Table Of ContentGrzimek’s Animal Life Encyclopedia
Evolution
Michael Hutchins, Series Editor
Valerius Geist and Erik Pianka, Advisory Editors
• • • •
In Association with The Wildlife Society
’
Grzimek s Animal Life Encyclopedia
Evolution
••••
(c) 2011 Cengage Learning. All Rights Reserved.
’
Grzimek s Animal Life Encyclopedia
Evolution
••••
Michael Hutchins, Series Editor
Valerius Geist and Eric R. Pianka, Advisory Editors
••••
In Association with The Wildlife Society
(c) 2011 Cengage Learning. All Rights Reserved.
Grzimek’sAnimalLifeEncyclopedia ©2011Gale,CengageLearning
Evolution
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LIBRARYOFCONGRESSCATALOGING-IN-PUBLICATIONDATA
Grzimek,Bernhard.[Animallifeencyclopediaevolution]Grzimek’s
animallifeencyclopediaevolution/MichaelHutchins,serieseditor;
ValeriusGeistandEricPianka,advisoryeditors;inassociationwithThe
WildlifeSociety.
p.cm.
Includesbibliographicalreferencesandindex.
ISBN978-1-4144-8669-7(hardcover)–ISBN978-1-4144-8670-3(e-book)
1.Evolution(Biology)–Encyclopedias.2.Evolution(Biology)–Studyand
teaching.I.Hutchins,Michael.II.Geist,Valerius.III.Pianka,EricR.IV.
WildlifeSociety.V.Title.VI.Title:Animallifeencyclopediaevolution.
QH360.2.G652011
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ISBN-13:978-1-4144-8669-7 ISBN-10:1-4144-8669-3
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•••••
Editorial board
EditorinChief
MichaelHutchins,Ph.D.
ExecutiveDirector/CEO
TheWildlifeSociety
AssociateEditors
ValeriusGeist,Ph.D.
ProfessorEmeritusof
EnvironmentalScience
UniversityofCalgary
EricR.Pianka,Ph.D.,D.Sc.
DentonA.CooleyCentennial
ProfessorofZoology
SectionofIntegrativeBiology
UniversityofTexasatAustin
Grzimek’sAnimalLifeEncyclopedia v
(c) 2011 Cengage Learning. All Rights Reserved.
•••••
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Sexual selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
The relevance of evolution to contemporary society . . . . xi Kin selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Contributing writers . . . . . . . . . . . . . . . . . . . . . . . . . . xvii Coevolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Optimal reproductive tactics and life history theory. . . . 201
Topic overviews Evolutionary physiology. . . . . . . . . . . . . . . . . . . . . . . . 207
Scientific methods and human knowledge. . . . . . . . . . . . .1 Industrial melanism in moths . . . . . . . . . . . . . . . . . . . . 215
Creation stories: The human journey to understand the Galápagos finches . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
origins of life on Earth . . . . . . . . . . . . . . . . . . . . . . . .9 Artificial selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
History of evolutionary thought. . . . . . . . . . . . . . . . . . .23 Convergent evolution and ecological equivalence. . . . . . 235
Charles Darwin: A life of discovery. . . . . . . . . . . . . . . . .31 Evolution of the animal eye . . . . . . . . . . . . . . . . . . . . . 243
Mechanisms of evolutionary change . . . . . . . . . . . . . . . .45 Evolution of flight. . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Origins of the universe. . . . . . . . . . . . . . . . . . . . . . . . . .55 Evolution of limblessness. . . . . . . . . . . . . . . . . . . . . . . 263
Origins of life on Earth . . . . . . . . . . . . . . . . . . . . . . . . .63 Canid evolution: From wolf to dog. . . . . . . . . . . . . . . . 271
Speciation: The origins of diversity. . . . . . . . . . . . . . . . .73 Dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
Sexual reproduction. . . . . . . . . . . . . . . . . . . . . . . . . . . .89 Megafauna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
Species nomenclature, classification and problems of Non-human primate and human evolution . . . . . . . . . . 299
application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95 Evolution of the brain and nervous system . . . . . . . . . . 311
The fossil record: A window to the past . . . . . . . . . . . . 103 Evolution of language . . . . . . . . . . . . . . . . . . . . . . . . . 319
What does the fossil record tell us about evolution?. . . . 111 Evolutionary medicine and the biology of health. . . . . . 325
Biogeography: The distribution of life . . . . . . . . . . . . . 123 Evolution and biodiversity conservation . . . . . . . . . . . . 335
Genetics: The blueprint of life. . . . . . . . . . . . . . . . . . . 133 Further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
Genes and development. . . . . . . . . . . . . . . . . . . . . . . . 141 Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
Evolutionary ecology. . . . . . . . . . . . . . . . . . . . . . . . . . 151 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
Adaptation and evolutionary change . . . . . . . . . . . . . . . 169 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
Grzimek’sAnimalLifeEncyclopedia vii
(c) 2011 Cengage Learning. All Rights Reserved.
