ORGANICEVOLUTION

Teacher's Guide

The programs are broadcast by TVOntario, thetelevision service of The Ontario EducationalCommunications Authority. For broadcast datesconsult the TVOntario schedule in SchoolBroadcasts, which is published in September anddistributed to all teachers in Ontario.

The programs are available on videotape. Orderinginformation for videotapes and this publicationappear on page 26.

Canadian Cataloguing In Publication Data

Rosenberg, Barbara L.Organic evolution. Teacher's guide

To be used with the television program, Organicevolution.

The Series

Producer/Director: David ChamberlainWriter/Narrator: James MoriartyConsultants: Geza Kocsis

William MacPhersonAnimation: Les animations Drouin Inc.

The Guide

Writer: Barbara RosenbergEditor: Carol SevittDesigner: Tom PilsworthReviewer: H. Murray Lang

ContentsIntroduction............................................................ 1

Program 1: I n the Beginning............................... 2

Program 2: Darwin, Naturally............................. 4

Program 3: Factoring in Mendel......................... 9

Program 4: The Meiotic Mix................................ 12

Program 5: The Population Picture.................... 16

Program 6: Mutation and All That....................... 23

Further Reading..................................................... 26

Ordering Information.............................................. 26

I SBN 0-88944-109-X

1. Organic evolution (Television program)2. Evolution-Study and teaching. I. TVOntario.II. Title.

QH362.R671986 575'.0071 C87-099604-5

© Copyright 1987 by The Ontario EducationalCommunications Authority.

All rights reserved.

Printed in Canada 2388/87

Series Outline

For centuries the Biblical account of creation wasfirmly fixed inthe minds of most individuals. But inthe early part of the eighteenth century, thedoctrine of special creation was questioned bynaturalists such as Carolus Linnaeus, GeorgesLeclerc, comle de Buffon, and Jean- Baptiste deLamarck. The Lamarckian theory of the inheritanceof acquired characteristics was attractive butunacceptable to most people. However, the workof Gregor Mendel, which was rediscovered about100 years after Lamarck, indicated that acquiredcharacteristics could not-be inherited.

Charles Darwin's contribution to science wastwofold:-he presented evidence to: prove that organic

evolution has occurred;-he devised a theory to explain how it operates.Darwin referred to this evolutionary mechanism asnatural selection. One of the major weaknesses inDarwin's evolutionary theory was that it could notaccount forthe occurrence of variation.

It was Mendel's experimentswith the common peaplant that later came to support -Darwin's theory.From the principles of Mendelian genetics theHardy-Weinberg law was established. This lawstates that if mating is random, if mutations andselection do not occur, and if the population islarge, then gene frequencies in a, populationremain constant from generation to generation.Variation occurs due to the process of meiosis andthe phenomenon of crossing over. The thorough

shuffling of the paternal and maternal genes in thegametes results in an astronomical number of genecombinations that could occur in a populatlon.ofinterbreeding organisms. Although continualreshuffling will aker the combinations of the genes,according to the Hardy-Weinberg law thefrequencywith which the different kinds of genesappear in the population will not be altered.

If the law fails, -itmust be due to some internal orexternal pressures that disturb the equilibrium ofthe -population. Factors, such as natural selection,sexual selection, directional and disruptiveselection, and geographic isolation resulting inrandom genetic drift are all possible causes of achange in the gene frequencies.

But the ultimate source of evolution is a mutationthat establishes an alternative allele at a given siteon the. chromosome and makes possible analtemative phenotype. The alteration of the DNAmolecule's structure and the consequent alterationin the genetic code result in a mutation.

The change in the gene pool resulting in anincreased frequency of the mutant gene in thepopulation is a gradual process that may occur overeons. This idea -of Darwin's is referred to asgradualism. Other scientists challenged this ideaand suggested that major changes occur suddenlyand are then followed by long periods of stability,hence the theory of punctuatedequilibrium. Thedebate on :these two theories continues.

INTRODUCTION

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ObjectivesStudents should be able to do the following

1. Describe the work of Bishop Paley.2. Describe the contributions of Carolus

Linnaeus.3. State the views put forth by le comte de

Buffon.4. Describe Lamarck's theory.

Program DescriptionI n the beginning God created the heaven and theearth and the living things that populate it. Thevery vivid Biblical account remained firmly fixed inthe minds of most people for centuries. It was notuntil the eighteenth century that some peoplebegan to think more deeply about aspects of theorigins of life.

Bishop Paley attempted to survey the facts ofnatural history to show that a grand design existed.He commented on the remarkable ways in whichplants and animals were adapted to theirenvironments and said that these adaptationscould not have arisen by chance. According toPaley, this proved the existence of a creatorguiding the natural world. Fixed and unchanging,all beings were ranked in ascending order froml owest to highest. This divine ranking he called thegreat chain of being.

Carolus Linnaeus was convinced of the essentialcorrectness of the Biblical views. He believed that

all species were created and had changed verylittle, if at all, since creation. Linnaeus recognizedthe species as the basic natural unit of life, and thekey to his system was structural similarity. Hegrouped plants and animals according to theirshared characteristics and divided them intoorders, families, genera, and species. Thisprovided the basis of today's expanded taxonomy.

Linnaeus gave every organism he knew twonames. The first word of the name is the genustowhich it belongs. The second is the so-calleddescriptive name or species.

But if species were related through genus, couldthe species have evolved from some form ofprototype? Linnaeus believed that the number ofspecies was fixed, and these species did notchange. Organisms that did not fit into his systemhe simply ignored. If he had understood that suchorganisms were in the process of change, the

theory of evolution might have developed longbefore Darwin. The idea of spontaneousgeneration suggested serious flaws in the theoryof fixed species.

Georges Leclerc, comte de Buffon, believed thatancestral forms of life arose spontaneously-thatone form of life became modified by theorganism's response to its environment to giverise to another form. He thought the originalorganism acted as a template to create adegenerate type. Therefore, a species coulddiverge from the ancestral form and could bemodified by its environment. Buffon arguedagainst the theory of catastrophism thatmaintained the changes that had occurred on theearth since its creation were the result of a numberof "catastrophies," of which the Biblical flood wasone. He stated in his Histoire Naturelle

We ought not to be affected by causes whichseldom act and whose action is alwayssudden and violent. These have no place inthe ordinary course of nature. But operationsuniformly repeated, motions which succeedone another without interruption, are causeswhich alone ought to be the foundation of ourreasoning.

One of the greatest naturalists, Jean-Baptiste deLamarck, thought of evolution in terms of indepen-dent lines ratherthan common ancestry. Each lineoriginated by spontaneous generation, with themost complex originating earlier and the simpleforms originating quite recently. He believed thatorganisms had an inner drive for perfection and

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PROGRAM 1 /In The Beginning

were driven by some need to become complex. Inhis Histoire Naturelle he wrote:

They feel certain needs, and each felt need,stirring their inner consciousness,immediately causes fluids and forces to bedirected toward the point of the body wherean action capable of satisfying the need cantake place. But if there exists at this point anorgan appropriate to such an action, it isstimulated to act; and if no organ exists andthe felt need is pressing and sustained, littleby little the organ is produced and developsby reason of its constant vigorous use.

Lamarck proposed that the environment broughtabout evolutionary changes. The famous exampleof the giraffe's long neck illustrates Lamarck'stheory. He believed that as giraffes fed on theleaves of trees, they stretched their necks a little toreach the highest leaves. These slightly elongatednecks were inherited by their offspring who in turnstretched their necks even more. Supposedly,over generations the giraffe acquired anelongated neck. Through this example hedeveloped the theory of inheritance of acquiredcharacteristics. Lamarck thought that by using acertain part of its body, an animal could changethat part to fit the environment better. Conversely,by disuse a body part would begin to disappear.

Lamarck's theory as well as the work of otherscientists paved the way for further thought anddiscovery in organic evolution.

Before ViewingIntroduce the topic of organic evolution bysuggesting that many biologists, as well as

theologians, believe in God as the creator of theworld. But they also believe that God gave livingcreatures the ability to adapt to changingenvironments and that because of this ability,living creatures have evolved into today's species.

After Viewing

Many experiments have been performed to lookfor evidence suggesting that characteristics couldbe affected by the individual's life experiences oractivities. Discuss the famous experiments per-formed by German biologist August Weismann inthe 1870s. In his experiments he cut off tails ofmice for several successive generations. Accord-ing to Lamarck, these tailless populations shouldhave started to produce offspring with shorter tailsbecause the tails were not used. Unfortunately forLamarck, all the mice in all the generations had tailsof the same length as those of the original mice.

