Teaching Evolutionary Mechanisms: Genetic Drift and M&M’s®

To promote evolution literacy, it is important to teach evolutionary principles in basic biology classes with an “ active teach ” approach in which students pose and answer questions, solve problems, and hash out and explain issues ( dependable 1992, McComas 1994, Linhart 1997, Smith 2000 ). however, there is not much literature on the use of active voice learn to present evolutionary principles. Most students begin their introductory biota classes in college having hear of natural selection, even few appreciate that there are other significant mechanisms of evolution, namely, familial drift, mutation, and gene hang. In fact, many students start off equating natural selection with development. Students need a solid framework of teach ( i.e., a well-organized class with a defined course of study and stated objectives to help students build their own framework of cognition ) to correct such misconceptions about development, and offering them memorable exercises can help provide that framework. This article describes one such classroom exercise that efficaciously teaches the mechanism of genetic drift to undergraduates. It illustrates, by sampling M & M ‘s milk cocoa candies, a phone number of concepts that are critical in developing development literacy. Although natural excerpt is significant because it is the only evolutionary mechanism to produce adaptation, other forces—genetic drift, mutation, and gene flow—can variety allele frequencies in a population over time and frankincense result in evolution. Students are much surprise to learn that even Darwin ( 1858 ) recognized that mechanisms other than natural choice were at work : “ I am convinced that Natural Selection has been the most authoritative, but not the exclusive, means of alteration ” ( p. 30 ). The neutral theory of molecular development postulates that genetic drift is responsible for the great majority of evolutionary changes at the molecular floor ( Kimura 1989 ). Kimura argues that this mechanism of development is so crucial that an apt description of the evolutionary action is “ survival of the luckiest. ” Of course, some molecular variation can be explained by fitness differences ( Messier and Stewart 1997 ), but the significant point is to introduce to students the idea that not all evolutionary change is adaptive, and thus not all change involves natural choice. familial drift is defined as random changes in allele frequencies in a population. The mechanism is so named because the radiation pattern shows the drift of allele frequencies, up and down over time—there is no predictable directional part to change from generation to generation. genic drift occurs in all populations that are not boundlessly large. It has particularly potent effects when populations are small over several generations. The founder effect and the constriction effect are special cases of genetic drift. The bottleneck effect occurs when a bombastic population undergoes a drastic reduction in size. For model, natural disasters such as hurricanes, fires, or floods can wipe out big areas of habitat but, by luck, leave modest areas undisturbed. The surviving organisms in these undisturbed areas are the golden founders ( their traits have no relation to their survival ) of the subsequent populations. The founder effect ( which is simulated in the M & M ‘s exercise ) is alike to the bottleneck impression, except that the establish individuals are in a geographic location different from that of the master population ( for exercise, a few individuals are blown to an uncolonized island and breed there ). These individuals that founded the new population carry fair a fraction of the familial mutant of the ancestral population. Random changes in the allele frequencies of the new population, or genetic freewheel, is the consequence of this sampling of the total genetic pas seul. Sewall Wright ( 1932, 1982 ) developed the hypothesis and established the importance of genetic drift. He described genic drift as a herculean “ trial and error ” mechanism for exploring different fitnesses. genic drift shifts demes ( local subpopulations in which random match occurs ) around on the seaworthiness landscape and occasionally places one within the survival gradient of a local fitness flower. Natural choice then moves the deme up the bill. Wright besides pointed out that these accidents of sample could at times overpower the forces of natural choice ( Wright 1982 ). ( See Futuyma [ 1998 ] for a discussion of Wright ‘s fitness landscape model. )

