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Card Lab: A Populaton Genetcs Simulation Exercise

Christopher M. Easton

Population genetics labs in introduc- the small population could not even longer contribute alleles directly to the tory laboratory manuals usually rely sustain itself, and numbers continued next generations. This age structure is Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/518/48275/4450369.pdf by guest on 29 September 2021 on jars filled with different colored to decline. Eventually, the remaining similar to that observed with insects beads from which students draw their wild condors were captured, and a which produce large numbers of eggs, breeding population (McCourt 1983; zoo-based breeding program begun, in have a high juvenile mortality, and Dolphin 1992). This type of lab as- an attempt to rebuild a population suf- die shortly after laying eggs as adults. sumes that all individuals drawn are of ficient to maintain itself in the wild. The age structure is also similar to reproductive age. In an actual popula- A population bottleneck also ran- many populations undergoing explo- tion, a range of ages is represented. domly samples alleles from the general sive growth, where the number of in- Populations can be divided into ju- population. Frequently rare alleles are dividuals beyond reproductive age is a veniles (too young to reproduce), indi- lost, reducing the variability of the minute fraction of the total. viduals or reproductive age, and indi- population (Hartl 1980). After the bot- The traditional 'sex' of the face cards viduals that are senescent (too old to tleneck, the diminished effective popu- was preserved, resulting in 16.66% of breed). The senescent organisms have lation size is now vulnerable to the the females being reproductive age, already passed on their alleles, and will effects of genetic drift. More rare alleles while 33.33% of the males are of repro- not do so again. However, the juveniles may be lost, reducing the genetic diver- ductive age. This would seem to be an that will become reproductive and the sity of the population even further. unusual and artificial distribution, but reproductive-age individuals ulti- The reduced diversity may become does characterize human populations mately determine which alleles can be evident as normally recessive and det- in the U.S. and other countries with passed on to the next generations. rimental alleles are expressed in a moderate population growth. Human A precipitous decline in population majority of the remaining population females have a finite reproductive life number (population bottleneck) typi- (inbreeding effects). For example, chee- span and then enter a period of post- cally impacts all ages, with no guaran- tahs were reduced by an unknown reproductive life. In contrast, males can tee that the surviving individuals will catastrophe about 10,000 years ago and remain reproductive throughout their be of reproductive age. If few repro- again by human hunting during the physiological life span, assuming good ductive individuals survive the bottle- 19th and early 20th centuries to a hand- health. Thus, the males have as much neck, the effective population size ful of small breeding populations in as twice the reproductive life span as (Hartl 1980; Hill 1972; Felsenstein South Africa (Campbell 1990). This bot- females, and may, conceivably, consti- 1971), and therefore population fecun- tleneck resulted in decreased general tute twice the reproductive population dity, will be reduced for several breed- fecundity, little genetic variability, and in some countries. In fact, 1985 data ing seasons. In this way, a population high susceptibility to disease. Similar indicate that as much as 70% of the bottleneck may be projected through congenital problems are evident in a males and 40% of the females are of several generations, even after the en- number of domestic dog breeds in reproductive age in highly developed vironmental conditions that imposed it which inbreeding is a common prac- countries, like the United States and have been ameliorated. tice. Sweden (Solomon et al. 1993). Such has been the plight of the Cal- Card lab demonstrates how a popu- Card lab also demonstrates the ef- ifornia condor. Hunting and DDT re- lation bottleneck impacts genetic diver- fects of migration, mutation and selec- duced the numbers to only a few re- sity and even survival of a population. tion on the allelic frequency and diver- maining in the wild. Although DDT It uses a standard deck of playing cards sity of a population. Card lab may was banned and the condors protected, to show how the age structure of a stand alone or supplement "bead labs" population can magnify the effect of a and computer simulations, which offer bottleneck by restricting reproduction more diverse choices for allelic fre- Christopher M. Easton is an Adjunct over several seasons. quency. Instructorin the Department of Biol- The face cards of the deck were cho- Lab Procedures ogy at Tompkins Cortland Commu- sen to represent the reproductive aged nity College and a Ph.D. student in the Department of Biological Sci- individuals purely for the simplicity in Students were given a lab handout, ences at Binghamton University,Bing- recognizing them. The remaining cards which not only provided basic instruc- hamton, NY 13902-6000, e-mail: represent juveniles. Individuals be- tions for the lab, but also asked leading [email protected]. yond reproductive age are not in- questions to stimulate thought and dis- cluded in this model because they no cussion about results. Students were