ACTIVITY 1: Field TripTake the students to a museum of natural history.Have them find out how fossils are formed. Askthem to sketch two orthree extinct animals andexplain why they thinkthese animals becameextinct.

ACTIVITY 2: Field TripTake the students through a wooded area or field.Note the organisms that have special character-istics to help them survive in this environment.

ACTIVITY 3: Guest Speakerinvite an expert in the area of evolution or anenvironmental scientist to speak to your class andanswer any questions on the effects of environ-ment on evolution.

ACTIVITY 4: ReviewDescribe Lamarck's theory of how species acquirenew characteristics. Use an example in yourdiscussion.

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ObjectivesAfter viewing the program and completing severalactivities, students should be able to do thefollowing:

1. Recognize the implications of evolution for allliving things.

2. Recognize that natural selection is theselection of the more "fit" over the less "fit"organisms.

3. Recognize that evolution is an essentiallyirreversible process.

4. State the physical and biological checks thatslow down or prevent some changes.

5. Summarize how natural selection works.

Program DescriptionIt is important to see Darwin's revolutionary ideas inthe context of the scientific information available tohim. Darwin was unaware of Mendel's ideas ongenetics and unable to explain his observations interms of genes and chromosomes. However, hedid think in terms of inherited variations passedfrom one generation to the next.

Darwin held that all modern living forms descen-ded with modifications from extinct earlier forms,which in turn had descended from extinct ances-tral forms. The modifications were random or"undirected"; that is, they rose without referenceto the needs of the plants and animals bearingthem. Darwin hoped to prove that the idea ofindependently created creatures-the Biblical

theory-was incorrect. He knew that breederscould "create" desirable plants and animals byselecting parents that already had the desiredtraits. These traits were likely to be inherited bythe offspring.

Darwin noted the close resemblance in the"selection" that occurs in the regulated breedingof horses and the "selection" that occurs in nature.There must be genetic variability in both cases.Breeders "select" what they desire from existingstock; the horses that best meet these require-ments are the most likely to pass on desirable traitsto their offspring. In nature, the selection-naturalselection-is done by the environment (climate,soil, food, competition with organisms, etc.). Theorganisms that survive and produce offspring arethe ones that are best adapted to their naturalenvironment.

Malthus suggested in his "Essay on the Principleof Population" that mankind multipliesgeometrically, although the means of subsistencedo not. He maintained that the human race wasreproducing at such a rate that inevitably it wouldoutgrow its food resources, and a calamitousstruggle for survival would occur. In Darwin's mind,this generated the notion that an averagei ndividual produces more offspring than cansurvive, permitting selection to occur. Hisdefinition of natural selection started from thepremise that there are checks on the number ofsurviving offspring an individual produces. Thesei nclude physical factors such as climate, disease,limited food supply, and predation. Darwin'sconclusion was:

As many more individuals of each species areborn than can possibly survive, and as,consequently, there is a frequent recurringstruggle for existence, it follows that anybeing, if it vary however slightly in any mannerprofitable to itself, under the complex andsomewhat varying conditions of life, will havea better chance of surviving and thus benaturally selected. From the strong principleof inheritance, any selected variety will tendto propagate its new and modified form.

Darwin applied the terms natural selection andsurvival of the fittest (a term that Darwin did notcoin, but accepted) to this preservation offavorable individual variations and the destructionof injurious variations. This means, as the programillustrates, that organisms with variations that helpthem cope with their environment have a betterchance of surviving; consequently they leavemore offspring, some of which may retain the

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useful variations. Organisms with variations notsuited to their environment leave few, if any,offspring and lose in the struggle for existence.Eventually, the naturally selected variety becomesthe dominant variety. Darwin called this process ofsurvival and reproduction of species best adaptedto their environment descent with modification.

Massive testimony to the evolutionary theory isfound in fossils; before the advent of the scienceof genetics, paleontology supplied the mostconvincing evidence for the principle. Fossils inrock layers of different ages show a distinctprogression of change from the simpler to themore complex forms of life. An incomplete butconsistent history of life on earth has beenrevealed through dating of fossilized remains ofextinct forms.

I n Darwin's time, many scientists incorporatedfossil evidence into the theory of catastrophism .This theory posited that great cataclysms (such asthe Old Testament flood) wiped out all life forms,which were gradually replaced by entirely newspecies, most built on previous designs. Witheach succeeding geological era, these scientistsreasoned, a number of species built on a newdesign.

Charles Lyell countered catastrophism with thetheory of uniformitarianism, which held thathistorical changes on earth were due not to aseries of catastrophes but to regular, gradualchanges. Having long observed such processesas erosion, Lyell proposed that the features of theearth's surface were the result of physical forcesoperating slowly over enormous periods of time.Given enough time, mountains could disappear inthe sea or new ones could emerge on land, andrivers could change their courses or form deepcanyons, as the program's animation shows.

Darwin applied the general principle of Lyell'stheory of geological evolution to living things.Like geological change, he reasoned, organicevolution proceeds slowly and relentlessly.

in summary, Darwin believed that evolution worksi n the following way.

- Populations tend to overproduce. Theyproduce more offspring than are likely tosurvive.

- The members of a population vary one fromanother and these variations are inherited.

- In nature, members of a population muststruggle to survive, and not all will survive.

- Since members of a population are notidentical, some are likely to be better suited tosurvival. Those best suited to the environmentwill survive and have offspring. This ensuresthat variation is preserved if it is useful andrejected if it is injurious.

Before Viewing1. Ask students to observe a local bird group or a

common local plant and watch for evidence ofvariation and the struggle to survive. Suggestthat they conjecture which individuals are bestadapted for survival and how this might lead totrait changes in the population as a whole.Another way to approach this topic is to showpictures of fossils that trace gradual changesor examine a local fossil record to show howvarieties of organisms have changed.

2. Ask the class to name some well-knownorganisms that failed to adapt and becameextinct. The mammoth, mastodon, pterodactyl,brontosaurus, tyrannosaurus, and sabre-

toothed tiger became extinct in prehistorictimes. The passenger pigeon and puptishbecame extinct while our very recent ancestorsstood by and watched-or helped. Why didthey disappear? Change of climate, decreasein food supply, sluggish activity, low intelli-gence and overhunting are some of thereasons.

After Viewing1. Begin discussion of the theories relating to

evolution, perhaps using as a springboard thefact that although every female frog lays severalhundred eggs each spring, the population offrogs in a pond remains relatively constant.Establish the facts about overproductioni nclud ing the role of enemies and unfavorableenvi ronment. What is meant by the "fittest"organisms?

2. A wide variety of discussions and researchprojects can proceed from the question "Howdo humans make use of selection?" Breedingplants and animals for characteristics moredesirable to humans has produced more andbetter food. Microbes used in the preparationof foods, chemicals, and pharmaceuticals areunder constant observation for the appearanceand selection of more useful strains. Newtechniques in genetic engineering expand theusefulness of selection in exciting, contro-versial ways.

ACTIVITY 1. SimulatingFormation of Fossils inSedimentary RockDemonstrate one or more of these methods inclass, before assigning individual projects.

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A. "Fossils" In plaster

In this demonstration, the plaster represents thesediments that are deposited by a slow-movingstream, a muddy embankment, or a swamp.

Method

1. Prepare a mixture of plaster of Paris and waterto the consistency of pancake batter; stir untilsmooth and pour a layer 2.5 cm deep into an

aluminum foil pan.

2. Coat a variety of leaves or shells with petroleumjelly and lay them on the plaster.

3. Coverthese with another layer of plaster ofParis.

4. After it has hardened, remove the plaster fromthe pan and crack it open to find the casts orimprints made by the embedded objects. Theactual molds, especially of solid objects such asshells, can be likened to what might be foundwhen a fragment of sedimentary rock is crackedopen.

B. "Fossils" in ice

This experiment shows how no decay takes placewhen bacteria do not have access to tissue, orwhen conditions are unfavorable to their growth.

Method

1. Place objects (leaf, insect, fresh fruit, etc.) inwater in the compartments of an ice-cube trayand freeze.

2. When partially frozen, add more water toensure that the embedded objects arecompletely covered in ice. Each ice cubeshows the "fossil" embedded in ice.

