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Exploring Genetic Drift & Natural Selection Through a Simulation

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Exploring Genetic Drift & Natural Selection Through a Simulation 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 natural selection 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 evolution 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 species 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 biology 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 allele 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 speciation.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 drift is 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 sampling error 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 population size 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 habitat. 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 adaptation, 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 reproduction 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.
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