1963). Animal Species and A simple demonstration.The Ameri- of genetic variationfound in the entire Evolution. Cambridge, MA: Harvard can Biology Teacher, 38, 551-553. species now, much less before it University Press. Wright,S. (1931).Evolution in Mende- became endangered. Not only do you Mayr, E. (1954). Change in genetic lian populations. Genetics, 97-159. not know what range of genetic vari- environment and evolution. In J.S. ance is currently adaptive in some parts of the species range, you have no hope of knowing what genetic variants that are currentlyneutral or even mal-

adaptive may be strongly favored by Downloaded from http://online.ucpress.edu/abt/article-pdf/60/9/681/10396/4450580.pdf by guest on 30 September 2021 some future natural selective forces. As more species become endangered Pixera Professional and their ranges fragmented,concerns of and genetic Digital MicroscopyCamera drift will become paramountin efforts Catre niages fron your i p to ur cmputer. to them. preserve _orpItible with most EM & MACB,dop & la"tqn Canbe i wth or withouta micrpe. resokn Note up to 1.2 milionpixels. Unlimited edusial uss inid- rg the intrnet desktoppubshing, communaurtions and A complete copy of this exercise is datbdset use. Compke System hdudes camera, cary availablefrom S. Rissingat SRISSING@ cas, software,cables, JPC or RMOA irterface,rbiri ASU.EDU. poddmao and les. Spedal EducationalPrbe: $1,145 Acknowledgments The motivation for developing this simulationcame from a summer work- shop in the Departmentof at Arizona State University led by Anton Lawson. We also thank the students sX)~~~~~~~~~~~~Wot oft aft fm 4t who participated in trial runs of this t aft a exercise and provided valuable input. Support for this work was provided by a grant from the Howard Hughes Medical Institute through the Under- graduate Biological Sciences Educa- tion Program. fr .00 fro $199.00 References GUARANTEEDLOWEST PRICES! Baker,J.J.W. & Allen, G.E. (1971). The We will beat any major catalog's published prices! Studyof Biology.Reading, MA: Addi- son-Wesley Publishing Co. "THEON-LINE MICROSCOPE STORE" Benner, D.B. (1987). A genetic drift exercise. The American Biology ON THEWEB Teacher,49, 244-245. http://www.melsobel.com Eldridge,N. & Gould, S.J.(1972). Punc- on-line tuated equilibria:An altemative to orders can be placed . In T.J.M. ViewHundreds of Microscopes(n Line Schopf (Ed.), Modelsin Paleobiology. New& Usd/TradesAccEqted/AJI Brands/All Dicunted! San Francisco: Freeman Cooper Amcup KenA-is, Lem, Meq,amft SR andothn and Co. Hammersmith, R.L. & Mertens, T.R. (1990). Teaching the concept of Toll Free 1-888-ALL-SCOPEScaEsTi genetic drift using a simulation. The fac 516.9356131 e-mail:info@mb ob.com AmericanBiology Teacher, 52, 497-500. Md Sbd Micros, Ltd. Estabishd 1 940 Keeton, W.T. (1967). BiologicalScience. New York: W.W. Norton and Co.

EXPLORINGGENETIC DRIFT & NATURALSELECTION 683