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8 Evolution and Diversity Evolution and diversity result from the interactions between organisms and their environments and the consequences of these interactions over long periods of time. Organisms continually adapt to their environments, and the diversity of environments that exists promotes a diversity of organisms adapted to them. In recent years, new techniques and approaches have opened exciting new avenues of investigation of the processes that generate evolution and diversity. As a result, greater opportunities exist now for advancing knowledge than during any period since the 1930s and 1940s, when evolutionary biology and genetics became united in what came to be called the modern synthesis of evolutionary biology. The Processes and Results of Evolution Are Exen~lif ed in the Evolution of Insecticide Resistance in Insects and Antibiotic Resistance in Bacteria The first synthetic organic insecticide to be adopted for practical use was DDT, which was introduced in 1941. DDT appeared to have many advantages because, in proper dose, it was toxic to insects but not to humans. As a conse- quence, DDT was quickly employed worldwide to control houseflies, mosqui- toes, and a variety of other insect pests. After the initial success of DDT, many other exotic chemical compounds were introduced as insecticides. The introduc- tion and widespread use of each of these was quickly followed by the evolution of resistance in large numbers of insect species. In fact, more than 200 species of insects had become resistant to DDT by 1976; some species have evolved mul- tiple resistance to four or more groups of chemical insecticides. In many cases, the insecticide resistance results from the action of a single gene, although multiple other genetic changes that can modify the response to insecticides also occur. In the common housefly, resistance results from the 260
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EVOLUTION AND DIVERSITY 261 presence of an enzyme called DDTase, the natural function of which is unknown. Mutant forms of the enzyme convert DDT into the relatively harmless compound DDE. Resistance in the mosquito Aedes aegypti is also associated with a DDTase enzyme, but not the one found in the housefly. The evolution of resistance to insecticides is so common because the insect populations often contain rare mutant variants that are already resistant. Exposure to the insecticide gives an advantage to these mutants, and over several genera- tions, they gradually increase in frequency at the expense of the normal Apes until very few of the normal sensitive types remain. A remarkable principle in population genetics states that insecticide resis- tance can be expected to evolve in approximately 5 to 50 pest generations, irrespective of the insect species, geographical region, nature of the pesticide, frequency and method of application, and other seemingly important variables. The phenomenon occurs because the time required to evolve significant resis- tance depends on the logarithm of the total increase in frequency of the resistance gene as a result of the pesticide application, which over a wide range of realistic values is effectively limited to 5 to 50 generations. The rapid, repeated evolution of insecticide resistance in many parts of the world reflects the operation of this simple mathematical principle. A similar situation accounts for the repeated evolution of antibiotic resistance in bacteria: Rare bacterial types containing genes for resistance are favored in the presence of the antibiotic and eventually displace the normal sensitive types. In this case, the overuse of inexpensive antibiotics, not only in medicine but in animal feed, fish culture, and agriculture, has promoted the evolution of antibiotic resistance in a wide spectrum of microorganisms. In many cases, the resistance genes are contained in mobile genetic elements that can be transmitted from one organism to the next, and their spread has resulted in the wide dissemination of the resistance genes among pathogenic and nonpathogenic forms. The molecular evolution of antibiotic resistance is similar to the process that bacteria have used for millenia to evolve resistance to naturally occurring antibi- otics and to soil contaminated with lethal concentrations of heavy metals. A resistance gene that evolves in one bacterial species can potentially be dissemi- nated to many others by means of molecules known as plasmids, which are transmitted among suitable hosts by cell contact. These plasmids occasionally pick up transposable DNA sequences that contain genes resistant to antibiotics, and they confer resistance upon host cells. When the antibiotics are widely used and present in the environment, cells containing the resistance plasmids are favored, and the plasmid spreads. In many cases resistance plasmids have acquired genes for simultaneous resistance to five or more chemically unrelated antibiotics. For some pathogenic bacteria, such as gonorrhea, antibiotic resistance has become so widespread that clinical treatment is severely compromised. The evolution of insecticide resistance in insect populations, antibiotic resis- tance in microbial populations, herbicide resistance in plant populations, and