•••••
Preface
The idea that species were divinely created and immutable westerncoastofSouthAmerica.Herecordedseveraldifferent
persisted for centuries. Then, a studious young man decided species of giant tortoises, each inhabiting different islands in
to leave his comfortable home in England and take a long, thearchipelago, whichsuggested thatthey allhadacommon
difficult voyage around the world. Charles Darwin’s now- ancestor but had somehow changed over time. Darwin
famous trip on H.M.S. Beagle from 1831–1836 would observed a new and unique species of iguana, which, unlike
ultimately alter our view of the origin of species. itsmainland ancestor,swam underwaterandgrazedonalgae.
He documented a new species of cormorant that possessed
Of course others—such as Darwin’s grandfather, Erasmus
under-developed wings and had completely lost theability to
Darwin, and the well-known French naturalist, Jean-Baptiste
fly.Furthermore,Darwinstudiedthemanyspeciesoffinches
Lamarck—hadspeculatedthatspeciesmightchangeovertime,
that,althoughtheylookedverysimilar,haddifferentlyshaped
but they could not identify the mechanism. Darwin’s keen
beaksthatallowedthemtoexploitdifferentfoodsourcesmore
observations of nature and his landmark 1859 book—The
efficiently.Oneevenusedcactusspinesasatooltopryinsects
OriginofSpeciesbyMeansofNaturalSelection,orthePreservation
from cracks and crevices.
of Favoured Races in the Struggle for Life—would provide that
answerandrockthescientificworld.Hiswritingsandthoseof IhadtheprivilegeofvisitingtheGalápagosIslandsin1979
the many scientists that followed would forever alter our and of observing these natural wonders firsthand. For
perceptions of ourselves and the other living creatures that biologists like me, a visit to the Galápagos is analogous to a
sharethisplanet. pilgrimage,butunlikeDarwin,Iknewwhattoexpectbasedon
alonglineofscientistswhohadgonebeforeme.Whilesailing
Darwin’s most significant contribution to the biological
through these unique desert islands, I often wondered how
scienceswashistheoryofevolutionthroughnaturalselection.
Darwin felt during his first encounters with these unusual
The basic concept is simple and elegant: Individual animals
naturalphenomena.Iassumehewasthrilledbythediscoveries,
possessing heritable traits that allow them to survive and
butIalsosuspectthatitchallengedhispreconceivedviews,and
reproduce will pass those traits on to the next generation,
began his remarkable intellectual journey. It also shaped his
while those that do not survive and reproduce will not pass
philosophy of science and life. Once a deeply religious man
along their traits. The result is a gradual adaptation of a
who accepted what he was told purely on the basis of faith,
species to its environment, and the evolution of traits that
Darwingraduallybecamemoreskeptical,listeningtowhatthe
promote survival and reproduction. natural world “told” him through direct observation and
experimentation.Heoncesaid:“Greatisthepowerofsteady
Ofcourse,environmentsdonotremainstatic,afactoralso
misrepresentation, but the history of science shows how,
seen asfueling evolutionary change and ultimately leading to
fortunately,thispowerdoesnotlongendure.”
the creation of new species. Amazingly, this concept was
“discovered”independentlybyanotheravidnaturalist,Alfred
Itisalsoremarkablethat neitherDarwinnorWallacehad
RussellWallace,acontemporary ofDarwin’s,whoalsospent
anyideaofhowphysical(phenotypic)traitswerepassedfrom
manyyears inthefieldobservingandrecordingdetails about
one generation to the next. That had to wait until the
the natural world.