C. "Fossils" in gelatin

Method

1. Soak 8 ml of unflavored gelatin in 60 ml of coldwater.

2. When the water has been absorbed, stir in180 ml of hot water.

3. Pour this into waterproof paper boxes or glassdishes and refrigerate.

4. When the mixture is partially congealed,embed a small insect (preferably hard-bodied,such as a beetle) in the gelatin.

5. After the gelatin has hardened, tear off thepaper box or remove from dish.

The product is an insect "fossil," resembling actualfossils created when ancient insects, mainly antsand small flies, became embedded in drops ofresin that trickled down the bark of evergreens.

ACTIVITY 2. ObservingVariations Due to Environment

Members of the same species of plant may showvariations due to environmental factors such as theamount of sunlight they receive or the amount ofwater in their growing medium. These variationscan be observed on a field trip to a stream or atake's edge and in the laboratory. Have studentsobserve plants in one or both of these environ-ments and make careful notes on their observa-tions.

1. On afield trip to a stream or lake, have studentscollect specimens of the plants that grow bothin water and on land. How do leaf shape and

color vary as the plant's environment becomesdryer?

2. In the laboratory, have students monitor thegrowth of plant seedlings in brilliant sunlight (orartificial light) and that of seedlings in relativedarkness. How do leaf color and plant shapevary with decreasing light? (Variegated coleusprovide spectacular results.)

Make certain that students understand that whatthe environment is changing is not the geneticpattern of the species, but the expression of thispattern. A range of variation already exists in theplant's genetic makeup, permitting the plant tomanifest itself according to the demands of itsenvironment. To test this point, gradually changethe environment of a coleus plant grown in relativedarkness by exposing it to increasing amounts oflight. After several weeks does the appearance ofthe plant change substantially? (Before and afterphotographs may prove useful here.) Environ-mental adaptation, then, is not the same asgenetic adaptation.

When the organism is in an environment wherethis variation is not a hindrance, the organismsurvives and reproduces more of its kind. In fact,the variation may be beneficial and thereby havespecial survival value. Have students explain suchexamples of adaptation in cacti, sun-loving plants,shade-loving plants, and dandelions.

ACTIVITY 3. Natural Selectionin Tadpoles

A few years ago a biologist looked into a smallpond with a gray, muddy bottom and countedabout 500 tadpoles. Most were dark, but 75 werewhite (albinos). The next morning tracks of araccoon were seen at the edge of the pool. The

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biologist counted only 49 white tadpoles that day;the next morning only 27; and the third morningonly 9. On the fourth morning he found only 7albino tadpoles. On that day he carefully estimatedthe number of dark tadpoles to be about 400. Oneach of the four mornings there had been freshraccoon tracks.

1. If the raccoon was wholly responsible for thedisappearance of tadpoles, how many of thedark ones had it eaten? How many white oneshad it eaten?

2. Why do you suppose the raccoon caught morewhite tadpoles than dark ones?

3. What prediction would you make for the pond'stadpole population the following year?

4. if this pond had a white, sandy bottom, whichcolor tadpole would the raccoon probably catchmore frequently?

Note: It is reasonable to assume that after fourdays the raccoon continued to reduce the size ofthe tadpole population. Observation of suchpopulations indicates that predators can reducethem to one-quarter of their size. There is almostno chance that the seven white tadpoles willsurvive to become adult toads. Therefore thepopulation next year will probably contain feweralbino tadpoles.

ACTIVITY 4. Evolution of aNew SpeciesSpecies A lives and flourishes in Zone 1, which isin the lowlands and has a warm, moist climate.Zone 2, also in the lowlands, is too dry forSpecies A to live successfully. Zone 3 is in the

mountains,where the environment is too harsh.Zone 4 is in the lowlands, but its climate is too-cool.

Let us now assume that the lowlands of Zone 1begin to sink slowly; probably just a few centi-metres every century. The sea will someday beginto cover Zone 1, and Species A's territory will grad-ually become smaller and smaller. Thus, not allSpecies A individuals will be able to find food orplaces to live. In time the species may becomeextinct, when the sea covers Zone 1 entirely.

But let us try to imagine what is occurring overtimewhere Zones 1 and 2 meet. Zone 2 has an environ-ment that is warm and dry; one in which Species Acannot flourish. But Species A has a largepopulation. When individuals mate, some of theresulting offspring may be a little more successfuli n a warm, dry climate than the rest of Species A.As the generations pass, those variations thatallow individuals in Species A to live fairly well inwarm-dry environments will increase as geologicalconditions change.

After many generations we would expect to findsome individuals with the characteristics thatenable them to live in a warm, dry climate. Thesecan then push theirway into Zone 2. The sameprocess could be happening where Zone 1 meetsZone 4. In other words, some of the individuals ofSpecies A may have changed sufficiently to live ina cool, moist environment.

After thousands of generations, the sea will haveentirely covered Zone 1, and all living individuals inSpecies A will be in Zones 2 and 4, and can becalled Species A-2 and Species A-4, respectively.By now these two groups will be completelyseparate-geological change has heightenedthe mountain barrier between them, preventingany combination of groups. After thousands more

years, the A-2 population will have changed inmany ways that have better adapted them to thewarm-dry environment. Similar changes will haveoccurred with A-4. But, as A-2 and A-4 becomei ncreasingly adapted to their different environ-ments, they become less like each other. In fact,they may become so different that they can berecognized as two separate species.

Discussion

1. I n the evolution of Species A into A-2 and A-4,what was the effect of the land's sinking?

2. What was the environmental effect of themountain range in Zone 3?

3. Draw a map showing land, water, and speciesdistribution at the "start" of the evolutionprocess and another showing them at the"end."

4. The ancestor of all domesticated dogs isthought to have been a wild animal resemblinga wolf. You have seen many different breeds ofdogs. How do you think they came to exist?

This study is adapted from Interaction of Man andthe Biosphere: Inquiry in Life Science. pp. 324-327 (see Further Reading).

ACTIVITY 5. ReviewQuestionsAsk the students to answer the followingquestions.

1. Explain the term natural selection. What roledoes it play in evolution?

2. How would you compare the selective

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breeding of horses with evolution of horses asit occurs in nature?

3. How would Charles Darwin explain the devel-opment of long necks in giraffes?

4. What is artificial selection? How has selectivebreeding been used?

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ObjectivesStudents should be able to do the following:

1. Recognize the fact that natural selection byitself cannot account for evolution.

2. Describe Mendel's experiment dealing withround and wrinkled seeds in the common peaplant.

3. Recognize that characteristics are preservedfrom generation to generation.

4. State the Hardy-Weinberg law and recognizeits limitations.

Program DescriptionNatural selection by itself cannot account forevolution - hereditary transmission and somevariation must be present. Inheritance is nowunderstood very clearly, due to Mendel and hissuccessors, and to workers in molecular genetics.Variation, in large part, is due to the phenomenonof mutation, which is discussed in Program 6.Some of the other evolutionary forces areisolation, which prevents interbreeding betweenspecies, and population sizes. This is discussed inProgram 5. The solution to the problem of howvariation is maintained arose when the Mendelianlaws of inheritance were established. The units ofhereditary information received by the zygote fromone gamete do not blend with the units receivedfrom the other gamete but retain their identity. Theinformation is reassorted and may be manifested inthe following generation.

The program describes Mendel's experiments with

the common pea plant where he cross-pollinatedround peas with wrinkled peas. The outcome ofthe F1 generation was 100 percent round peas.Self-pollinating the F1 peas, Mendel then notedthat in the offspring of the F2 generation, thewrinkled characteristic reappeared in the ratio of3:1 - round to wrinkled. The crossing of twoheterozygous pea plants resulted in the well-known Mendelian ratio of 3:1 for the dominantcharacteristic. These experiments established thefact that factors-units of heredity we now callgenes- are neither destroyed nor blended, butrather preserved from generation to generation.

The program concludes by demonstratingmathematically how the Hardy-Weinberg modelproves that the rata of dominant to recessive traitsremains constant in a large breeding population.The concept of genetic equilibrium refers to astate of balance. Gene frequencies are said to bein equilibrium when they remain constant fromgeneration to generation. Such equilibrium, underrandom mating and free from external forces,i mplies that genotypic frequencies will also beconstant in successive generations. The Hardy-Weinberg law makes use of a formula derived fromthe F2 results of a monohybrid cross. Recall thatwhen Mendel crossed a smooth seed with a wrin-kled seed, he found the following results in the F2:

RR +Rr+rr

Algebraically, RR is R2 and rr is r2 . Using theseexpressions we get the formula: R2 + 2Rr + r2. Thisrepresents the frequency of the genes andgenotypes in a stable population. "R" and °r" are

gene frequencies, R2, 2Rr, and r2 are thefrequencies of the phenotypes.