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262 OPPORTUNITIES IN BIOLOGY ~ ~EW:)~LUTIO~N~:OF:~IN:S:E~ICIDE~:~RESIS~NCE :: ~ : I: : ::: . , ~ ~ ~ ~ ~ , , ::~::~ ~ ~Some~:of~th~e~most: ~!amat~exa~mp Test ot~evo utionin~act~ionresu tram ~::the~n~u~ral sel - ion~fo~r~chemical~peiti~£id~e~:resistance~in :natu:ral~populutions ~ : ::: of insets stand other~pgricultural pests. ~ lint The ~1 940s,::~when Chemical Ski- a: ~C:~QS~ we: f~i~ ~ - plied on a Lade S=~IQ ah: An e~i~mmed~:::~7 percent: the ::: ~ :~ ~ ~ : ~ ~ ~ ~ ? ~ ~ :: ~ :~ag~ricultural~:cropp in the lU~nited~States~were~ bst~to~:in~s~ects.: ::lnitiai:su~esses : ~ thin ~chem~al~ peat ~managemant~ ~were~llow~d by ~a~gr~dual loss of Effective : ~ ~ nests::: Mama more than: 400~ ~pest::~:s~p~les ~ ~ave::~:evo iv"~;:signdicant~ :resis :~ ~ ~ · ~ ~ ~ ~ ~ ~ ~ i: ~:~:tan~ce~to onset: or::~more:~pesticides :~ and~:13~, - Rents of ~the:~crop:: of ~::yiel:ds:: in theft: :~: ~ : :: :: :: :~ :::: Ad: ::~: ::~: ~ :~: ::: ~ :~ :::: :~ : :: I: :: :: :: :: :: ::: : : ::: ~:~ :~ ~ An: At::: ::: ~ : ~ : : :: ::: ~:~ :: ::~u~ndec : ~tates~'OS:t ~ O~::~OS~C IS.: ~ : ~ ~ : of:::: ::::: :~ :~ ~:~ : :: ~ amp: ::::: :: ~ : : :~ :: i: ~ I:: a: ~ ~ :: i:: : ~ ~ ~ I:: ~ : :~ :::: ~ ~ ~ ~ :: ~ ~ : : ~ ~ ~ ~ ~ :~: ~ ~ in Many cases, significant pesticide resistance has en 5 To ~50 ~ ~ : ige:n~e~rations~'n~spits of great:~::variation~;~n~:~the~i:nse~ species, :the~:insect:icide, ~ ~: l and tithed methods ~ application. ~meoret~l population genetics ~ helps furs ~ ~ ~ ~ ~ ~: ~ ~ ~ ~ . ~ ~: ~ ~ ~ ~ ~ ~ At: ~ ~ . - ~ ; ~ , ~ ~ . . . . ~ ~ I. :~ ~ ~ ~ ~ :u~n~de~rstand ~ these apparent paradox.: : Many ~::oT: tog :~InsectIciae r~s~stances :: ~ ~ ~ ~ ~ ~ ~ ~ ~ :: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ :~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~re~s~u~tt::~from::~si~ngle~ m~:utant~ :9~en~es~ :~:: Th~Q:~ resistance genes am often~part~'al~y~ :: ~ Dominant, ~so~:~the~change ~in:~th~e ~frequen~of the ~resistance~:g~ne~is~go~rern~ed: ~ ::; : : ~: ~ : LO : V_ :- I~ ~ ~ . ~ u ~ ~ ~ ~ ~ new- =: Bent _ + ~ ~ i: ~_ . ir\~w:h~ich~p::and~q~are,~re~spe~ive~ly~::~thie::~gen~e~frequQ~nciesof:~th~e:resista~nt:~:and~: :~:~sensitive genes, i~n~i~a~ily Limed and at timed generations After Insecticide ~ ~:~ applications, aft: And ~ ~ ~s measures ~ it he ~ degree to wh ich~;~ ~ resistant insects ~ caret : ~ ~favo~md~over:sensiti:~ o:n~a~s. ~ ~::~::~ ~ :: ~ i:: ~ ~:~ ~:~ ~ ~:~ :: i: i: ~ ~ :: ~Prio:r~:~to: appi~icatio~n ~:~ ~th~e~pesticid~e$~i~:t~he~ ~gen~:~f:requency~p :~:::~6f;:the~ ~:~ : : : : : ~ : ~ 0 : : ~ ~ ~resist~ant~mut~n~is:::generally~:~close~to: 0.:~ ~:::~Appli~io~n:~:of~ th~e~pe:sticide~ i: :: i: incre~ases~the: gene :freq~uen~ey, so~m~t:imes :by many :ord~ers~of~ mpg Etude:, ~b:ut: :: i:~: significant~res~istance~ ~is::~noticed in~th~e:~:ppst~:popul~ation even boor the gene: : i:: frequency up ~ increases above :a~ few percent. :~Thu~s :::as rough:~approx'~ma-:: i: ~ ~ t ~ , Tons: We may assume: that :4~:::~ano 9 :~are ~:=t ~ close::::~:nough :to :~1::~:~:that~ Ink t :~::~:: ~ ~ v ~ . ~ ~ ~ v a)::= In(pO): and :In(p ~/q ):=: Into J.: Using these ::approxim~i:ons: ~the::~equation ~ : a ~ t t t :~ impiles~thatt: to (=ln:(p~::: Ad). I:n many cases,~the:~::ratio p ~'qDo:~:may range from:: ~ ~ ~ ~ t ~ ~ 1 x 1 of to~perhaps 1; x 107,: and s~m:ay typically be 0.5: or greater. deer this::: i:: ~wide: range parameter values, time this ~:effective:ly~: limited~to~5~ to ~50 ~::~: i: generations for t lie appearance TOTS ~a~s~g~n~'cant~pgree 0 ~ :post~c~c We resins- ; : ~ ::tance.~ Details :in:~act~ual: cases well:: depend on: such~f~adtors:as:t~he~ size :of~the~: ~ insect::~popula I: io n :an Dot: t h:e exten t :~:~:9 ene t iC isol~;o no :b~ee:n local Hula-: : tions,:~ and: the ~volut:io:n Of :polyg~en~ic resistance::: may bd:::::::expected:~:to stake: ::: somewhat Longer than :singlegene resistance. ~ ~ ~Nevertheless, the example ~ :: :demonstrates the pr~ed~ictive:~power~ of mathem~atical:~approaches: in :ewlu-: ::
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EVOLUTION AND DIVERSITY 263 heavy-metal tolerance in plant and bacterial populations has been demonstrated repeatedly. In every case, genetic variation and natural selection provide an amazingly effective process for promoting the adaptation of organisms to their environments. The study of evolution and diversity of life on earth is concerned with the tempo, mode, and patterns of such adaptations. Some of the Most Exciting Current Research Opportunities in Evolution and Diversity Result from Technical Innovations in Molecular Biology The techniques of molecular biology have revealed a rich level of detail in studies of DNA variation and its analysis. They have uncovered an unexpected avenue of genomic evolution through the activities of transposable elements. They have opened up the transfer of individual genes between species as a major new tool for the study of the mechanisms and subsequent events in speciation. And they have made possible an integration of