disciplineofgeneticsdeveloped,whichbeganwithworkofan
obscureAustrianmonk,GregorMendel,whostudiedpatterns
Darwin and Wallace based their theories on direct
of inheritance in garden peas. The development of this
observationsofnature.However,itwasDarwinwhoamassed
discipline eventually led, in the late1930s and early1940s,
the largest amount of evidence. This, and his prolific writing
on the topic, explains why his name, not Wallace’s, has to another great synthesis between modern genetics and
evolutionary theory. Led by such notables as Theodosius
become most closely associated with the discovery of
Dobzhansky and Ernst Mayer, this period produced the
evolution through natural selection.
Hardy-Weinberg Equation, which provided a greater under-
ManyofDarwin’smostsignificantandperceptiveobserva- standing of how species change as the result of population-
tionsoccurredduringhisvisittotheGalápagosIslandsoffthe level changes in gene frequencies that code for particular
Grzimek’sAnimalLifeEncyclopedia ix
(c) 2011 Cengage Learning. All Rights Reserved.
Preface Evolution
traits.Theseconceptswerefurtherconfirmedbythediscovery Evolutionarytheoryisoftenmisinterpreted.Inthepast,some
of DNA—the building block of all life—by James Watson, peoplehaveusedittopromoteandjustifysocialdominanceof
FrancisCrick and RosalindFranklin in 1953. onegroupoveranother,soitisimportantthatweremainvigilant.
However, it is also important to note the positive message of
Since then, the field of evolutionary biology and all of its evolution:thatalllivingorganismsarerelatedanddependenton
sister disciplines—including geology, paleontology, genetics, manyofthesamethingsfortheirsurvival.Thisshouldbeaneven
ecology, animal behavior, zoology, botany and others—have more powerful argument that humans should show greater
accumulatedvaststoresofknowledgeaboutthenaturalworld. respectfornatureandnotdriveotherspeciestowardirreversible
The power of evolutionary theory has been demonstrated extinction.Giventheinterdependenciesthatexistinnature,such
time and time again through both observation and predictive attitudesmaybecriticalforthefutureofHomosapiens.
experimental studies.
Grzimek’s Animal Life Encyclopedia is a 17-volume set, co-
Does this mean that Darwinian natural selection can veringtheentireanimalkingdom,fromprotozoatomammals.
explain everything about the natural world? No. There are Originally edited by the famous Frankfurt Zoo director and
manynuancestoscience,andweadmittedlystillhavemuchto conservationist, Bernhard Grzimek, and translated into
learn. As an example: Niles Eldredge and Stephen J. Gould Englishintheearly1970s,thefirstEnglishedition,consisting
introduced the concept of “punctuated equilibrium” in 1972, of 11 volumes, was published by Van Nostrand Reinhold in
a comparatively rapid form of evolution that can occur in both England and the United States. That version was very
response to environmental change and lead to speciation. popular, but after 30 years the series was in urgent need of
Though it did not change the basic concept behind natural updating. During that time, thousands of biologists, working
selection,itdidchallengetheideathatallevolutionisuniform inboth thefield andlaboratory,had generated atremendous
and gradual. amount ofnew information covering awiderangeofspecies.
Recognizing an urgent need to include the new information,
This volume is an attempt to summarize our current theentirecollectionwasupdated,reformatted,andcomplete-
knowledge about the evolutionary process and the evidence ly rewritten by English-speaking experts from 2001 to 2003
that has accumulated over the past century and a half since under the direction of new owner and publisher, Gale. I had
Darwin’s writings. With the discovery of genetics and DNA, the privilege of serving as the primary Series Editor for that
which is common to all living organisms, evolution through project. However, the project was not complete, as the
natural selection has become the unifying theory in all of original series also included three additional topical volumes:
biology. As such, it is a powerful tool with which to Ethology (animal behavior), Ecology, and Evolution.
understand the origin of many of nature’s wonders, whether
it be fish that change sex, flowers that resemble bees, or the WiththepublicationofGrzimek’sEvolution,SecondEdition,we
colorful plumage and bizarre courtship behavior of New havenowupdated,completelyrewritten,andreformattedoneof
Guinea’s birds of paradise. thesetopicalvolumes.Yetdespitethemanyscientificadvances
detailedinthisbook,thebasicgoalsandperspectiveremainthe
Darwin’s revelation challenged prevailing non-scientific sameasthoseexpressedbytheeditorofthe1976firstedition,
explanationsfortheoriginoflifeonEarth,includinghumans. Herbert Wendt, who said: “In the individual articles here the
His theory generates public controversy even to this day. readerwillfindmaterialhehasnotencounteredinotherworks.