Let us examine a gene pool in which the genefrequencies are p(R) = 0.8 and q(r) = 0.2. These, inthe absence of selective advantage of onegeneration type over another and of mutation, arealso the gametic frequencies. The genotypicfrequencies are readily obtained from the followingtable:

The population thus consists of 64% RR's, 32%Rr's, and 4% rr's. As long as the population is notdisturbed, the gene frequency will remainconstant.

Before ViewingReview the principles of inheritance as revealed byMendel. Refresh the students' memories by citingexamples of monohybrid crosses. A review of thebinomial expression would also enhance the valueof the Hardy-Weinberg law.

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PROGRAM 3 /Factoring In Mendel

1 0

After ViewingDuring the Industrial Revolution in England, thesoot particles in the air and the pollutant-ladenrains changed the color of tree trunks. Thischange in color was to have an influence onthe color of certain populations of moths.H.B. Kettlewell felt he had gathered directevidence to support the idea of natural selection.He had shown that the genetic makeup of apopulation of living things can change undercertain environmental conditions.

Discuss population genetics and the Hardy-Weinberg law by analyzing the story of the mothsas described in H.B. Kettlewell, "Insect Survivaland Selection for Patter" (Science 148:1290-96,1965) or in the BSCS biology texts.

ACTIVITY 1: Why Are SomePea Seeds Wrinkled and SomeNot?

Mendel discovered that the shape of pea seeds isunder genetic control. Plants with genes forwrinkled seeds always produced offspring withwrinkled seeds, and plants with genes for smoothseeds always produced offspring with smoothseeds. How then does the gene affect the seedcoat to cause it to be wrinkled or smooth? Aftercompleting this investigation, students should beable to explain how the variation between roundand wrinkled peas is the result of differences in themolecular activity of enzymes. Students shouldalso have a better idea of how genes exert theirinfluence in determining characteristics ofi ndividuals. This investigation is found in BiologicalScience -A Molecular Approach, 4th ed.Investigation 14-A, pp.634-636. (see FurtherReading)

ACTIVITY 2: Inheritance ofTraits in Fruit Flies)

This activity will demonstrate how the wing shapetrait is inherited in fruit flies. Having completed thisactivity, students should be able to distinguishbetween male and female fruit flies, describe thedifferences between fruit flies showing normal andvestigial wings, trace the life cycle of a fruit fly, tellwhich trait is recessive and explain the ratio ofeach type of offspring resulting in each genera-tion. The investigation may be found in Biology,The Science of Life, Investigation 10-1, pp.276-281. Further investigations with Drosphilia maybefound in Biological Science -A MolecularApproach, 4th ed. Investigation 13-B, p.630 (seeFurther Reading).

ACTIVITY 3: Random Selec-lion in a Large Population

Upon completion of this activity, students shouldbe able to explain how a population's geneticmakeup remains constant if no external forces areinvolved. This activity is in the form of a game.

There are 120 cards (similar to playing cards)representing genes for color in an insect. Thereare 60 light gene cards (recessive) and 60darkgene cards (dominant). Another 20 cards aremarked "eaten by birds" and 10 cards are marked"escaped from birds." Shuffle 30 light and 30 darkcards and give each student a pair of cards. Re-cord the number of dark insects and light insectson the chalkboard. Shuffle the cards marked"eaten by birds" and "escaped from birds" togeth-er and deal one card to each student. Since it isassumed that the birds show no preference incolor of insects eaten, this is called "random selec-tion." During Summer 1 only those insects withescape cards live to reproduce and donate genesto the next generation. Those insects that receiveeaten cards do not reproduce, and their genes areremoved from the population. Record the numberof dark and light insects that escape as well as thenumber of dark and light alleles that remained inthe population during Summer 1. Collect the cardsand make up a new set of 60 cards, having a dark-to-light ratio identical to the ratio of dark-to-lightalleles that were retained the previous summer.The game continues for four or five summers.

Questions

1. What determines the selection of insects to beeaten?

2. What happened to the ratio of dark to lighti nsects between Summer 1 and Summer 5?

3. What happened to the ratio of dark to lightalleles in the population during the fivesummers?

4. Predict what might happen to the colorcomposition of the population over the next20 years.

For further details refer to Interaction of Man andthe Biosphere - Inquiry in Life Science,I nvestigation 12.3, p.321. You may wish to useI nvestigation 12.4, Nonrandom Selection in aLarge Population, as a follow-up. (see FurtherReading)

ACTIVITY 4: Review

Have your students do the following activities.

1. With specific reference to Mendel'sexperiments with the seeds of pea plants,describe how characteristics are preservedfrom generation to generation.

2. State the Hardy-Weinberg law. What is itssignificance in evolutionary theory?

1 1

1 2

ObjectivesStudents should be able to do the following:

1. Describe the process of mitosis.2. Describe the process of meiosis.3. Explain the significance of melosis.4. Describe the process of crossing over.5. State why the potential genotypes in the

human are so great.6. Identify the genetic material found in

chromosomes.

Program DescriptionMendel demonstrated through his experimentswith pea plants that individual characteristics werenot blended but survived from generation togeneration. He explained how a particularphenotype could suddenly reappear in apopulation, the specific gene forthat phenotypehaving been masked in previous generations.

Once in a while a gene will produce a differentphenotype and due to some change in theenvironment, this particular variation may befavored over another. As a result, natural selectionacting on this specific phenotype could bringabout evolution of the species.

I n the early part of the twentieth century, biologistsdiscovered that chromosomes contained specificgenetic information that determined the functionand appearance of an organism. They observedthat in order for two nuclei derived from one

nucleus in the parent cell to contain the samequantity and quality of chromosomes as theparent, each of the original chromosomes had toreplicate itself. The chromosomes would then lineup along the centre of the cell; each half of thechromosomes would be pulled to opposites endsof the cell and the cytoplasm would divide into twonew cells. As a result, two daughter cells areproduced, each identical to the parent cell. Thename given to this process is mitosis. The replica-tion of chromosomes and the equal separation ofthese duplicated chromosomes into the daughtercells are the key features of mitosis. The chromo-somes provide a "blueprint" for the proper func-tioning of the cell, and therefore, forthe whole or-ganism. However, while continuity is guaranteed,the process of mitosis does not explain variability.

It was nature's invention of sexual reproductionthat enabled organisms to generate variability.When two gametes fuse during fertilization, eachparent contributes one member of each pair ofchromosomes to the offspring. As a result ofmitosis and cell division, all cells, except thegametes of an individual, contain the same amountof genetic material. These cells are said to containthe diploid number (2N) of chromosomes. Ifgametes were each to contain 2N chromosomes,upon fusion of the two gametes (egg and sperm)to form a zygote, the zygote would contain 4Nchromosomes and each cell derived from it wouldcontain 4N-an abnormal amount. Therefore, ifsexual reproduction is to be successful, thenumber of chromosomes must be halved by aspecial type of nuclear division known as meiosis.When the now haploid (N) egg combines with the

haploid (N) sperm, the diploid number (2N) ofchromosomes is restored, with chromosomes fromeach parent. The stages of meiosis (as well ascrossing over) are illustrated in Figure 4-1 on thefollowing page.

Meiosis selects chromosomes by random assort-ment. If we considerthe number of chromosomesin the human gamete - 23, due to randomassortment there are 223 , or in excess of eightmillion possible combinations of chromosomes ineach gamete. Crossing over, the exchange ofsections of genetic material between members ofhomologous pairs of chromosomes, leads to aneven greater variability in the possible genecombinations of a gamete. The potential diversityof human genotypes becomes so enormous that itexceeds the total number of molecules in theuniverse. Crossing over, coupled with randomassortment, produces an astronomical number ofpotential genotypes.

Although Darwin was not aware of meiosis at thetime, he realized that some mechanism must bepresent to account for the countless variations,thus giving rise to the evolution of a new species.