the techniques of molecular biology with questions of field natural history, such as in the use of mitochondrial DNA polymorphisms to study population structure and migration in fish and other organisms. Application of the techniques of molecular biology has made possible, for the first time, the beginnings of a synthesis of microbiology and evolutionary biol- ogy. These two fields have developed in almost complete isolation from each other. Microbiology is among the least evolutionarily oriented of biological disciplines, and evolutionary biology is the evolutionary biology of metazoans. Studies at the interface of these disciplines will result in the definition of new evolutionary principles and a deeper understanding of principles already estab- lished. Perhaps the most surprising initial result of studies in microbial evolution has been the discovery of what some scientists regard as a distinct kingdom of organisms, the archaebacteria, which combine some features of more familiar kinds of bacteria (eubacteria) with others characteristic of eukaryotic organisms. Indeed, paleobiologists now believe that the earth's biota was composed solely of bacteria for at least two-thirds of its total history. Many evolutionary innovations have been powered by changes in intracellular biochemistry rather than by changes in the shape, size, or physical organization of organisms. More- over, the global biota, especially bacteria, with their diverse physiological capa- bilities, have interacted with and changed the global environment in numerous crucial ways, such as in the creation of our oxygen-rich atmosphere. The application of molecular techniques has also contributed to the current revolution in systematics. Molecular studies of DNA and proteins are now used routinely to distinguish species and to estimate phylogenetic relations among closely related species. Direct DNA sequencing is providing phylogenetically useful data almost faster than they can be analyzed. The inferred genealogical relations based on macromolecules are usually consistent with those based on
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264 OPPORTUNITIES IN BIOLOGY morphology, but molecular studies often help to resolve relations that are mor- phologically ambiguous. Overall similarity in macromolecules provides a reliable measure of evolu- tionary time only when the molecules being studied evolve at much the same rate in different lineages and at different times. Whether DNA sequences actually evolve with regular rates like molecular clocks is still much debated, but the data so far suggest at least moderate regularity. The concept of the molecular clock has provided a unique and powerful time dimension in evolutionary studies and has augmented as well as complemented inferences from the fossil record. How- ever, not all of the evidence is consistent with the hypothesis that molecular evolution occurs at a nearly constant rate, and further evidence is needed to establish the validity of the hypothesis and to determine its range of application. The explosive increase in knowledge of DNA sequences has created an acute need for new kinds of computational technologies and algorithms, as well as new statistical approaches, so that the data can be interpreted to maximum advantage. Appropriately analyzed, the new kinds of data will reveal, with a level of detail never before possible, patterns in the history of evolution; the new data will thus shed light not only on the evolution of macromolecules, but also on the processes of evolution of morphology, life histories, and physiology. Regrettably, the analysis of sequence data, which must bring together experts in statistics, com- puter science, mathematics, molecular biology, population genetics, developmen- tal biology, and systematics, has lagged behind as ever more data have accumu- lated. At the same time, more extensive data on the extent of DNA sequence variation within species are badly needed. Technical Innovations That Have Transformed Studies of Evolution and Diversity Are Not Limited to Molecular Biology Studies in biomechanics, ecology, and behavior have profited tremendously from improved techniques of photography and telemetry, and in almost every area of study, modem computers facilitate sophisticated simulation modeling and data analysis. Paleobiology has benefited from methods of organic geochemistry that enable the determination of the nature and isotopic characteristics of biologically derived organic compounds preserved in ancient sediments and also from new techniques of radiometric age determination and new data regarding the geologic and plate-tectonic history of the earth. These approaches, when combined with information derived from molecular biology, promise to promote new knowledge about such fundamental evolutionary events as the origin of invertebrates, verte- brates, plants, and human beings. Even earlier PI nbrian events, such as the advent of photosynthesis, oxygen-dependent respiration, nucleated cells, eukar- yotic sexual reproduction, and the modern type of anaerobic-aerobic global envi- ronment, may be within the reach of the new approaches.