Some U.S. states have attempted to ban the teaching of However,sciencenevercomestoastandstill;itknowsnofinal
evolutioninelementaryschools,harkeningbacktotheheady conclusion, and each new work brings forth new information
days of the Scopes Monkey Trial and the spirited debates of compiledfromthelatestresearchinthatfield....Thereadercan
WilliamJenningsBryantandClarenceDarrow.However,the become acquainted with current theoretical thinking about
courts have consistently upheld the differences between evolution in reading this volume.” This volume is the next
science-based knowledge and religion, which is not testable iterationinthecontinuingaccumulationofknowledgeaboutthe
via the scientific method. That being said, controversy originsandinnerworkingsofournaturalworld.
persists, and education is key, making the dissemination of
information about evolution vitallyimportant.Indeed, evolu- I’dliketoexpressmydeepappreciationtopublisherGale;
tion is not just about dinosaurs and other extinct species. On toTheWildlifeSocietyCouncilforallowingmyparticipation
the contrary, it is highly relevant to today’s society and we in this project; to my topic editors Valerius Geist and Eric
ignoreitatourperil.Ourunderstandingofmodernmedicine, Pianka; and to the many expert authors who contributed
agriculture, and conservation—just to name a few—are essays that make up this new volume.
dependent on our knowledge of biological evolution. How
else can we combat disease organisms that quickly adapt to
Bethesda,Maryland,2010
antibiotics, ensure the future of domestic animals and plants,
MichaelHutchins
andconservesomesemblanceofnatureforfuturegenerations
SeriesEditor
in a world dominated by human influences?
x Grzimek’sAnimalLifeEncyclopedia
(c) 2011 Cengage Learning. All Rights Reserved.
•••••
The relevance of evolution to contemporary society
A lot of time and energy has been consumed debating the transmitted,andtodesigneffectiveprogramstohaltepidemics.
question of whether evolution has happened or not, but to Several fields of study outside of biology have benefited from
peoplewhoareneitherevolutionarybiologistsnorcreationists,a theinsightthatself-replicatingsystemsevolveandhaveactually
better question might be “why does it matter?” In many harnessedtheevolutionaryprocesstoimprovethedesignof,for
branchesofscience,includingevolutionarybiology,theimpacts example, computer programs. Finally, evolutionary biology
ofaparticulartheoreticaladvanceornewbitofdatacanseem may become a key tool to confront the challenge of a rapidly
obscure, orevenirrelevant, to manypeople.Toillustratethis, changingclimate.
considerthefollowingtitlesfromtheprestigiousjournalNature
on the topic of evolution from the first half of 2010: “Allelic This entry offers examples of some of the benefits that
VariationinaFatty-AcylReductaseGeneCausesDivergencein evolutionary biology has bestowed on humanity. Many such
MothSexPheromones”;“Post-copulatorySexualSelectionand examplesexist,butonlyalimitednumberhavebeenhighlighted
Sexual Conflict in the Evolution of Male Pregnancy”; “Co- here, including some of the major contributions evolutionary
optionoftheHormone-SignallingModuleDafachronicAcid– biology has made to contemporary life. Some of the ways in
DAF-12 in Nematode Evolution”; “The Ectocarpus Genome whichevolutionarybiologyisexpectedtoassisthumankindinthe
and the Independent Evolution of Multicellularity in Brown futurearealsodiscussed.