Accidents of sampling ( sampling erroneousness ) are based on sample size. Smaller samples are subjugate to more chance variation than are larger ones. As with flipping a coin, big deviations from the 50:50 heads-to-tails ratio are not surprise when humble sample distribution sizes are involved. As the sample distribution size increases—that is, the more times the coin is tossed—the frequencies tend to approximate the 50:50 theoretical expectation. In a gene pool, the alleles of the adults are a sample of alleles from all the gametes of the former generation, and frankincense they are affected by sampling erroneousness ( Futuyma 1998 ). The smaller the sample of adults is relative to the original population, the larger the possible deviation, just by luck. This sampling process starts at conception—not all gametes make it to the zygote stage—and continues throughout life ( Ridley 1996 ). Because genetic float is based on a random sampling process quite than a deterministic process, students frequently have a difficult fourth dimension understand and appreciating its character in development. Although there are creative classroom exercises depicting evolutionary timelines with toilet newspaper ( O’Brien 2000 ), demonstrating types of natural excerpt with jelly beans ( Lauer 2000 ), and showing how building complex adaptations can be built via natural excerpt using the Yahtzee® bet on ( Dickinson 1998 ), few exercises have focused on genetic drift. The drill described in the adjacent section simulates one type of genetic drift, the founder effect, by sampling M & M ‘s randomly.

The exercise

The goal is to measure the allelic diversity of one marker gene in numerous little fall through populations compared to a big ancestral population. The material needed is one large bag ( 1 pound of M & M ‘s works well for a course of 35–40 ). The bag of candies represents a gene pond. The different colors of M & M ‘s represent unlike alleles of one gene. therefore two M & M ‘s compose one individual. Individual organism, however, are not explicitly represented in this exercise because its focus is on allele frequencies. The exercise involves passing the cup of tea of M & M ‘s around the classroom and having students pour a relatively little sample on their desk. Students are simulating the founder impression, what happens genetically ( to the allele frequencies of our one marker gene ) when a few individuals are blown in a ramp from the mainland to an island, for exemplar. An assumption of this exercise is that sampling the pocket without successor does not affect the results—in other words, the base of M & M ‘s is behaving genetically as if it were infinitely big. Have students tally their sample distribution size of individuals ( phone number of sample alleles/2 alleles per person = count of individuals ) and the color frequencies within the sample. Either teach students to choose an evening act of M & M ‘s from the bag ( which slows down the sampling prison term ) or provide a strategy if an leftover number of alleles are sampled ( although dealing with half an individual is not a problem because this is theoretical sample, students could round up or toss off for the number of individuals or merely eat one randomly chosen M & M ). Students do not sort the alleles into homozygote or heterozygote individuals ; they just compute the overall allele frequencies in their fresh humble gene pool. While students are sampling M & M ‘s and determining the allele frequencies, write on the board the frequencies of the original population. Determine this before the exercise for your particular bag or use the expected frequencies in a udder of M & M ‘s : 30 percentage brown, 20 percentage each of scandalmongering and red, 10 percentage each of orange, park, and blue ( Eleanor Eggert [ Mars, Inc. ], personal communication, 2001 ). table 1 provides an example of color frequencies for a one-pound bulge and a format for recording results. ( A new coloring material [ purple, greenish blue, or pink ] will be added to bags after consumer preference is determined. The expected color frequencies of these bags will therefore be 20 % each of loss, yellow, and the new coloring material and 10 % each of orange, k, blue, and brown [ Eleanor Eggert ( Mars, Inc. ), personal communication, 2002 ]. ) randomly call on students to provide results for your data sheet. besides ask for results from the extremes—the largest and the smallest samples. The results should illustrate the points below .