I n 1953 Watson and Crick reported that variabilitywas due to the genetic material, DNA, within thechromosomes. The DNA double helix moleculeresembles a spiral ladder, the rungs composed offour nitrogenous bases, adenine, thymine,guanine, and cytosine, that are bonded togetherin varying sequences to form the genetic code.During both mitosis and meiosis, the DNAmolecule unzips, each strand rebuilding its

PROGRAM 4 /The Meiotic Mix

Figure 4-1

a) The diploid number ofchromosomes

d) Chromosomes align alongcentre of cell

g) Chromatids are pulled toopposite ends

Meiosis

b) Replication and pairing ofhomologous chromosomes

s'.~V1 2)

e) Spindle formation homologouschromosomes move to oppositeends of the cell

c) Crossing over has occurred

f) Homologous chromosomes areseparated

h) Four nuclei appear, each i) Cytoplasm divides to form fourenclosing the haploid number of daughter cellschromosomes

organism. This DNA replication is the foundation ofmeiosis- a way of reshuffling genes to produceconstant variation.

Before ViewingReview the processes of mitosis and meiosis. Thisprogram briefly mentions mitosis and emphasizesthe fact that meiosis is the foundation of geneticvariability.

After ViewingDiscuss the process of meiosis emphasizing thetwo main ideas: random assortment and crossingover. These two factors (as well as mutation) arethe key to genetic variation. Refer to Figure 4-2 fora more detailed picture of crossing over on thefollowing page.

ACTIVITY 1: The Site ofGenetic MaterialI n order to see chromosomes within the nucleus ofa cell, the cell must be treated and stained. Thechromosomes will take on the stain only if the cellis undergoing cell division. Such cells are to befound near the tips of young growing roots. Beanor onion root tips are suitable forthis activity.

Method:

1. Collect in a "test tube four bean or onion roottips and soak them in acetic alcohol for fourhours. This "fixes" the cells.

2. Wash root tips thoroughly in water, transferthem to a small test tube and cover with dilutehydrochloric acid. Place the test tube in a waterbath at 60°C for exactly six minutes.

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3. Carefully transfer root tips to a second smalltest tube, cover with Feulgen's reagent and setaside for one hour in darkness. This stains theroots red.

4. Then cut 2 mm from the end of each root tip,place on separate slides and add one drop of

Figure 42Crossing Over

Chromatids breakat the chiasma,the point at whichthe exchange ofgenetic materialwill occur.

Portions of geneticmaterial havebecome attachedto oppositechromatids.

Homologouschromosomesbegin to repel eachother except at thepoint of chiasma.

Chromosomesseparate and arepulled to oppositeends of the cell.Note the rearrange-ment of the geneson the chromatidswhere crossingover has occurred.

45% ethanoic acid to each slide. Thenthoroughly squash each root tip with theflattened end of a glass rod.

5. Place a cover slip over each preparation, warmbriefly over a small flame. Squash material bygently pressing the slide between folds ofpapertissue.

6. Examine each slide under the high powerobjective of the microscope.

The threadlike structures inside the nuclei are thechromosomes. Have the students draw the cellsshowing the stages of mitosis.

ACTIVITY 2: MeiosisTo see chromosomes in meiosis, choosei mmature flowers that are higher up on the stalkrather than the ones in which the anthers arebeginning to turn yellow. Dissect out thei nflorescences, fixing them immediately in Clarke'sfluid (25 ml glacial acetic [ethanoic] acid, 25 mlethanol) for 24 hours.

Method

1. Dissect out the anthers, place them on a watchglass and cover them with nine drops of aceticorcein and one drop dilute hydrochloric acid.

2. Gently heat the watch glass until mist rises fromthe stain, but do not boil.

3. Leave the watch glass covered for at least fiveminutes.

4. Place anthers on separate slides. Add onedrop of 45% ethanoic acid. Place a cover slipover each preparation, press gently on

coverslip with the flattened end of a glass rod.

5. Observe under high power objective of themicroscope. Have students draw the differentstages of meiosis.

ACTIVITY 3: ObservingChromosomes in the SalivaryGlands of Fruit Fly LarvaeThe salivary gland chromosomes of the fruit fly arevery enlarged and can be observed with classroommicroscopes underthe high power objective. Ifyou are planning to do the activity for Program 3dealing with fruit flies, you should have a goodstock of fruit fly larvae at this time. This activity maybe found in Biology, The Science of Life, TeacherEd., p.T92 (see Further Reading). When the pro-cedure is completed, the students will be able toobserve banding within the chromosome. Doingthe demonstration is worthwhile because it clarifiessome of the abstract character of the chromo-somes that may exist in the minds of yourstudents.

ACTIVITY 4: Law ofIndependent AssortmentThis activity allows the student to use homemadechromosomes to aid in the understanding ofindependent assortment. Using the homemadechromosomes, the students will follow pea plantgenes through the process of meiosis. Bycomparing class results, the students will be ableto draw some important conclusions aboutindependent or random assortment. Although thisactivity is very simple, it is fun. It may be found inDiscovering Biological Science, Activity 30.4,p.441 (see Further Reading).

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ACTIVITY 5: DNA and InheritedTraits

This investigation is a dry lab. An excellentexample of the use of logic and the scientificmethod to solve a problem, this investigationenables student to see how the hypothesisconcerning the role of DNA in inherited traits wasdeveloped. When the students have completedthe activity, they should be able to:

fertilize a haploid ovum, a) what effect wouldyou expect this to have on the zygote andoffspring and, b) if the zygote grew into anormal individual, what might happen when theindividual produced gametes by meiosis?

4. A horse and donkey when mated produce amule that is healthy but sterile. With specificreference to chromosomes and meiosis,suggest an explanation for this phenomenon.

- describe the difference between bacteria thatcause pneumonia and bacteria that do not;

- describe part of an experiment thatdemonstrates that the presence or absence ofa capsule is an inherited trait;

- provide a hypothesis to explain how theoffspring of pneumonia bacteria withoutcapsules developed capsules. This activity isfound in Biology, The Science of Life,I nvestigation 10-2, p.285 (see FurtherReading).

5. A short-wing, grey-bodied Drosophila iscrossed with a normal-winged, black-bodiedDrosophila. A short-winged, black-bodiedoffspring is produced. Explain how this couldhappen if a) the genes for body color and wingsize are on different chromosomes, and b) ifthese genes are on the same chromosomes.

ACTIVITY 6: Review

Ask your students the following questions:

1. How does the pattern of inheritance help youto understand the mechanism of evolution?

2. Should the knowledge that you have nowconcerning inheritance lead you to renounceLamarck's theory, i.e., that stretching the neckin the giraffe could affect the DNA of thegametes, thereby affecting later generations?State your reasons.

3. Once in awhile, during meiosis, the chromo-somes fail to separate properly and the gameteso formed contains the diploid number ofchromosomes. If such a diploid sperm were to

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ObjectivesStudents should be able to do the following:

1. Explain the gene pool concept.2. Define organic evolution.3. State that selection and survival factors can

afterthe expected gene frequency distributionin a population.

4. Explain what is meant by sexual selection.5. Describe how divergence leads to speciation.6. Use the laws of probability to predict the likeli-

hood of the occurrence of a particular event.

Program DescriptionThe population represents an immense genepool, an enormous reservoir of genetic variation.As individuals die, their genes leave the pool; asthey reproduce, new combinations of their genesare added to the pool. Evolution consists of overallchanges in the frequencies of genes in thepopulation pool. According to the Hardy-Weinberg law, the population of alleles shouldremain constant in the population generation aftergeneration. If for some reason the Hardy-Weinberg law fails, it must be due to some internalor external force that has disrupted the stability ofthe gene pool. For example, if a particulartraft suchas fleetness of foot is favored, the individualpossessing it lives to pass on his attributes, i.e.,the allele for fleetness gains a larger constituencyi n the gene pool. A change in the frequency ofvarious alleles in the population is termed organicevolution.

Gene frequencies are said to be in equilibriumwhen they remain constant from generation togeneration. Such equilibrium under randombreeding and free from disturbing forces impliesthat genotypic frequencies will also be constant infuture generations. But this equilibrium can bedistributed by what Darwin referred to as sexualselection.

There are well- established behavior patterns ofcourtship and mating in many species. Thei ndividual who displays a more vigorous courtshipmay confer selective advantages and donate agreater share of his genes to the gene pool. Thissuccess has been termed survival of the fittest.But natural selection tends to favor the averageand removes the extremes. Species tend to bevery stable. Natural selection fine tunes thepopulation according to environmental conditions.