Algae”;and“EvolutionofSelf-CompatibilityinArabidopsisbya
MutationintheMaleSpecificityGene.”Asimplesearchofany Topic overviewsFirst, however, it is critical to define the
recentscientificjournalwilllikelyrevealequallydensetitles. term evolution. Evolution simply refers toapatternofchange
in the phenotype of members of a population (e.g., any
These topics are certainly interesting to scientists who characteristic of its behavior or anatomy) through time.
work on similar questions, and they may even be inherently Evolution refers only to changes in traits that are inherited
interesting to people with a desire to understand the way the fromonegenerationtothenext,usuallybecausesuchchanges
world works, but it can be hard to see how these discoveries are the result of genetic differences, though cultures can also
will have a material impact on people’s day-to-day lives. If evolvebyprocessesanalogoustobiologicalevolution(seethe
there is no impact—if the study of evolution is merely entry “Evolution oflanguage”).Several processes canlead to
interesting but not important—then why is the topic subject evolution. Genetic drift, the change in the genetic composi-
tosuchimpassioneddebate?Whydomanyindividualsdevote tion of a population due to random variation in birth and
theirentirecareerstostudyingtheevolutionaryprocess?Why death, is one such process. Mutation, genetic changes due to
do taxpayers invest hundreds of thousands of dollars funding errors during DNA replication, is another. However, the
evolutionary research? process that most evolutionary biologists credit with produc-
ing broad evolutionary patterns is natural selection. First
The answer is that the study of evolution has important
elucidated by Charles Darwin in 1859 in his book On the
implicationsforhumanwelfare.Thehumanspecies(andevery
Origin of Species, natural selection is the process by which
otherspeciesontheplanet)istheoutcomeofmillionsofyearsof
individualswithadvantageousvariations(forexample,animals
evolution;everyaspectofourbodiesandmindshasbeenhoned
that are faster or stronger) succeed in producing more
bynaturalselection.Understandinghumanevolutionarybiolo-
offspring, while individuals with harmful variations fail to
gyallowsustocareforourbodiesandmindsmoreeffectively.
survive or reproduce. For natural selection to lead to
Learningabouttheevolutionaryprocesshasalsoenabledusto
evolution, individual variation must be passed from parent
harnessthisforceforourownends.Evolutionaryprincipleshave
to offspring; in other words, it must be heritable. Over time,
unwittinglybeeninuseforthousandsofyearsinthepracticeof
many small advantageous variations can accumulate, leading
agriculture; more recently, an explicit understanding of evolu-
tolargerpatternsofchangewithinthepopulation.Evolution-
tion has allowed us to advance agricultural science. Pathogens ary biologists propose that these processes acting together—
areconstantlyevolvingtothwartoureffortstocontrolthem,but naturalselection,geneticdrift,andmutation—canexplainthe
evolutionary studies of pathogens have enabled scientists to
diversity and complexity of all life on Earth.
identifynewemergingdiseases,todeterminehowdiseasesare
Grzimek’sAnimalLifeEncyclopedia xi
(c) 2011 Cengage Learning. All Rights Reserved.
Therelevanceofevolutiontocontemporarysociety Evolution
A variety of methods are used to study the evolutionary increaseinthesizeandnumberofkernelsperear,adecrease
process. Paleontologists study evolution using fossils to in the height and number of branches of the plant, and a
observe some of the different life-forms that have existed reduction in the strength of the protective coat surrounding
throughhistory,includingtheancestorsofplantsandanimals each kernel.
alive today (see the entries “The fossil record: A window to
the past” and “What does the fossil record tell us about Research conducted since the early twentieth century has
evolution?). Phylogeneticists use similarities and differences helped to identify the genes that are responsible for these
between organisms toarrangespeciesinto phylogenetic trees changes. For example, it appears that a single amino acid
that reflect the historical relationships among groups. change in the gene teosinte glume architechture1 (tga1) is
Evolutionary ecologists and experimental evolutionary biolo- responsibleforthereductionofthethicknessoftheseedcoat
gists measure selection within or among natural or experi- in corn. Changes in the gene Teosinte branched1 are responsi-
mental populations in real time (see the entry ”Evolutionary ble for a reduction in the height and number of branches in
ecology“).Evolutionary geneticistsusemolecularmethods to domestic corn versus wild corn. Genes have also been
tracethegeneticchangesthatallowanorganismtotakeona identified that control kernel color, starch content, inflores-
new appearance (see the entry ”Genetics: The blueprint of cence structure, and the circadian rhythms of domestic corn.