  • The allele frequencies of each student ‘s sample are about constantly different from the original population ‘s frequencies ( evolution via genetic drift ) .
  • The allele frequencies of each scholar ‘s sample are unlike from frequencies of other samples ( genetic roll increases variation among populations ). This period is significant for understanding processes such as population deviation within a species .
  • Alleles with the lowest frequency in the original population have the lowest probability of becoming fixed ( reaching 100 percentage frequency ) and the highest probability of getting lost ( reaching 0 percentage frequency ). Alleles with the highest frequency in the original population have the highest probability of becoming fixed and the lowest probability of getting lost. The probability of arrested development equals the frequency of the allele ‘s happening in the population ( Futuyma 1998 ). Extrapolating over clock, there is a march to homozygosity for all achromatic genes occurring in all populations ( Ridley 1996 ). This parade is counteracted by mutant and gene flow. In short, genetic drift decreases version within a population over clock .
  • By casual, a small population may have an exception ally senior high school frequency of a rare allele. This point can be observed in many human populations that were founded by relatively few individuals. The amish population of Lancaster County, Pennsylvania, has a very high frequency ( 0.07 ) of the Ellis-van Creveld syndrome ( polydactyl dwarfism ) compared with that in most populations ( 0.001 ) because the allele was portray in the establish population ( Freeman and Herron 1998 ) .
  • familial drift has a stronger effect in small populations than in big ones. The differences from the ancestral population will be greatest, on modal, in the smallest samples. Have students predict the allele frequencies if their sample distribution from the M & M ‘s bag had been 300 candies alternatively of 13 .
  • Remember that this practice follows lone one familial locus—genetic freewheel affects allele frequencies at all locus simultaneously .

Follow-up homework assignments

In-class work and homework assignments enable students to process the material and construct a framework of cognition. The classic experiments of Peter Buri ( 1956 ), a student of Sewall Wright, are useful in this context and have the total benefit of helping students with graph rendition. Buri studied the effects of genic drift in numerous small populations over meter. He started with 107 lines of Drosophila, each one starting with a 0.5 frequency of two different mutant alleles, biological warfare and bw75. Homozygotes of each type and the heterozygotes have distinct body color. The bw75/bw75 homozygote is bright red-orange ( in new flies ), heterozygotes are light orange, and the bw/bw homozygotes are white. eight males and eight females ( 32 alleles ), chosen at random, were used to start each subsequent generation. Buri followed his Drosophila lines for 19 generations. Allele frequencies became more evenly distributed from 0 to 1, with many populations losing or fixing the bw75 allele ( and vice versa for the other allele ). typically, students are merely presented with all the results from Buri ‘s ( 1956 ) experiments. rather, give students the startle frequency histogram of share of flies with different alleles and ask them to predict, using frequency histograms, what will happen in 5, 10, and 20 generations. In the next course, have students compare their predictions with Buri ‘s results. many evolutionary texts provide clear compendious graph of the results ( for example, Freeman and Herron 1998, Strickberger 2000 ).

Additional concepts for discussion

After this introduction to familial drift using the M & M ‘s exercise, the follow concepts can help reinforce and elaborate your points.

Time

The sampling done for the M & M ‘s exercise represents the starting population on a new island. What will happen over time to this population ? here you can discuss the effects of little population sizes over long periods of time, the population dynamics of founder–flush cycles, the extinction of most establish populations, and the increase in linear familial variability in founding populations ( or save this last degree for your high-level development class ).

Random versus nonrandom processes: Natural selection compared with genetic drift

This exercise can be used to reinforce the difference between random and nonrandom processes by contrasting genetic drift and lifelike survival. It helps to point out that both genetic drift and natural excerpt are sampling processes ; genic drift is a random sample action, and natural choice is a bias sample distribution process. While natural excerpt ultimately depends on the random version that mutation produces, the summons of natural selection itself is nonrandom. With natural excerpt, individuals differentially reproduce and survive as determined by the interaction between their phenotype and the environment—this is not random. Describe to the students a genetic situation in which the different alleles, alternatively of being neutral in effect, have unlike fitnesses because they influence a significant phenotypical trait. For example, if a student ( acting as “ the environment ” ) intentionally chose all red M & M’s—because individuals with those alleles were observed most promptly or tasted the best or ran the slowest or had the showiest courtship display—instead of taking a random sample distribution, it would be a sheath of identical strong natural choice, not genetic drift. In this example, the crimson M & M ‘s were chosen deterministically, not randomly.