As there is a "struggle for survival," theenvironmental factors such as climate, food,

disease, and predators will determine whichparticular varieties of the organisms of a populationwill survive to reproduce. In this competition,individuals with certain characteristics will moreoften survive to have offspring than the individualswithout these valuable traits. If selection is of thestabilizing kind, an equilibrium may still persist.Under directional selection this would not be thecase. At some point, natural selection may favorone extreme and the frequency of the allele thatconferred the selective advantage would increase.Over time the population would shift in onedirection.

Natural selection may also split a population intotwo directions -this is called disruptive selection.As time goes on, each pool evolves in response toits environment. Eventually the two groups havetheir own set of traits. If the two groups werebrought back together, they could still interbreedto produce fertile offspring and thus would beconsidered one sexually reproductive species.But if isolation persists and further changes occur,eventually the two groups would not be able tointerbreed at all, even if they could get together.At this point the two groups have becomeseparate species.

Evolution of a new species is called speciation.Speciation occurs as a result of geographicisolation and the evolution of separate gene poolsin response to their own environments. Speciationusually results from divergence. In thereproduction of small populations, chance alonecan play a significant role in determining the genepool. The equilibrium of the genetic pool can be

PROGRAM 5 /The Population Picture

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changed by chance processes rather than bynatural selection. When the population multiplies,certain genes may appear more often, others mayappear much less often until they eventuallydisappear altogether. This phenomenon is knownas random genetic drift.

Although meiosis is constantly shuffling andreshuffling genes, this process alone cannotaccount forthe progression of a species from alowerform to a more highly-developed individual.As you will observe in Program 6, mutation is also aprime source of variation.

Before ViewingReview the principles set forth by Hardy andWeinberg before viewing the program.

After ViewingFollow up the ideas of geological disruption byworking through the following problem: A canyonin North America separates two demes of a

species of chipmunk (Species A). The chipmunkson the north side of the canyon are adapted to lifei n a forest. The deme of chipmunks on the southside is adapted to desert life. Another species ofchipmunk (Species B) is adapted only to thedesert.

1. Which species probably has the greaternumber of genetic variations?

2. Describe the probable genetic variations ofthis species.

3. Which species has the greater gene flow?What is the effect of this on the population?

4. Which species is better protected againstenvironmental change? Explain why.

5. Which species demonstrates the better traitsfor survival? Explain why.

ACTIVITY 1: Is "Different"Better?

I n this investigation the students examine datadealing with humans, the seeds of a plant and thefinches of the Galapagos Islands. The studentscompare variations within each of the areas,measure the variations, and graph the data. Theusefulness of such data in supporting hypothesesabout how species are formed should bediscussed.

Materials

ruler

25 cm of string

graph paper

10 bean seeds

Design (Part A)

Eye See!

Tie a knot 5 cm from the end of the string. Hold thestring across the bridge of your partner's nose andposition the knot exactly at the outer corner of oneeye. Extend the string to the outer comer of theother eye. Mark the string at that point. Measurethe distance between the knot and the mark to thenearest millimetre and record the information. Yourpartner now repeats the procedure on you,measuring your eye-width.

Interpreting the Data

1. Collect the data from the entire class andarrange all of the eye-width measurements in atable. Make your table similar to the followingone, substituting eye-width measurements forheight measurements. Determine the smallestand the greatest eye-width measurements inthe class. The table of heights of Canadianmen uses intervals of 2 cm. In your eye-widthtable, use intervals of 3 mm. Suppose that98 mm and 101 mm are two columns in yourtable. To which would you assign a measure-ment of 100 mm?

2. Prepare a graph of these eye-widthmeasurements. Record the range ofmeasurements, from the smallest to thegreatest, on the horizontal axis. Record thenumber of individuals per 3 mm interval on thevertical axis. Examine the axes of the graph ofheights of Canadian men. Plot your data foreye-widths. Connect the plot-points with aseries of straight lines, in what is called astraight-line graph. Now, note the smoothcurve of the graph of heights of Canadianmen. How does it differ from your straight-line

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3. An optometrist measures the width of youreyes before he fits you with glasses. Suppose thatall of the thousands of measurements of eye-widththat have been made by Canadian optometristswere plotted on a graph similar to the one youhave made for your class data. Would the generalshape of the optometrists' graph be more like thestraight-line graph or the smooth curve? Explainyour answer and describe one advantage of usinga large number of measurements.

graph? Your plot-points for eye-width shouldenable you to draw, free-hand, a similar curve.

Height of 50 Canadian Men (All Heights to the Nearest 2 cm)

Design (Part B)

Obtain 10 dry bean seeds(e.g., lima bean seeds) andplace each one on a smallsquare of paper. Number thesquares of paperfrom 1 to 10.

Record in a table the length of each seed, to thenearest millimetre. Identify each seed in your tableby its number.

Determine the greatest and smallestmeasurements recorded in the class. Select asuitable interval of measurement and label a set ofjars for the whole range of measurements. Sort theseeds into the jars according to length. Count theseeds in each jar and record this information in atable.

Alike as Peas in a Pod

Design (Part C)

Every Finch Has its Beak!

i magine that you have been given an opportunityto go to the Galapagos to study variation inDarwin's finches. Soon after arrival, you spot threeground finches (see below?). The three birds areidentical in plumage. Their bills vary considerably insize and shape. Are these three ground finchesmembers of the same species? One way oftackling this question would be to examine theshape of the beaks of a large number of birds.From your experience with eye-width and seed-length measurements, you would expect the beak-measurement data to provide a bell-shaped curve.Many characteristics of a species provide such acurve when large numbers of individuals areexamined.

Beak Widths in Darwin's Finches: Identical Birdsor Different Species

expected distribution of beakwidths if all birds measured belongto same species

Height Distribution of 50 Canadian MenInterpreting the Data

4. Prepare a graph of the seed measurements.What descriptive labels will you put on thehorizontal axis and the vertical axis? Draw astraight-line graph connecting all of the points.Draw a smooth curve, free-hand.

5. Compare your graphs for Part A and Part B. Inwhich of them-the one for eye-width or theone for seed- length - is there less differencebetween the smooth curve and the straight-line graph? Account for your answer.

6. Which seeds contain the most stored food?Would this variation be advantageous to theembryonic plants inside such seeds? Explainyour answer.

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The finches of the Galapagos have been attractingthe attention of biology students ever sinceDarwin's visit, and in 1939 the English biologistDavid Lack went to the islands to study variation inthe birds. He collected data on the size of thebeaks of many ground finches like thoseillustrated. When he made a graph of a largenumber of measurements, instead of getting a bell-shaped curve he got three distinct bell-shapedcurves that did not overlap.

A COMPARISON OF DARWIN'S FINCHES (BASED ON DATA GATHERED BY DAVID LACK)

Group name Chief food Comments

A. Ground finches Seeds

B. Vegetarian finches Fleshy part of cactus,buds, leaves, fruit

Four types that differ in size and habit

Parrot-like bills, feed on cacti or trees

C. Insectivorous tree finches I nsects

D. Insectivorous ground finches Insects

E. Woodpecker finches Insects from under thebark of tree cacti

Feed on insects found in trees

Warbler-like, search ground vegetation for insects

Lack the long tongue of the woodpecker; usecactus spine to probe trunk for insects underthe bark

Predominant beak-shape

What did this mean? Lack wondered whether itmight mean that there were three distinct species,rather than a single one with a variable beak-size.To determine that, Lack would have to collectother data about the finches. Suggest what otherstudies would be necessary.

Now, focus your attention on the variation in beak-size in finches, and its possible significance. Somedata about the sizes and shapes of the beaks ofother finches is given in this table:

Interpreting the Data

7. Explain why Lack's data on variations in beak-size enabled him to hypothesize that there arethree distinct species of ground finches.

8. Examine the data in the table on four distinctkinds of finches, and identify three obviousdifferences in beak-structure.

9. Note the diet of each group of finches. How arethe structure of the beak and (perhaps) thefeeding habits suited to the diet in each case?

10. One speculation about the origin of Darwin'sfinches is that a flock of finches reached theGalapagos from the mainland of South Americaa very long time ago - perhaps a million yearsor more.

Answer the following questions with speculations.

a) What do you think such "ancestral" finchesmight have looked like?

b) Could the ancestral finches have been birds ofvarious species that arrived on the islands atdifferent times? Explain your answer.

c) Assume that a single small flock of colonizersdid give rise to the 14 different species thatnow exist on the islands. Forthis to occur, howimportant would be the fact that the Galapagosconsists of many small islands and not just onelarge one?

d) How important would it be to the colonizers toarrive on the islands at a time when there were:i) no other species with exactly the same

requirements,ii) few or no predators, andiii) no parasites that could use the finches as

host organisms?Explain your answers.