518 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 8, OCTOBER1997 encouraged to work in pairs or small single mated pair colonize a new area, reduced by population bottlenecks. groups, and to actively discuss their starting a new population. Approximatelyhalf the students using answers. In addition to the lab hand- Each suit (hearts,spades, diamonds, this lab immediately extrapolated to out, student groups needed a standard clubs) in the deck represents a genetic analyzing the age structure of initial deck of 52 playing cards and a calcula- variant (allele) of the population. Aces deals that did not contain reproductive tor. represent infants of both sexes. Even age individuals. Most of these students The lab handout is reproduced be- numbers (2, 4, 6, 8, 10, Q) represent assumed that a '10' would require one low as part of the text. Generalanswers females, but only the Queens are of breeding season to reach reproductive are provided immediately following reproductive age. Odd numbers (3, 5, age. They then looked at the numberof each question or set of questions.These 7, 9, J, K) represent males, with Jacks reproductive individuals available for answers may be readily derived from and Kings being reproductive age. the next five seasons, realizing that the the concepts and principles described Laterin the semesterwe may make use single bottleneck would thus be pro- in any introductorylevel text (e.g. So- of the age structureand introduce dif- jected through several generations of lomon et al. 1993; Campbell 1990). ferential reproduction between Jacks the population. The remaining stu- Some concepts were not mentioned in and Kings, but for now they may be dents required additional coaching to the lab handout, but were emphasized considered identical. realize the severity of the effect they to the students during lab as each had created.Discussions of population group reached certain points in the Questions for the Students genetics and microevolutionfollowing exercise. This fostered communication 1) What is the frequency of each this lab reinforced the concepts, and between individual students, the genetic variant(suit) in the population? integratedthem into the frameworkof Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/518/48275/4450369.pdf by guest on 29 September 2021 group, and the instructor. It also al- This simply requires dividing the evolutionarybiology. lowed the instructorto assess individ- numberof cardsin each suit (13)by the ual participationwithin the group. total number of cards in the deck (52), 13/52 = 1/4 = 0.25 for each suit. PartII: The Number of Caution to Instructors 2) What percent of the population is SurvivorsNeeded To Individuals from foreign countries of reproductiveage? or from strict religious backgrounds This requirescounting the face cards Avoid a Boffleneck may lack any exposure to playing (12) and again dividing by the total Instructions cards. Instructorsshould be prepared number of cards (12/52 = 0.23), then to explain some fundamentals,such as multiplying by 100% to convert the Using trial and repetition,determine what a suit is, what a face card is, and frequency to a percentage (23%). your answers to the following two how to shuffle, without embarrassing questions.You should be able to repeat the student. Instructions your result in order to call it consistent. The following rules shall apply to One method to assure this is to shuffle deal Introduction to Population this lab: A) Females may mate with and the cards, counting the cards Genetics only one male. B) Hybridization will until the stated requirementsare met. Write down this number. Shuffle the Population genetics is a study of the preserve both genetic variants. To simulate a population bottleneck: cardsback into the deck and repeatthis distribution of genes and alleles (pos- a total of 10 times. Shuffle the cards well, then deal 10 process Then,calcu- sible alternategene forms) within pop- late the mean (average), mode (most ulations. Population geneticists also cards face up. Record your results by writing down the face value and suit of frequent), and median (middle in a study the circumstancesthat alter the ranked of 10 each card dealt. Repeat this procedure list) your counts. [Instruc- distribution, such as mutation, migra- tor's note: Instructorsshould be two more times, shuffling dealt cards pre- tion, selection,and genetic drift.A pop- cal- back into the deck each