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EVOLUTION AND DIVERSITY 265 Progress in the study of evolution and diversity does not require technical innovations, although it frequently benefits from them. Advances also come from the synthesis of previously disconnected areas, from new ways of looking at problems, or from new concepts. Therefore, in evolution and diversity, too much stress on technical virtuosity and trendiness runs the risk of promoting a kind of brush-fire pattern of scientific advance, with great activity and excitement near the front but little behind in the area where the practical applications of basic discoveries are often developed. Although many exciting directions in evolution and diversity have been opened by advances in molecular biology, numerous fundamental problems occur at levels of biological organization above that of molecules. The evolution of populations of organisms is affected by the interactions with the environment of physiology, development, and behavior at levels that are not amenable to molecu- lar analysis. Molecular biology is an aid but not a panacea in the discovery and classification of organisms. And the processes of speciation and extinction, while fundamental to evolution and diversity, are population, not molecular, processes. THE EVOLUTIONARY PROCESS Population Genetics Continues to Emphasize Genetic Variation Its Nature, Causes, and Maintenance in Populations Studies of population genetics or genetic variation have become significantly more sophisticated with the use of molecular techniques and new types of mate- rial, including microbial organisms and chloroplast and mitochondrial DNA. Progress in molecular biology has been especially helpful for population genetics and promises to aid in the resolution of several outstanding problems in the field. Genetic variation can be resolved at the ultimate level of the DNA sequence. With this level of resolution, it becomes possible to determine whether genes that are highly variable within populations also evolve rapidly. The distribution of DNA polymorphisms within and among species results from the operation of evolutionary forces that are in many cases too weak or too difficult to measure in the laboratory or field; it may be possible to infer their magnitude from analysis of the sequences themselves. The rich possibilities of inferences that may be made from DNA sequence data warrant significant efforts in this direction. The Technique of Site-Directed Mutagenesis Also Opens New Possibilitiesfor Population Genetics Traditionally, inferences about evolutionary constraints on molecular struc- ture have been gathered from comparisons of homologous molecules among species. With site-directed mutagenesis, the inferences can be tested directly by
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: ::: : :: :~ :~ 266 OPPORTUNITIES IN BIOLOGY ~ ~ GINA :SEQILIENCE:S~ :I:N :E~LlI I:IONA~ STUDIES ~ :~ ~ ~ ~m~parisons~of DANA find protein sequences have revolutionized the :reconstruction:~:of~: the :evolut~io:nary: relations among :organisms because::the ~ ~ , . ~ sequences: th~e~m~selvesl~contain~ :~i~orm~at~lon a bout t heir ~ancedt:ra ~ History: t hat: I: :~ can be Extracted by appropr'3to ~stat~st~lea met Ox s. ~ quay y~pow~ertu~: ~ In err:::: offices can ~ - a~rawn~:~trom Comparisons o sequences among ~ln it,'< us s ~: ~ ~wh;hin~ species.: ~:Th~i~s~is :~ - bible ~becaus~e:the:~sequences :~also:~contain~ inform: : i: ~md:tion~-ut:the~ev~o~lutIona~rees:~:th~bt::~mo:ld~ed~:~them,::~wh~ich can:~bb:~stud-~: ::~ ~ :~ ::~:td~m~a~ inferences~a~ut~th~e~ magnitude~:~of natural selection ~th~e~:l~impor-~ i:: :~ Stances Loft random :~p~rocesses~ ~ the role: of recom~bi~nat~on ~ and:~:solo:n.::: . . ~ . i: ~ ~ ~ ~ ~ Iwo:~stuc l~e:s~of ~ ~ A~s~uences~am~ong natural::lm atQS~Of Byte 1Q ~ Ja~enum :~ ~ ma ~ ~ ~wnc :e~rsco~re ~ t ,~ Over of ~ t ,~:~ ~ ma ecu are ~approac~ A. : one ~::~ ::::~:~:st:udv:~focu:sed~::o~n~: ~:3~NA~:~sea:~u~Q:nces:~:in~t:he::~:asne.: ~wh~h~:~:code~s: :for:::~th:e::::: : ~:~ :~n~rn~e 6-phosp~h~og~lu£on~dto d~ehydroge~nas~.:; The purpose~was: to :~est:in~ate: . ~ l :~ Ha freon: of bused: amino acid: polymorphisms :th~ad War: selectively I neutral. ~ levels has been a central issue: In population genet'¢s~tor more than a I :: d~e~d~e:~but it: has Edified ~resolOtion~:~b~ause:: most statistical tQStS of:: ob-: I ~ :: ~ ~1 : i: sewed gene::::frequ~e~ncies~::and:~:~:mos t labbrdtory~exper:iments :lack ~su~ff~ient :~:::po~r to :detect~:selection:~co~f:icie:nts of~:t~he: relevant :magn~itude~. ~ ~AIt~hbug:h: :~:most random :amino:acid substitutions might be:~expected~to ~ harmful kinky::: ::~a :small proportion of harmful mutations~ever become: established ~as~pply ~:morphi:sms In Natal: ,mpu~l~at~ns.~ A ~:sig~nifica~nt proportion~of alleles that :~bemme ~Iy~morphic~ might therefore be expected to :be s~elebti~vely:~or nearly::: :: :~: :~:Th~e idea~:~behin:d~ the: study of: DNA sequencers is:~:t:h:at~:nucleotide poly-: morphisms bt~:~si~lent~sitas, which do not change amino acid sequences, can ~DQ :useo ashy internal standards for comparison wan amino acid ~:lymor- i: ~ ~p~h:ism~s in :the same gene. When 768:: nucleot:ides in the gndge:ne:in~::seven~ :: ~ ~ i: strains:of E: co§~:were: compared, J 2 am~ino~:~acid: polyn~o::rphisn,:s and 78 silent:::: : : : . . . ~ . ~ . . .. , _ . ~ . . . . . . :: polymarpn~sms ;:were: found. ~ AIDING 1 ~ Amino ac~a:: ~Iymorpn~sms: ~ur:reo In singleton: oo~nfi~g~u~dt~ions Meaning theta six strains shared a common~a~mino :::: Waco at~t~he~:sit~e ~and~on~l~i one strain was :~ d~ifferent3,~where~as~ only about haH~of : : : :~ th~e~:si~:le~nt:~:polymorphi~s:ms exhibited this:oonfiguration.~::~::Bas~ed on::th:is~:d~iffer-::::: :~: eTnce~::~:alone,:~o~ne::can::conclude:th~e~::no~more~t:h~an:six~:~6f::~the~::am:ino~:acid:: substitutions: oou:ld by: selective:l~' neutral. Alternativelv:. if all amino acid polymorphisms are assumed to be mildly harmful the amount of selection ~ . ~ ~ ~ n;ecessa~ to account for the preponderance of singleton configurations is: ::~ ~ only dbo:ut ::1 .6~x:~0-7:,~:an: amount of selection: much :too:smali~:~ d:etett~:~except::~; :: Dy means or uNA:seguence comparisons, : : The second: study concern the occurrence of genetic recombination ~ . . . _ .. _ . . . . . . . . among natural ~isolates~of: E: cab. Evidence for recombination was found in a Revlon o' 1~.~1 nucleotloe Fairs around the Know Gunnel wn~cn coaes tar aIkaline~ phos:phdtase, ~ inky 10 ~Rat~ural isolates. ~ The regional oontained~ ~87~ i: polymorphic nucleot~ae: sites Ot WnlCn 4Z were snared oy mo or more