This activity can be found in Understanding LivingThings, Section 12.6, p.405 ©Copyright 1977 byD.C. Heath & Co. (see Further Reading).Reprinted with permission.

ACTIVITY 2: Changing GeneFrequencies in a Population byGenetic DriftAny force that disrupts the stability of a gene poolis an evolutionary factor. In this activity the specificfactor that will alter the frequencies of genes in apopulation is the size of the population. It is a verysimple activity but the questions that arise from theexercise are excellent.

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Purpose

One way in which the frequencies of genes in apopulation can be modified is by changing the sizeof the population. For example, if the number ofrabbits in Australia were reduced from fifty millionto forty million, the frequencies of all genes in thegene pool would be reduced by 20%. Suchchanges in the population are of little importancebecause, as is well known, rabbits could quicklyreplenish or even surpass the lost numbers andreconstruct the former gene pool.

A different kind of effect begins to appear, if onereduces a population to very small numbers. Thenthe relative frequencies, as well as the absolutefrequencies, may be changed merely by chance.I n the same way, if one flips a coin fifty milliontimes, he will get very close to hall heads; but if heflips it only four times, he may very well get headsthree or one out of four trials. When the populationagain multiplies, certain genes may be much less,others much more, frequent.

may be alleles - alternate genes for the sameposition. In our model we shall study only onegene locus, and at that locus there are only twoalleles - red and white.

We shall make the assumption that the organismsi n our population, like ourselves, are cross-fertilizing and interbreed freely. These are impor-tant assumptions of the Hardy-Weinberg principle.

In the present investigation we shall produce smallpopulations from a large parent population. Startby placing 50 white beans and 50 red beans in abag. These are the parent population.

Then draw at random 20 beans from the bag andplace them in an empty can. Draw 4 other beansand place them in the other empty can. The beansi n these cans represent two small populations thatare derived from the larger parent population.

Record your team's results in Table 1. Add theresults of the other teams in your class.

Questions

1. According to the Hardy-Weinberg principle,what proportions of red and white beans wouldyou expect in the derived populations?

2. How do the actual results compare with theexpected results?

3. Do the results deviate less if you average theresults of all teams?

4. In which of the derived populations do theresults more nearly approach the expectedproportions, the 20 bean or the 4 beanpopulation?

5. How do you explain the deviation of yourresults from the expected results?

6. Is it possible by reproducing rapidly, that one ofthe small derived populations could produceanother large population in a single genera-

This phenomenon is known as geneticdrift. It willbe studied by means of the model gene pool.

Materials

(per team)

1 bag of 50 red beans1 bag of 50 white beans2 empty cans

Procedure

In this and several later studies we will utilize amodel gene pool consisting of two colors of beansi n a bag. Our population consists of some speciesof diploid organism. This organism, like ourselves,has paired chromosomes with two genes at eachl ocus. The two genes may be the same or they

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Table 1Results of Genetic Drift Study

LARGE POPULATION SMALL POPULATION

100 GENES 20 GENES 4 GENES

Red White Red White Red White

50 50

tion? How would the large third generationpopulation differ from the large parental popu-l ation?

Procedure

1. Use red beads to represent the T(taster) alleleand white to designate the t(nontaster) allele.

natural selection? (You may want to use aspecific example such as the sickling cellwhen you considerthis question.)

7. In what natural situations might a populationundergo similar fluctuations?

8. What precautions did you take to insure thatthe derived populations were producedrandomly?

This activity is found in BSCS BiologyLab Block,Evolution, I nvestigation 1, p.1 (see FurtherReading). Reprinted by permission of D.C. Heath& Co.

ACTIVITY 3:

a) A Population ModelThis exercise provides students with anopportunity to compare the mathematical results ofthe Hardy- Weinberg formula with the results of apopulation model activity. The taster versusnontaster trait is considered.

I n this investigation you will compare genotypefrequencies for two generations in a modelpopulation.

Materials

(per team)

paperpencil100 red and 100 white beads of uniform size

stringboxcalculator, if available

2. Tie the beads together in 100 pairs torepresent individuals:

20 pairs of 2 red beads: homozygous tasters, TT;50 pairs of 1 red and 1 white: heterozygous

tasters, Tt;30 pairs of 2 white beads: homozygous

nontasters, ft.

3. Place these 100 pairs of beads in a box, mixthem thoroughly, and have one person at atime withdraw, at random, two pairs of beads.Record each mating.

4. Assume that each pairof parents producesfour offspring and that the genotypes of thesefour progeny are those that are theoreticallypossible in single gene pair inheritance. Tallythe offspring from each of the 50 matings; thentotal the number of TT, Tt, and, tt offspring.

5. Calculate the genotypic frequencies of theoffspring. (You will need these calculations tocompare with your results in Activity 3b).)

Questions and Discussion

1. How do the genotypic frequencies of theoffspring in your matings compare with thegenotypic frequencies of the parents? Explainyour results.

2. The Hardy-Weinberg formula is a theoreticalmodel that works if a population fulfills twocriteria. These were met in the modelpopulation you have just studied. What arethese criteria, and how are they related to

3. Why is a theoretical model, such as the Hardy-Weinberg formula, an important tool in biology?

b) Selection

This investigation takes a look at what wouldhappen to the frequencies of tasters andnontasters if individuals with one or the other genehad a survival advantage. This should be used as afollow-up to 3a) and a comparsion of the datashould be made.

Assume an extreme hypothesis-individuals withthe ttgenotype are also sterile.

Materials

(per team)

paperpencil200 red beads and 100 white beads of the same

sizestring

boxcalculator, if available

Procedure

1. Begin, as you did in Activity 3a), with initialgenotype frequencies of 20 TT, 50 Tt, and 30

tt (Go original generation). Calculate genefrequencies in the reproductive gene pool forthe next generation (G1) using the Hardy-Weinberg checkerboard and a table likeTable 2. Rememberthat tt individuals aresterile.

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2. Represent the original generation (Go ) asfollows:

20 pairs of red beads- TT50 pairs of 1 red and 1 white bead - Tt30 pairs of white beads- tt

3. Place 100 pairs in a box, mix them thoroughly,and have one person at a time withdraw, atrandom, two pairs of beads to represent amating. Record all matings.

4. Assume that each pair of fertile parentsproduces four offspring, and the genotypes of

these four progeny are those that aretheoretically possible in single gene pairinheritance. (Fertile pairs are TTx TT, TTx Tt,and Ttx Tt, all other pairs are sterile.)

Calculate gene and genotype frequencies for theoffspring and set up a table like Table 2.

5. Using the frequencies you determined instep 4, set up 100 bead pairs to represent thesecond generation (G1).

6. Repeat steps 3 through 5 until you have datafor 5 generations (Gothrough G4).

Investigations 11-11 and 11-13. (See FurtherReading.) Reprinted by permission of Prentice HallI nc. and Biological Sciences Curriculum Study.

ACTIVITY 4: ReviewAsk your students to do the following reviewactivities.

1. Explain the gene pool concept.2. Discuss the evolutionary factors that could up-

set the principles of the Hardy-Weinberg law.3. Explain the origin of species by means of

natural selection.

Questions and Discussion

How do your calculations (step 1) forthesecond generation (G 1) compare with yoursampling results for G1

2. What is happening to the frequency of therecessive gene? In which genotype in G4 aremost of the recessive genes found? How doesthis compare with G o?

3. Compare your data from Activity 3a) with thosefrom this investigation. Is selection aloneenough to explain the differences? Explainyour answer.

4. Even under extreme selection, can a recessivegene be eliminated from a gene pool?

Both activities are found in BSCS, BiologicalScience, Interaction of Experiments and Ideas,

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PROGRAM 6 /Mutation And All ThatObjectives

Students should be able to do the following:

1. Explain the significance of DNA.2. Define the term mutation.3. List some causes of mutations.4. Recognize the fact that most mutations are

harmful.5. Explain the concepts of gradualism versus

punctuated equilibrium.

Program Description

It is the recombination of genes that continuallycreates new phenotypes. Nature then selectswhich organisms will survive and thrive and whichwill eventually become extinct. We have alreadylearned that meiosis reshuffles genes and sortsthem into new combinations and thus causesvariation. But the prime source of variation amongmembers of a population is mutation.