time. pared to assist students with these ulation bottleneckis when a large pop- culations as necessary.] ulation undergoes a sudden and dramaticdecrease in size, due to some Questions catastrophe, disease, or human inter- 3) Did you get any individuals of Questions vention. Such a bottleneck preserves a reproductive age? How many? Of 5) How many cards must be dealt to random sample of the original popula- what suit(s)? Did you get both a male consistently get at least two reproduc- tion's alleles in the surviving members. and a female? tive-age individuals, one male and one Some alleles increasein frequency;oth- This reduces the population to ap- female of any suit (to ensure survival ers decrease. Some rare alleles are lost proximately 1/5 its former numbers of the species)? What percent of the altogether. with a random cut (no selection by age population is this? or genetic variant).Approximately 20% Students will deal an average of 12 of all deals will produce both a male cards to get both a male and female of Part1: Population and a female. Cards from all four suits reproductive age (23%of the popula- Boffleneck Efects will be present in most of the deals, tion). However, student results may on & though many will be juveniles. vary considerably depending on the Reproduction 4) What effectdoes such a bottleneck distributionof cards within individual Genetic Diversity have on the future population?Give a decks. brief descriptionof its effecton the size, 6) How many cards must be dealt to Instructionsto Students ability to reproduce,and genetic diver- consistently get at least four reproduc- This exercise is intended to illustrate sity. tive-age individuals, two females and the effect of population bottlenecks on Students should write a brief para- two males each from a different suit? species survival (reproduction)and ge- graph demonstrating a basic under- [Note: As defined in the rules, all four netic diversity. Many of these concepts standing that genetic diversity and re- genetic variants must be represented in also apply to the founder effect,when a productive capacitymay be drastically the breeding pairs in order to preserve

POPULATIONGENETICS 519 genetic diversity.] What percent of the Part II, determine your answer to the populationmust survive to preservean population is this? following questions: allele present in 1% of the population To preserve diversity in the breeding versus an allele present in 25% of the population students will have to deal population. Of course, individuals are an average of 26 cards, 50% of the Questions a product of all their alleles. Instructors population. Again, this may vary be- 9) How many cards must be dealt to should emphasize that each individual tween groups, with some having to ensure preservation of the species (one representsa unique combinationof al- deal nearly the entire deck. male and one female of any suit)? What leles, which is really what defines indi- Instructorsshould point out that the percent of the reproductive population vidual fitness. mean, median and mode are measures is this? 13) Allelic diversity affects individ- of central tendencies of a distribution. On average, students will have to ual variability.What effect is a bottle- That is, if students were to draw that deal three cards (25% of the popula- neck likely to have on a population's number of cards, they will only satisfy tion). Students should note that this is ability to evolve? the conditions approximately 50% of close to the percentage of cards re- Obviously, any reduction in allelic the time. In order to "guarantee"sur- quired for the whole population (23%). diversity reduces the variation in the vival or diversity, the students would In fact, it is as close as possible given population, and therefore reduces the have to draw the maximum number only 12 cards. ability of a population to evolve. required to satisfy conditions (within 10) How many cards must be dealt to their experience). preserve genetic diversity (two females

and two males, each from a different Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/518/48275/4450369.pdf by guest on 29 September 2021 suit)? What percent of the reproductive Part IV: The Efects of Part Ill: The Number of population is this? Migration, Mutation Reproductive Survivors To satisfy these criteria, students will & Selection Needed To Avoid a have to deal exactly the same average percentage of cards as for the full pop- Instructions Bottleneck ulation (50%, or six cards). 