Effective population size

not all individuals in a population reproduce. Some are besides young or besides old or lack the opportunity. The generative individuals are the alone ones, obviously, that contribute genes to the next genesis. The number of generative individuals is broadly much lower than the absolute issue of observe adults in the population. The number of reproducing individuals is one factor that determines how the population behaves genetically over time ( how strong the effects of familial roll are ). A genetically meaningful theatrical performance of this smaller population size is called the effective population size. It is the number of individuals in a theoretical population ( random checkmate among adults ) in which the sum of genetic drift equals that of the actual population. The effective population size is reduced by a number of factors—a skewed sex ratio, fluctuations in population size, small breed groups, overlapping generations, and variable richness, for case ( Futuyma 1998, Ridley 1998 ). The difference between the absolute population size and the effective population size can be demonstrated using M & M ‘s. Use two bags to represent two populations having different sex ratios of breeding adults. For one population with a sex ratio of breeding individuals of 1:1 ( one male to one female ), 50 percentage of the M & M ‘s represent electric potential alleles from the breeding male population and 50 percentage represent the female alleles. For a population with a skew arouse proportion of breeding individuals—for model, a polygynous system with a 1:9 arouse proportion ( one male to 9 females ) —the male pond of alleles would be merely 10 percentage of the udder and the female pool of alleles the remaining 90 percentage. For each population, 50 percentage of the alleles for the adjacent generation will come from the male gene pool and 50 percentage will come from the female gene pool. Have students take founding populations from the different populations and compare the results. For the population with the skew sex proportion, the 50 percentage contribution to the future genesis from the males will come from the relatively small sample distribution of alleles ( 10 percentage of the bag ). In contrast, for the population with an even arouse proportion, the male contribution to the adjacent generation will come from a relatively large sample ( 50 percentage of the bag ). The population with the skew sex proportion will be influenced more dramatically by genic freewheel.

Real examples

The history of the northern elephant navy seal ( Mirounga angustirostris ) provides a classical case of the consequences of familial drift through the constriction effect. In the 1890s, overhunting purportedly reduced this species to about 20 animals. The effective population size at this time was fewer than 20 because the northern elephant seal has a polygynous coupling system. The current population size is approximately 30,000. Of 24 enzymatic venue analyzed, none shows variation—an extreme position that suggests a history of familial drift ( Futuyma 1998 ). More holocene analyses of mtDNA confirm the highly low levels of genetic variation and provide estimates of population sizes for the bottleneck consequence ( Hoelzel et aluminum. 1993, Hedrick 1995, Weber et alabama. 2000 ). interestingly, the lack of genetic variation at allozyme locus is not amply explained by the estimated bottleneck size and duration ( Hedrick 1995 ). The high count of endemic species of Drosophila ( more than 100 in the picture-winged group, more than 800 in the family Drosophilidae ) in the hawaiian Islands is a solution of numerous founder events. analysis of salivary chromosome ring patterns reveals the perennial pattern of colonization between older and younger islands ( Strickberger 2000 ). Darwin ‘s finches ( Geospiza magnirostris ) in the Galápagos provide an example of the collapse consequence and resultant nonselected morphologic change. The population of finches created by the founder effect had larger bills than the source population. Song type, a nongenetic culturally familial trait, was affected deoxyadenosine monophosphate well ( Grant and Grant 1995 ). Collared lizards ( Crotaphytus collaris ) in the Ozark Mountains live in little, isolate remnants of desert habitat. population sizes are minor, and 11 of the 14 populations examined were fixed for a specific multilocus genotype ( Freeman and Herron 1998 ). Examples that specifically relate to conservation biota promptly get student interest. The 35-year study on the Illinois prairie chicken ( Tympanuchus cupido pinnatus ) intelligibly demonstrates the genetic and seaworthiness consequences of the bottleneck effect, american samoa well as the challenges of conserving small populations ( Westmeier et aluminum. 1998 ). Habitat loss is correlated with a drastic reduction in population size ( N < 50 ) of this once widespread species. Population size and fitness ( as measured by the number of fat incubated eggs per full eggs and phone number of think up eggs per full ) steadily declined, as did genic diverseness ( measured by allelic diverseness of fewer microsatellites ). Despite conservation efforts that enlarged available habitat, overall population size and fitness decreased. Neighboring populations went extinct and frankincense genetic change via gene hang stopped. once this happened, the seaworthiness of the focal population decreased dramatically. however, conservation efforts improved the chickens ' fitness ( increased fertility and hatching achiever ) by introducing individuals from genetically divers populations from other Midwestern states ( Westmeier et aluminum. 1998 ). This cogitation is noteworthy in that it provides data on respective of the variables associated with the extinction vortex threatening humble populations and shows that inbreeding depression is a real refer ( Soulé and Mills 1998 ).