The DNA molecule is made up of a sequence ofbases that form the genetic code. During proteinsynthesis the code is transferred to a messengerRNA and carried to the ribosome where aminoacids are linked up in proper sequence to form aspecific protein. The amino acids are the buildingblocks of proteins and proteins are the criticalmolecules that determine the form and function ofthe organism. Thus, one could say that the DNAcode is the essence of life. But genes can changeas a result of errors in the replication of the DNA.Short wave ionization can alter the DNA and

ultraviolet light can break the DNA strands. It is ofobvious importance to assess the effect of anincrease in radiation on the health of individualsand, as a result of mutations in their reproductivecells, the health of their offspring. Chemicals withinthe cells can mimic the bases and be substitutedfor the regular base pairing, thus rendering the celldysfunctional.

A mutation occurring in a gamete can affect theentire organism arising from this cell. Since themutant form of the gene is inherited, the mutationwill persist in subsequent generations. In this waywholly new evolutionary "ideas" are introducedinto the gene pool occasionally. A mutation isdefined as a spontaneous change in a gene or achromosome that may produce an alteration in thecharacteristics under its control and be passed onto the offspring. Mutations may be consideredbeneficial if the result is advantageous to theoffspring. But in most instances new mutations are

harmful to the organisms in which they arise. Thisis because they are changes to a genetic makeupthat has been refined by natural selection for along time. The changes in living species take placeover many millions of years and new species arisethrough the gradual accumulation of tinyadvantageous modifications. Darwin termed thisprocess gradualism.

But according to fossil evidence there were gapsand very little evidence of gradual change.Therefore, some biologists and paleontologistsproposed a different view of evolutionary changethat they called punctuated equilibrium. Accordingto this view, new species do not arise gradually butappear by the development of major variations in arelatively short period of time. As a result, recordsshow long periods of equilibrium or stability,punctuated by occasional major changes.However, the rate of mutation and the rate ofevolution are still a subject of speculation.

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Before ViewingReview the DNA molecule and briefly discuss itsstructure, replication, the role of messenger RNA,the function of the ribosome, and the fact thatamino acids are the building blocks of protein. Theprogram makes considerable reference to theDNA molecule.

After ViewingStudents may have difficulty grasping the idea thatmost mutations are harmful. The program doesstress that mutations are the source of new genesand new species. Therefore, how can mutationsbe harmful? Emphasize the fact that present-dayplants and animals are highly specialized and welladapted to their environment. Further mutationsare more likely to be disadvantageous thanhelpful.

ACTIVITY 1: DebatesStudents could plan short debates on topics suchas: the role of mutations and recombination inevolution, the significance of barriers in theformation of new species, random genetic drift andits effect on populations of organisms, thedifferences between natural selection and artificialselection, and the role of lethal mutations.

ACTIVITY 2: Changing GeneFrequencies in a Population byMutation.I n this activity the students study the manner inwhich mutations can help produce evolutionarychange. It is a simple but effective activity. Theconcepts derived from the exercise are verysignificant.

Procedure

I n order to facilitate the procedure, we shallassume an unnaturally high mutation rate in ourmodel gene pool. Let the red genes mutate towhite at the rate of one mutation for every ten redgenes each generation. And let one in every tengenes mutate to red in each generation.

Begin with a population of 50 red beans.Represent the passage of a generation bytransferring the beans from one can to the other.For each generation replace every tenth red beanwith a white bean. And when there are sufficientwhite beans, round the total to the nearest 10 andreplace every tenth white bean with a red bean.Count the totals in each generation. Graph thechanges that occur in each generation asmutations change the gene frequencies. SeeFigure 6-1. Stop when you reach an equilibriumbetween reds and whites.

In the great majority of instances new mutationsare harmful to the organisms in which they arise.Let us suppose that the white mutants in thisinvestigation are extremely disadvantageous and,therefore, each organism that possesses twogenes dies before he can reproduce.

In our model this is accomplished by picking twobeans at a time from the population just graphed.Each pair of beans represents a diploid individualorganism. If you pick two white beans together,discard them. Place all the other pairs in a can.When you have finished, count the beans of eachcolor and record the data for this generation on theappropriate graph. See Figure 6-2.

In the can you have just filled, let mutations occur;that is, replace every tenth red bean with a whitebean and vice versa. Then again transfer thebeans two at a time to another can, discarding the

white pairs. Record the results after eachgeneration on the graph. See Figure 6-2.

Carry this procedure on until the number of whitebeans stops decreasing.

Questions

1. Why do you suppose mutations are usuallyharmful?

2. How can mutant genes be maintained in apopulation when they are selectively destroyed?

3. Why are there more red-to-white mutations inan all-red population than in an even red-whitepopulation?

4. If two alleles occur at the same frequencies andmutate at the same rates, how can mutationschange the gene pool?

5. What are the major roles of mutations inevolution?

6. What advantage might be attached tomaintaining at low frequencies, genes that aregenerally harmful?

This activity may be found in BSCS, Biological LabBlock, Evolution, I nvestigation 4, p.6. (see FurtherReading) Reprinted by permission of D.C. Heath& Co.

ACTIVITY 3: ReviewExplain how knowledge of DNA and mutation aidsi n the understanding of the mechanism ofevolution.

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GENERATIONS GENERATIONSFigure 6-1 Changing gene frequencies due to mutations Figure 6-2 Changing gene frequencies due to mutations

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FURTHER READINGAbraham, et al. Interaction of Man and the

Biosphere: Inquiry in Life Science. Chicago:Rand McNally and Co., 1979.

Andrews, W.A. et al. Discovering BiologicalScience. Scarborough, Ontario: Prentice-HallCanada Inc., 1983.

Ayala, F.J. Molecular Evolution. Sunderland,Massachusetts: Sinauer Associates, 1976.

BSCS. Biological Science A MolecularApproach, 4th ed. Lexington, Massachusetts:D.C. Heath and Company, 1980.

BSCS. Biological Science-Interaction ofExperiments and Ideas, 3rd. ed. EnglewoodCliffs, New Jersey: Prentice-Hall Inc., 1977.

BSCS. Biological Science - Patterns andProcesses. Teachers' Ed. Dubuque, Iowa:Kendall/Hunt,1985.

Darwin, Charles. Voyage of the Beagle. TotowaNewJersey: Biblio Dist,1979.

Futuyama, Douglas J. Science on Trial: The CaseforEvolution. Toronto: Random House of

Canada Ltd., 1983.Gould, Stephen J. Ever Since Darwin. New York.

W.W. Norton, 1977.Gould, Stephen J. Hen's Teeth and Horses' Toes-

NewYork: W.W. Norton, 1983.Gould, Stephen J. The Pandas Thumb. New York:

W.W. Norton, 1980.Guring, Joel. "In the Beginning." Science, July-

August 1980.Hutchins, Ross E. Nature Invented it First. New

York: Dodd Mead and Co., 1980.Jastrow, Robert. "Evolution: Selection for

Perfection." Science Digest, December 1981.Jensch, Peter F. Biology, the Science of Life,

revised ed. Boston: Houghton Mifflin Co.,1980.

Lemer, I. Michael and Libby, William J. Heredity,Evolution and Society. San Francisco: W.H.Freeman and Company, 1976.

Lewontin, Richard C. "Adaptation." ScientificAmerican, September 1978.

Luria, Salvador E. et al. A View of Life. Menlo Park,California: The Benjamin/CummingsPublishing Company Inc., 1981.

Morholt, Evelyn, et al. A Sourcebook for theBiological Sciences, 2nd ed. New York:Harcourt, Brace & World Inc., 1966.

Oram, R. Biology. Living Things, 4th ed.Columbus, Ohio: Charles E. MerillPublishing Co., 1983.

Otto, James, Towle Albert, and Madnick, Myra.Modem Biology. New York: Holt, Rinehart andWinston,1977.

Reimer, John D. and Wilson, William G.Understanding Living Things. Toronto:D.C. Heath Canada Ltd., 1977.

Sisson, Robert F. "Deception: FormulaforSurvival." National Geographic, March 1980.

Watson, James D. and Tooze, John. The DNAStory. San Francisco: W. H. Freeman andCompany, 1981.

Webb, S. David. BS CS. Biology LaboratoryBlock,Evolution. Boulder, Colorado: D.C. Heath &Co., 1968.

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Videotapes BPN

Program 1: In the Beginning 265501Program 2: Darwin, Naturally 265502Program 3: Factoring in Mendel 265503Program 4: The Meiotic Mix 265504Program 5: The Population Picture 265505Program 6: Mutation and All That 265506

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