11) How does "guaranteeing" the Migrationinvolves the movement of Instructions survival of some reproductive-age in- individuals into or out of the popula- Now separateall the face cards from dividuals through a bottleneck change tion, bringing in or taking out all of the rest. We will now apply our bottle- the effect of that bottleneck? Or does it? their alleles. Migration(gene flow) be- neck while considering reproductive- Look at the percent of relevant popu- tween populations alters the allelic fre- in age individuals separate from juve- lations required to ensure survival and quency both populations, but, gen- niles. diversity. erally, without serious impact on Shuffle the two partial decks well. Although applying a bottleneck to reproduction. Draw three from the reproductive reproductive-age individuals separate Shuffle the cards well and cut the deck, and 10 from the juvenile deck to from juveniles reduces the number of deck into two stacks (they do not need reduce the population to 1/4 of its cards that need to be dealt, it does not to be equal). You and a partnershould count the number of each suit original size. change the percent of each population (genetic that must pass through the bottleneck. variant) and the number of reproduc- Questions Instructors should emphasize that na- tive individuals (face cards) in each ture often (but not always) selects stack.Now, deal the cards into a pile in 7) Of the reproductive-ageindividu- against the very young and very old. front of you, keeping your piles sepa- als, how many suits are represented? However, major catastrophes (e.g. hab- rate. Every tenth card deal one of your Did you get a female? and a male? Of itat destruction) are likely to be nonse- cards onto your partner's pile. This differentsuits? lective; while certain human activities simulates a migration rate of 0.1 (1 Students will find that dealing three (e.g. trophy hunting) select against the migrant per 10 individuals). Pick up cards from the face cards frequently biggest and best of reproductive age. your pile and again count suits and (50%)produces a male and female of Changing the type of selection occur- reproductive individuals. Compare different suits. Typically, two or three ring during a bottleneck may alter the these values to the values you had suits will be represented. effect of that bottleneck on future gen- before migration. 8) Of the juveniles (which represent erations. the future reproductive individuals), 12) We have used a small population suits are Are Questions how many represented? (52) of consistent and known age struc- the numbers of males and females ture, with only four possible genetic 14) Did your allelic frequency equal? states (four alleles of one gene). What change? Did you gain (or lose) any The number of males and females does this exercise suggest to you about reproductiveindividuals? will usually be nearly equal. All suits the effects of a bottleneck in popula- The answer to this will depend en- typically will be present. However, the tions involving hundreds of genes and tirely on the distributionof cards each distribution of suits within the age alleles, some of which may be very rare student received in his/her initial cut. structurewill not be equal, and should (frequency 0.01 or less)? In other Typically, the allele frequencies will be emphasized by the instructor.Any words, what are the limitations of this change 0.05 to 0.1, depending on the predation or accident affecting the ju- model compared to real populations? initial number of cards present, for venile population could reduce varia- The percent of a population that suits that gain or lose migrants. The tion by eliminating rare alleles. must be preserved to maintain all of its other suits will remain unchanged. In- diversity is inversely proportional to structors should point out that mi- Instructions the frequencies of its rarest alleles. In- grants are usually of young reproduc- Using only the face cards and the tuitively, students should understand tive age, or nearly reproductive age, trial and repetition method used in that a much greater percentage of the individuals. Therefore, alleles carried

520 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 8, OCTOBER1997 by these individuals are often immedi- Questions Conclusion ately injectedinto the future gene pool of the population. 19) What is the effect of selection on This lab effectivelydemonstrates the 15) How could migration preserve reproduction,allelic diversity, and fre- specific effects of declining population genetic diversity? quency? numbers on isolated populations. Stu- Bringing in new alleles, or more in- 20) What would happen if similar dents quickly grasp how the age struc- dividuals with rare alleles, will pre- selection continued over several gener- ture of a population may magnify the serve, or even enhance, existing diver- ations? effects of a moderate decrease in pop- sity. As applied in this exercise, selection ulation over successive breeding sea- is extreme and confined to one suit. sons. Students also develop a clear un- Students will see that allelic frequency derstandingof the Instructions drops precipitously under such selec- effects of migration, tion. If applied over several genera- mutation and selection on allelic fre- Mutationis a change from one allele quency and diversity. to another (usually unique) due to a tions, this degree of selection would result in the elimination of that allele The lab exerciseis amenableto coop- change in the gene itself (altering the erative