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EVOLUTION AND DIVERSITY 267 strains. ~ Comparisons of the shar~pplymorphisms gave clear Evident forty ~ i Up:. large y conal, clonal rear u n Is neve heoss:consisen w h r mbi- nation involving short strelchos:of DNA because most' genes are shill trans- | mitted~uniparentally. ~ This is yet another example Reflow a ONA sequence can contain information amp its h '§to ry that cannot easily ted inferred from [ direr experiments. : deliberately altering parts of the molecule of interest, reintroducing the gene for the altered molecule into living organisms, and studying the effects of the changes. Such experiments reveal not only which changes affect the molecule, but also the magnitude of these effects. For the first time, population geneticists are able to study a collection of mutant molecules that are well characterized at the molecular level. The process of mutation, which until recently seemed to result from an essentially simple process of nucleotide substitution or rearrangement, is now appreciated to include mechanisms for creating evolutionary novelty through the movement and other activities of transposable elements. Indeed, virtually all proteins may have been created in evolution by the rearrangement of exon units, which code for smaller structural domains able to fold autonomously and carry out elementary functions such as ligand binding. If true, this would mean that the evolution of new functions cannot be likened to the proverbial monkey pecking away at a typewriter in hope of creating something meaningful; the analogy should rather be to a monkey that can shuffle complete words and entire sentences and paragraphs. Recombination, traditionally viewed as important from the standpoint of creating genetic variation through new combinations of genes, has taken on a new dimension in population genetics because of its conservative role in maintaining similarity between members of multigene families. However, little is known about the rate of gene conversion in multigene families or about the role of intragenic recombination in creating new genetic variation. The Study of Natural Selection Remains One of the Principal Preoccupations of Evolutionary Biologists The understanding of the mechanisms of selection in natural populations is still inadequate. At the molecular level, it is necessary to understand how changes
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268 OPPORTUNITIES IN BIOLOGY in protein molecules affect fitness and to critically evaluate the contribution of selectively neutral mutations to molecular evolution. These problems are ideal for the application of site-directed mutagenesis in experimental organisms such as bacteria, yeast, and Drosophila. At the phenotypic level, it is necessary to understand how genes affecting quantitative characters respond to natural selec- tion. This is an area in which substantial advances in the theory have been made recently and in which further progress can be expected. Analysis of multifactorial traits is essential to understanding the genetic basis and inheritance of many genetically complex disease traits in humans, including the most common birth defects and adult disorders. It is also important in evolution and diversity in interpreting the evolution of such multifactorial traits as morphology. Significant methodological problems in natural selection include difficulties in measuring reproductive components, including fertility selection and sexual asymmetry in selection, nuclear-cytoplasmic gene interactions in fitness, and the elaboration of statistical models and experimental designs to estimate fitness components when there is inbreeding (as in some plants). Studies of selection in natural habitats are often hampered by lack of a rigorous, quantitative approach to studying the environment and its variation. Evaluation of the role of population structure in evolution is also marred by important unresolved problems, such as the need to improve methods of estimat- ing migration rate, to define the role of interactions between genotypes in selec- tion, and to evaluate the significance of selection among demes (a local popula- tion of closely related organisms) is the genetic divergence and transformation of populations. Genetic differentiation results in variation among populations, and methods for the statistical analysis of such spatial patterning are now being developed. Progress Is Being Made in Our Understanding of Speciation and the Evolution and Maintenance of Diversity Organismal diversity is a direct and inevitable outcome of speciation, the process whereby a single species evolves into two or more distinct ones. The conditions required to initiate, promote, and complete the speciation process are still poorly understood and hence controversial. To resolve this problem, a major effort has been made in recent years to examine the biological and genetic attributes of closely related taxa actively undergoing various degrees of differen- tiation. Three approaches are taken in these investigations: field, experimental, and theoretical. Significant advances have come from the analysis of the genetic variation and structure in natural populations. Many of these studies are of insects. For example the Hawaiian Drosophila, which have proliferated rapidly on the emerg- ing islands of the archipelago, serve as an outstanding model system for examin- ing the relations among geographic isolation, population size, sexual selection,