life“). Finally, some evolutionary biologists use computer
simulationstopredictwhattheoutcomeofselectionmightbe These studies have increased our understanding of how
under given conditions. Together, these methods have been domestic corn differs from teosinte; they may also help
usedtoadvanceourunderstandingoftheprocessofevolution. scientists accelerate the development of new varieties. For
This body of knowledge has been put to use for human example,plantbreedingcompaniesareworkingtoidentifythe
benefit in a variety of ways. genetic changes that are responsible for their best varieties—
several of these changes could then be combined in a single
variety. They are also screening wild populations of teosinte
(and other wild progenitors of domestic crops) for new gene
Evolutionary agriculture
variants that may prove useful. In some cases evolutionary
ThousandsofyearsbeforeDarwinarticulatedthetheoryof geneticistshavebeenabletoincreasethenutritionalvalueofa
evolution by natural selection, humans were intuitively using cropbymanipulatinggeneexpression.Forexample,scientists
evolutionary principles to shape domestic plants and animals. havebeenabletoincreasetheprovitaminAcontentofseveral
In fact, Darwin’s knowledge of animal husbandry (including varietiesofriceusingavariantknownfromwildpopulations.
his own extensive work breeding pigeons) was integral to the
development of his theory. Breeding experiments at home Thetransformationsthathumanshavewroughtinanimals
helpedhimtoappreciatethevariationamongindividualsofa areinsomecasesfarmoredramaticthanthechangesobserved
speciesandthepotentialtoharnessthatvariationforavariety in plants. For example, all breeds of the domestic dog are
ofpurposes.Bykeepingandbreedinglivestockwithespecially believedtobetheresultofasingledomesticationofthegray
usefulcharacteristics,earlyhumans(andDarwin)intentionally wolf (see the entry “Canid evolution: From wolf to dog”).
or unintentionally preserved desirable variations and elimi-
nated undesirable ones. Likewise, by saving only the largest In some cases, the domestication of wild animals has
and most flavorful seeds to plant in the coming year, early resulted in genetic changes to both animals and humans. For
humans were sure to gradually improve the quality of their example, very few human cultures have begun using dairy
crops. The steady accumulation of variation in a particular products from cattle to supplement their nutritional needs.
direction ultimately produced plants and animals so different Dairy products such as milk and cheese contain a unique
from their wild counterparts that they could scarcely be sugar, lactose, than can be digested only with the enzyme
recognizedasrelatives.Differentvarietiesofthesamespecies lactase. All people have the gene for lactase and produce
became as different as distinct species, or even genera. copious quantities of the enzyme as infants when milk
comprises the majority of their diet. The adult human diet,
Although agriculture is no longer the primary occupation however,hasnothistoricallycontainedmilkormilkproducts,
ofcitizensinmanydevelopedcountries,agriculturalproducts andinmostpeoplethelactasegeneisshutofflaterinlife.For
are still one of the foundations on which civilization is built. adults who are not producing lactase (who are lactose
Consider corn, one of the most successful and widespread intolerant), the consumption of dairy products can result in
agricultural products in history. About 800 million tons of severe gastrointestinal distress.
corn are produced annually; corn is consumed directly,
modified to make a variety of food additives, used as feed Intheculturesinwhichdairycattleareused,adultswould
fordomesticanimals,andfermentedtomakeethanolforfuel. clearlybenefitfrombeingabletoconsumelactose-containing
Yet the wild progenitor of corn, teosinte grass, is a humble milkasadults.Thisability,alsocalledlactasepersistence,has
plant with little nutritional or economic value. The transfor- infactevolvedindependentlyinseveralregionsinwhichdairy
mation of corn began thousands of years ago when Central products are a part of the diet. Genetic mutations in the
American people began to harvest and plant the seeds of regulatory region of the gene have resulted in lactase
teosinte. In its wild form, teosinte has only five to ten small expression in adulthood, allowing comfortable digestion of
kernels,andeachkernelisencasedinahardshellthatmakesit lactose.Thegeneticchangesthatcauselactasepersistenceare
difficult and time consuming to extract. A relatively small currentlyunderpositiveselection,astheconsumptionofdairy
number of changes resulted in modern corn, including an has spread worldwide. Consequently, these genetic changes
xii Grzimek’sAnimalLifeEncyclopedia
(c) 2011 Cengage Learning. All Rights Reserved.