learning, fostering effective DNA). Mutation typically has a very from the reproductivepopulation, and, eventually, from the gene pool itself. group dynamics with participationby minor impact on allelic frequency,but all members. However, it is equally may affectvariability in a population. suitable for individual learning, and Separate your deck into red cards Evaluation can be carried out of the lab to a and black cards. This will provide two Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/518/48275/4450369.pdf by guest on 29 September 2021 student's populations each with two alleles This exercise was tested in an Intro- home for review and repeti- present at a frequency of 0.5 each. ductoryBiology Labof TompkinsCort- tion. Again, you and a partnerdeal the cards land CommunityCollege. Totaltime to into separate piles, dealing into the perform the exercise ranged from one other's pile at the 20th card. This sim- hour to slightly over two hours, de- ulates a mutation from red to black pending on the speed of dealing and Acknowledgments (and vice versa) with a rate of 0.05. familiaritywith cards. I wish to express my thanks to my [Note: This is an extremely high rate. The average values presented in this who Typical mutation rates are 0.0001 or paper are the result of actual perfor- colleagues expressed interest in less.] mance and are not calculatedprobabil- and offered suggestions for this lab, ities. Calculatingthe probabilityof sat- and to the students who were sub- jected to early versions of this lab, and Questions isfying the conditions for any of these draws is fairlycomplex, involving mul- whose questions were incorporated 16) Whateffect did mutationhave on tiple draws of multiple cards, with the into revisions of the handout. Special your allelic frequencies? base probabilitieschanging after each thanks are due Dr. Herb Posner of 17) Whateffect did mutationhave on draw. This goes beyond the scope of an Binghamton University for encourag- allelic diversity? introductorylab, though it might be an ing me to publish. Each population will lose 0.04 from interesting exercise for a class in pop- the one allele that "mutated,"and will ulation genetics. introducea new "mutant"allele with a The simulationsof populationbottle- frequency of 0.04. Thus, a minimal ef- neck, migration,mutation, and selection References fect is seen on allelic frequency, but all illustratedtheir points exceptionally allelic diversity (as number of alleles well to the students. Furthermore,the Campbell, N.A. (1990). Biology, 2nd Edi- present) increases by 50%. students reported enjoying the lab ex- tion. Redwood City, CA: Benjamin/ 18) Why is mutation important? ercise, perhaps in part due to its nov- Cummings Publishing. Allelic diversity and variability are elty. Instructors should beware of Dolphin, W.D. (1992). Biology Labora- essential to the process of microevolu- impromptupoker games and keep stu- tory Manual. Dubuque, IA: Wm. C. tion and population adaptability.Since dents focused on the lab problems. Brown Publishers. mutation increasesthe allelic diversity, Although this lab is capableof stand- Felsenstein, J. (1971). Inbreeding and it also increasesthe population'sability ing on its own as a lab exercise, com- variance effective numbers in popu- to adapt and evolve. puter simulations generally show the lations with overlapping genera- effects of migration, mutation and se- tions. Genetics, 68, 581-597. Instructions lection better. Such computer simu- Hartl, D.L. (1980). Principlesof Popula- lations can apply them over many tion Genetics. Sunderland, MA: Si- Selection confers a reproductivead- successive generations. Furthermore, nauer Associates, Inc. to individuals based on their simulations utilize rates of vantage computer Hill, W.G. (1972). Effective size of pop- adaptations, and therefore causes the migration,mutation and selection that ulations with alleles carried by those individuals to are closer to real values, and therefore overlapping genera- increase in the population. Selection may help students understandthe time tions. TheoreticalPopulation Biology, 3, can be simulated by having your part- scales involved in microevolution. 278-289. ner remove any three individuals of a However, computer simulations of McCourt, R.M. (1983). LaboratoryMan- particular suit as you deal the cards mutation typically flip one allele to the ual to AccompanyBiology (Wessels & into a pile. The fitness of that variantis other (e.g. a to A), which clearly does Hopson). New York, NY: Random therefore 0.77 (10/13 survive), while not representreality. Since mutation is House. the fitness of the other three variants known to enhance variability, the in- Solomon, E.P., Berg, L.R. Martin, D.W. remains at 1.0 (13/13 survive). Recal- troductionof a third allele better illus- & Villee, C. (1993). Biology, 3rd Edi- culate allelic (suit) frequencies after se- trates the real effect of mutation in a tion. Orlando, FL: Saunders College lection. population. Publishing.

POPULATIONGENETICS 521