Speciation

genic drift has been thought to play an important role in the geological formation of species ( Carson 1975, Templeton 1996 ), peculiarly in the peripheral isolation model proposed by Mayr ( 1954 ). In this model, relatively modest, isolated populations located on the periphery of an ancestral population diverge relatively quickly from the ancestral population, because of the charm of random genic changes. This mechanism of speciation has been reasonably controversial because species differences via genetic float would be dysfunctional. In general, the mechanism of natural choice is probably crucial in conjunction with familial drift in the stay divergence of little populations from ancestral populations ( for a general discussion of this topic, see Freeman and Heron 1998 and, particularly, Futuyma 1998 ).

Mutation, gene flow, and natural selection

other mechanisms of development can be incorporated into the discussion of the M & M ‘s drill. Some of us remember that there used to be light brown M & M ‘s. They have been lost from the population and replaced by blue ones. What is the simplest explanation for this design ? Mutation of faint brown to blue followed by obsession by genetic drift ( neutral alleles ) ? By natural survival on an allele that once had a selective advantage and is now achromatic ? Selection against light brown and choice for the newly arisen ( via mutation ) blue ( but no bags are known to have ever contained both blue and light embrown M & M ‘s ) ? choice for blue accompanied by gene flow that spread the gloomy allele to other populations ? Be prepared for potentially bouncy interactions if you ask students to demonstrate gene flow from a population on one english of the room to a population on the other. The allelic diversity of this genetic venue has increased dramatically recently. Purple, pink, and greenish blue alleles are nowadays promptly found in some bags.

Assessment

Although no formal assessment of this practice has been done, its potency can be evaluated by assigning homework questions about genic drift after the standard call on the carpet presentation of the topic and then again after the M & M ‘s sampling practice. informal evaluation indicates that this drill promptly captures student concern and helps clarify challenge concepts.

Summary points of this exercise

genic stray is a dysfunctional mechanism of development that deserves more dispatch and synergistic treatment in the classroom. The follow points about genic stray can be clearly illustrated using the M & M ‘s sampling exercise : furthermore, this exercise provides an introduction to respective other important evolutionary topics ( speciation, lifelike choice, gene flow, mutation ) .

  • genetic drift has a stronger effect in modest populations than in big ones .
  • Alleles can be fixed or lost by prospect .
  • genic drift increases variation among populations and decreases variation within a population over time .

The M & M ‘s sampling practice stimulates critical intelligent and develops evolution literacy. You will know that your students have grasped these concepts when they understand that a random sampling action can result in what appears to be a nonrandom assortment of M & M ‘s and when they are more matter to in counting M & M ‘s than in eating them.

Acknowledgements

I thank Alison Chubb, Dave Darda, Matthew Greenstone, Shawn Kutcha, David B. Wake, and an anonymous reviewer for helpful comments on the manuscript, and I thank Henry and Barbara Staub for the inspiration to develop this exercise.

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Table 1. Percent occurrence of different colors of M&M’s in a one-pound bag (determined by counting) and representative student samples

table 1. percentage occurrence of unlike colors of M & M ‘s in a one-pound cup of tea ( determined by counting ) and representative student samples

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Author notes

© 2002 American Institute of Biological Sciences

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