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EVOLUTION AND DIVERSITY 269 and genetic divergence. The fact that the islands can be accurately dated in geologic time provides a unique opportunity to ascertain how the species have evolved. Founder events followed by repeated population expansions and con- tractions accompanied by strong sexual selection appear to have promoted the rapid divergence of isolated populations of these flies. The causes of speciation are different in Rhagoletis, a group of economically important flies whose larvae infest the fruits of a wide range of plants. Within the past 150 years, species of these flies have formed genetically distinct host races on introduced plants, in the absence of any geographic barriers to gene flow. These races appear to be in the early stages of speciation. Detailed behavioral, ecologi- cal, biochemical, and molecular research has revealed that because mating in these flies occurs on the host fruit, genes that govern host choice directly affect mate choice. Another approach to the study of speciation in natural populations focuses on the genetic and biological outcome of hybridization in zones of overlap either between previously geographically isolated, but closely related, populations that have reestablished contact, or in zones of transition across a sharp ecological boundary between populations adapting to different habitats. These investiga- tions are being carried out on a wide range of animals and plants. The objective of such studies is to establish whether different mate recognition systems and repro- ductive isolation can evolve in zones of contact as a result of a selective process called reinforcement or develop as a by-product of genomic divergence in isola- tion. The increased genetic resolution recently provided by molecular techniques is contributing significantly to our understanding of how hybridization affects the speciation process. A third approach to the study of speciation involves direct laboratory selec- tion experiments. Such experiments suggest that considerable progress toward speciation can occur rapidly, even in the face of considerable gene flow. This experimental approach offers promise for testing some hypotheses of speciation mechanisms now being generated from studies on natural populations. In recent years, theoretical population genetic models, using analytical and computer stimulation approaches, have been developed in an attempt to under- stand under what conditions species evolve in nature. These models have become increasingly more sophisticated and biologically meaningful and have yielded insights into the speciation process as well as models for exploring, in nature or in the laboratory, the conditions under which speciation can occur. The study of speciation, one of the most important fields of research in evolutionary biology, has a direct bearing on our understanding of the origin of organismal diversity in the past, the present, and the future. It has left the descriptive, comparative phase that predominated in the past for a more empirical approach to the study of speciation mechanisms. Sufficient evidence has come from recent studies to indicate that we are on the threshold of resolving some of the most intractable problems concerning modes of speciation. The increasing
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270 OPPORTUNITIES IN BIOLOGY interest in microbial evolution also encourages a new analysis of the species concept and species formation in prokaryotes. The Study of Evolution of the Organization and Composition of the Genome Is Still in Its Infancy Even though we know that the overall genetic organization of the chromo- some in certain groups of bacteria is strongly conserved, the reasons are obscure. Similarly, in eukaryotes, no principles are known that govern conservation or changes in chromosome structure or organization. Genomic evolution also in- cludes unknown contributions from various localized and dispersed highly repeti- tive DNA families and numerous types of transposable elements with different characteristics and evolutionary implications. In a wider sense, genomic evolu- tion also includes that of viral genomes and the interactions with the host genomes. Recently it has become clear that certain plant genomes undergo a novel and potentially major mechanism of evolution in response to environmental stress. For example, plants under stress manifest marked phenotypic changes that are associated with heritable changes in copy number of several multigene families including ribosomal DNA sequences. New methods of manipulating and cloning large DNA molecules will be critical to the study of evolution at the level of the chromosome. Although ambitious, the synthesis of disciplines that characterize modem evolutionary biology should be extended to embrace areas such as developmental biology, neurobiology, and behavior. Little is known about possible developmen- tal sequences available to organisms with particular genotypes, or about new kinds of developmental pathways that are accessible by mutation from genotypes already existing. In addition, virtually nothing is known about the genetic deter- mination of complex animal behaviors and the manner in which these behaviors feed back on the evolution of molecular and morphological traits. THE RESULT OF EVOLUTION The Study of Adaptation Is Still a Pervasive Theme in Biology The most dramatic result of the evolutionary process is seen in the adapta- iions of organisms alive today. One of Darwin's chief accomplishments in The Origin of Species was to show that the exquisite adaptations of organisms that "so justly excite our admiration" could be explained by the purely mechanistic pro- cess of natural selection. Important research opportunities in studies of adaptation derive from both technical and conceptual innovations during the past several decades. Some of the technical advances have been mentioned. As an example of conceptual inno- vation, it is now generally agreed that traits do not typically evolve for the good of the group or species as a whole, but for the direct or indirect advantages they
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276 OPPORTUNITIES IN BIOLOGY Finally, it has Gently become clear that the behavioral and morphologically ~ ~ ~ . ~ ~ . ~ , ~ ~ ~ ,,, ~ ~ .~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: ~ . _~ ~:~ tic ~Iversity~ oat ~ t ,~ Scan: cichlids~is~`tro:n~gly iriflu~enc~; ruby environmental :~ All: : ~ I: ~ ::: ~ ~ ~ T: ~ ~ : ~ : : ~ : ~ ~ ~ I: ~ ~ ~ ~ ~ F: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ : ~ ~ factors. ~ :This Means that differences observed fin nature ~ might Not ~ · ~ ·E ~ ~ ~ ~ ~ , ~ ~ On l~rely~o:r~even~mos : y~e~net'~a SO ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: The African cichl ~ is remails i s they eve long Hen considered ~ of - ~ ~ ~ ~ ~' ~ ~ ~ ~ ~ ~ L.. ~ ~ ~ 11.. ~ ~ ~1,...:~ ~ ~e Q n o r m o u s ~ ~ ~ ~ e v o l u t l o n a ~ ; l n t e ~ r e s t . ~ ~ ~ B ~ t , ~ ~ r a t n ~ r ~ ~ t n ~ a n ~ c ~ e l n g a ~ ~ t e x t ~ O O k ~ x a m ~ p l ~ e ~ ~ ~ ~ ~ : ~ ~ ~ ~j ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ :: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ :~ :~ ~ ~ ~ ~ ~ jots anyone ~:partlcu a~r~p ~eno~manon~ ~ they ~se~em~to rQpresQnt~a: natu~ral~la~ra-~ ~ ~ ~ ~ ~ , ~ ~ ~ ~ ~ ~ ~ ~ If, ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~: . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ :~ Stow Tour stuc yang evolution, ~ecology, And mo~hol~gy.~ And that stay is~still :~in ache ~:v^:It;~^ :~^q::rl`:~ The ~ : I: ~ ~ ~ ~:~ ~ ~ go:::: ail: :~ ~:~ ~ ~ : :~ ~ i: ~ ~ ~: :~ :: ~ :: ::: I: ~ ~ :: :~ I: ~ aft: ~ ~:~ ~:~ ~ :: I: : ' il:: 16. ~V^ - I`II 1 - ::~1:1~ .~:~:~., 2 ~ I_ ~ ~ ~ I, ~ ~ _ ~: ~.~ ~ ~ :~ ~ ~ ~ :~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: :~:::~ ~ ::::: I: ~ :::::::::: ::: ::: ~:~ :::~: ~ : : ~ ~ I: ~ ~ ~:~ ~ I:: ~ : ~ ~ ~if: ~ ~ ~::~ I:: ::: ~ ~ : ~ ~ ~ ~ ~: ~: : ~ ~ . 1_ 1 1 1 1 ' number of species on earth. Although approximately 1.4 million species of all kinds of organisms have been formally named since Linnaeus inaugurated the binomial system of species identification in 1753, this figure grossly underesti mates the diversity of life. Considering the prodigious variety of insects alone and the underrepresentation in the catalogue of many types of organisms, such as microbes, it is reasonable to guess that, the absolute number of species of all groups on earth falls somewhere between 5 and 30 million. Proper species classification is important because a species is not like a molecule in a cloud of molecules, but is rather a unique population of organisms; the terminus of a lineage that split off from the most closely related species thousands or even millions of years ago. Species have been shaped into their present forms by mutations and natural selection, during which certain genetic combinations survived and reproduced differentially out of an almost inconceiv ably larger total. No two species, no matter how closely related, are any more interchangeable than are two Mozart sonatas. Each species of organism is incredibly rich in genetic information. The genetic information in the constituent bases that make up the DNA in a single mouse cell, if translated into ordinary letters of printed text, would nearly fill all 15 editions of the Encyclopedia Britannica published since 1768. Since other evolutionary disciplines, including ecology, biogeography, and behavioral biology, depend on systematics, an entire hierarchy of important problems must be addressed. Two stand out in the sense that progress toward their solutions is needed to put the other disciplines on a permanently sound basis. The first problem is to define the magnitude and causes of biological diversity, and the second is to determine the most reliable measures of homology and their implications for phylogenetic relationships. In defining the magnitude and causes of biological diversity, systematics will undoubtedly fall short of obtaining a complete catalog of life on earth, but a determined effort would pay many dividends. A greater understanding of biologi- cal diversity promises to resolve some of the conflicts in current theory and at the
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EVOLUTION AND DIVERSITY 277 same time to open productive new areas of research. In addition, the answers will influence a variety of related disciplines, affect our view of the place of humans in the order of things, and open opportunities for the development of new knowledge of social importance. For example, control of mosquito-borne diseases such as malaria has profited from the ability to define as separate species mosquitoes that are morphologically almost identical but that differ in behavior and in their ability to transmit the diseases. Systematics is also a discipline with a time limit because much diversity is being lost through extinction caused by the accelerating destruction of natural habitats. This is especially true in the tropical moist forests, where more than half of the world's species are thought to exist. Although extinction rates are difficult to estimate, in part owing to inadequate systematics, current rates of extinction seem to approach or exceed 1,000 times the average rate in past geological time. Tragically, and perhaps ominously for human welfare, most of the tropical for- ests, and with them many thousands of species of plants and animals, seem destined to disappear during the next 30 years. It is not too much to say that humanity is locked into a race in which systematics must play a crucial role. From a practical standpoint, plants provide many critical medicines and pharmaceuticals, many species contain genes for disease resistance and other desirable traits that can potentially improve agricultural varieties, and many could potentially be developed into important crops themselves. For example, the taxonomy of plants has stimulated and in turn been invigorated by the discovery of more than 10,000 secondary compounds scattered among a vast array of species. These substances (alkaloids, terpenes, phenolics, cyanogens, and glu- cosinolates) are equally crucial to the understanding of plant evolution and to the improvement of human welfare. Thus, the study of biological diversity and the desire for its preservation are not based on esthetic principles alone. Systematics Also Includes Studies of the Interrelationships Among Organisms Studies on phylogenetic relationships.among organisms aid in the develop- ment and evaluation of theories about evolutionary processes. Models of the origin of species have been stimulated as well as guided by the development of the species concept in systematics. Phylogeneiic information is important in many areas of evolutionary biology. For example, in biogeography, the occurrence of flightless ratite birds (ostrich, rhea, emu, and others) in Africa, South America, Australia, and New Zealand is apparently inconsistent with morphological and molecular data indicating that the birds share a common ancestry. The paradox is resolved by the knowledge that the birds all diverged from a common stock that inhabited the great southern continent of Gondwanaland before it split and be- came dispersed through continental drift. Phylogenetic information is the basis of the comparative method for the study of adaptation. A positive adaptive value for a particular characteristic is suggested when two or more distantly related organ
