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  • 1

    Biology 1201 LabArtificial Selection Experiment, Part I

    Artificial Selection inBrassica, Part I

    Goals of this activity:

    1. Gain general familiarity with the process ofartificial selection.

    2. Become acquainted with some of the diverseproducts of artificial selection in Brassica.

    3. Set up an artificial selection experimentinvolving a variety of Brassica rapa.

    Introduction

    A trip to a supermarket, farm, pet store or gardencenter will offer nearly endless examples of theproducts of selective plant and animal breedingby humans. Over hundreds, and in some casesthousands of years, humans have altered variousspecies of plants and animals for our own use byselecting individuals for breeding that possessedcertain desirable characteristics, and continuingthis selective breeding process generation aftergeneration. In many cases the end results havebeen dramatic. Domestic dog varieties, fromchihuahuas to great danes, trace their separatelineages to a common wild ancestor, the wolf(Canis lupus). Domestic fowl varieties are allderived from the wild jungle fowl (Gallus gallus),while most modern breeds of domestic cattleoriginate from the now-extinct giant wild ox (Bosprimigenius). One example that particularlyimpressed Charles Darwin was that of domesticpigeon varieties (such as tumblers, fantails,carriers, pouters, and many others), which arederived from wild rock doves (Columba livia) overa period of some 5000 years.

    There are many similar examples among plants,including those that humans have bred for food

    as well as beauty. One plant group especiallyimportant to humans for food is Brassica, a genusof plants in the mustard family. A wide variety offamiliar and highly nutritious vegetables originatefrom just a few species of wild Brassicas, inparticular B. rapa, B. oleracea, and B. juncea. Somevarieties have been bred specifically for rootproduction, others for leaves, flower buds, oilproduction, etc. The group is of great economicimportance, and because of this, the geneticrelationships among the various forms have beenthoroughly studied and are now fairly wellunderstood. It may surprise you to learn that thesefamiliar vegetables so different in appearance havethe same species as a common ancestor. Centuriesof artificial selection have produced such widelydivergent forms. For example, the followingvarieties originate from these wild species:

    Brassica oleracea: kale, cauliflower, broccoli, cabbage,Brussels sprouts, kohlrabi, collards, savoy cabbage.

    Brassica juncea: leaf mustard, root mustard, headmustard, and many more varieties of mustard.

    Brassica rapa: turnip, Chinese cabbage, pak choi,rapid-cycling (see Figure 4).

    The next time youre in a grocery with a largeproduce section, or a large farmers market, see howmany of these different varieties you can find; thereare numerous other varieties not listed above.

    That plant and animal breeders have been able todramatically change the appearance of variouslineages of organisms in a relatively short periodof time is an obvious yet profound fact. It certainlydid not escape the attention of Charles Darwin, whodevoted the first chapter of The Origin of Species tothe topic of artificial selection by humans(Variation Under Domestication). He used manyexamples of selection by humans to help supportthe case for his proposed mechanism resulting inevolution of natural populationsnatural selection.

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    Biology 1201 LabArtificial Selection Experiment, Part I

    In order to gain better understanding of selectionand inheritance, researchers have experimentedwith artificial selection involving a wide variety oftraits in many different species of plants andanimals. The results, obtained in a relatively shortperiod of time, are often impressive.

    For example, in one experiment (started in 1896),the average oil content of corn kernels wasincreased from 5% to 19% in 75 generations ofselective breeding. In another, average annual eggproduction in a flock of white leghorn chickens wasdoubled in only 33 years (from 126 to 250 eggs perhen). Similar examples abound, for charactersuseful to humans (as above) as well as more esotericones (such as the number of abdominal bristles onfruit flies, or the fecundity of female flour beetles).

    A general finding from these studies is that mostvariable traits in organisms respond to artificialselection (i.e., it is usually possibleeven easyto increase or decrease the frequency or averagevalue of a trait in a lineage through careful selectivebreeding). Starting today in a lab exercise, you willattempt to accomplish the same thing.

    How artificial selection is similar tonatural selection

    You will be learning a great deal about naturalselection in lecture and in later labs. Naturalselection is a deceptively simple concept, relativelyeasy to understand at a basic level, but withprofound implications that are intellectuallychallenging. The following exercise in artificialselection will serve as an introduction to naturalselection, and we hope will help you to betterunderstand this important concept.Fundamentally, artificial selection and naturalselection are quite similar, but there are a fewimportant differences.

    Very briefly, natural selection occurs when certainvariants within a population of organismsexperience consistently greater reproductivesuccess (i.e., leave more offspring) than do othervariants. Generally, within a population oforganisms (of the same species), there is variationamong individuals for a great variety of obviousand not so obvious traits. Some of this variabilityis a result of genetic differences among individuals,while some is probably a result of different

    turnip Chinese cabbage pak choi rapid-cycling

    Figure 4 Varieties (cultivar groups) of Brassica rapa.

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    Biology 1201 LabArtificial Selection Experiment, Part I

    environmental influences. Here we are concernedonly with variability that has a genetic basis (i.e.,is inheritablepassed on from parents tooffspring). As a general rule, organisms producemore (often far more) offspring than can surviveto reproduce themselves (in Darwins words, thereis a struggle for existence). If individuals withcertain inheritable characteristics have anadvantage in survival and reproduction overindividuals with other characteristics, then itfollows that the next generation will differ fromthe previous oneperhaps slightly, perhapsconsiderably. Why? Because individuals withthose certain favored inheritable characteristics aremore likely to be parents by virtue of possessingthese characteristics; the next generation willcontain a disproportionate number of offspring ofthese naturally selected parents, and theseoffspring will tend to resemble their parents. Asa result, over many generations the geneticstructure of the population changes (in otherwords, the population evolves).

    Artificial selection is essentially this same process.A population of organisms exhibits considerablevariability among individuals for a trait or traitsthat might be of interest to humans for one reasonor another (such as running speed in thoroughbredhorses, petal size and shape in tulips, number ofkernels in an ear of corn, egg size in chickens, etc.).By virtue of possession of certain traits, someindividuals are selected by the plant or animalbreeder or scientist to be parents of the nextgeneration, while the remainder of the populationnot possessing those traits are excluded from

    breeding. To the degree that the desired trait isinheritable, the offspring will tend to resemble theirparents, and hence on average the next generationwill differ from the previous one i.e., will befaster, or have larger petals, or more kernels, orlarger eggs, etc. Over time, substantial differencescan be achieved between the original populationand the artificially selected subsequent ones.

    How artificial selection differs fromnatural selection

    As noted above, there are some importantdifferences between artificial and natural selection.In contrast to natural selection, artificial selection:1) favors traits that for one reason or another arepreferred by humans; 2) has a goal or directiontoward which the selection process is directed; 3)generally is much faster than natural selection,because the next generation can be absolutelyrestricted to offspring of parents that meet thedesired criteria (rarely is natural selection such anall-or-none phenomenon). In artificial selection,humans are doing the selectingpurposelyrestricting breeding to individuals with certaincharacteristics. In natural selection, theenvironment does the selectingindividualsthat survive and reproduce better in a givenenvironment, for whatever reason, are naturallyselected. The environment can include such thingsas predators, food supply, climatic conditions, soilnutrients, and many, many others.

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    Biology 1201 LabArtificial Selection Experiment, Part I

    An Artificial SelectionExperiment UsingWisconsin FastPlantsTM

    Introduction:

    You and the other members of your lab section willparticipate as plant breeders in an attempt toartificially select for a particular variable trait in alineage of rapid-cycling Brassica rapa (WisconsinFast PlantsTM). This exercise will begin today, butwill not be completed until the last lab period thisquarter. The organism with which you willexperiment belongs to the same species as turnip,pak choi and chinese cabbage, each of which is adifferent artificially selected variety. The Brassicarapa you will be using is itself is a product of intense

    artificial selection over the past 20 years for thefollowing traits: rapid flowering and maturation;high seed production; short stature; and the abilityto thrive under artificial light. The result of theseefforts (at the Univer