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278 OPPORTUNITIES IN BIOLOGY isms have undergone parallel evolution in that characteristic, for example flower shape or color. Phylogenetic analysis is also necessary for understanding the sequence in which characteristics have undergone evolutionary transformation and for estimating rates of evolutionary change, be it morphological or molecular. One of the chief tasks of systematics is the elucidation of phylogenetic, or genealogical, relationships among organisms. Inference of genealogy is a desir- able goal both for fossilized forms and for living organisms whose ancestry is poorly documented in the fossil record. The reconstruction of phylogenetic history is often of great interest in itself, for example in determining the ancestral relationships among humans and other higher primates. But the reconstruction of phylogenetic trees has numerous other uses as well. Indeed, phylogenetic data are the source of almost everything we know about the patterns of evolutionary change over the course of millions of years, including convergent evolution, parallel evolution, adaptive radiation, and mosaic evolution. Phylogenetic studies have been essential to understanding how species have arrived at their present geographical distributions and to interpreting processes and rates of change at the level of the DNA. Only in the past 20 years have the logical and evidential criteria for establish- ing phylogenetic relationships been articulated. Through these sometimes contro- versial developments, systematics has become a highly sophisticated, rigorous science in which mathematics, statistics, and molecular biology play leading roles. Modern systematics differs greatly from what it was even 10 years ago and poses extremely complex questions. An important step in this revolution was the development of methods of classification that allowed beelike diagrams expressing the similarity among organisms to be derived by objective criteria through the use of appropriate mathematical expressions evaluated by computer algorithms. These kinds of clustering procedures first developed for biological classification have since been used in many other applications, for example, in linguistics. The beelike diagrams derived from clustering procedures do not necessarily reflect the genealogical relationships in phylogenetic trees unless the similarity of two species is directly proportional to how recently they diverged from their common ancestor. Proportionality does not exist when many characteristics undergo convergent evolution or when different evolutionary lineages evolve at different rates. However, methods have also been developed that aim to infer the correct phylogenetic relationships among species, although these methods are sometimes difficult to apply in practice because of uncertainties and ambiguities in the data. An important area of current research is the development of statistical techniques to evaluate the degree of uncertainty in estimates of phylogenetic relationship. Just as estimates of numerical quantities should be accompanied by confidence intervals giving the precision of the estimates, inferred patterns of phylogenetic relationship must be accompanied by some kind of measure of their reliability.
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EVOLl~ION AND DIVERSITY 279 Traditionally relying on the data most readily available, usually the morpho- logical characteristics of preserved museum specimens, modern systematics also includes other sources of data, such as ecology, behavior, genetics, and biochem- istry. The power of systematics has recently been augmented by data from molecular biology. Electrophoretically distinguishable proteins are now routinely used to distinguish species and to estimate phylogenetic relationships among closely related species, and restriction enzyme digests of DNA sequences such as mitochondrial DNA provide numerous systematic characters. EVOLUTIONARY HISTORY The Fossil Record Makes Special Contributions to Evolutionary Biology and to Knowledge of Preseni-Day Diversity Although questions of both process and result are central in evolution and diversity, the history of evolution has only one source of primary direct evidence, one court of last resort, which is the fossil record. Studies in paleobiology therefore directly affect all aspects of evolution and diversity. The fossil record provides the vital time dimension for the understanding of biological diversity and the history of life. The current data base of paleobiology consists of records of some 250,000 extinct species of plants, animals, and microorganisms occurring in deposits spanning more than 3.5 billion years of earth history. Although the record comprises only a small fraction of all the fossil taxa that ever lived, systematic collections in museums and universities contain tens of millions of documented specimens, in many cases with good representa- tion of individual species in space and time. With respect to the history of diversity, the fossil record can be analyzed to determine whether diversity is higher now than in the geological past' whether the evolution of diversity might be expected to reach a steady-state level, and whether community structure has changed over geological time. Paleobiology is unique in being the only source of data about certain evolu- tionary processes and events. For example, although the observational and experimental work of most biologists is necessarily limited to processes and phenomena that are relatively rapid or common, paleobiologists capitalizing on the depth of the geological record have access to much rarer events. The geological record also documents a unique and lengthy natural experi- ment in adaptation. Many biological innovations originated, flourished, and died out long before the modem biota emerged. Studies in paleobiology can shed light on when these lost adaptations originated and whether they were better solutions to functional problems than are found among living organisms today. Adaptive radiations-bursts of speciation in which the number of species in a biological group or adaptive zone increases exponentially during a relatively short time, with accompanying expansion in the diversity of structure and function is
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280 OPPORTUNITIES IN BIOLOGY ~ ~ ~ ~ ~ ~ ~ ~ THE RISE OF THE WANTS ~ ~ ~ I :~ ~ ~ :: n Unit:;: ~ ~ar\Jarc ~:~:unl Grsr :y rece:lven:t 1Q t~l:rst~ tnown::soecl:m~ens~:::ot fossil ~ ~ ~ ~ ~ ~ ~ ~ :: ~ ~ Ants of;~Mesozo~ic Age, Ohio Beautifully preserYQd~spQcime~n~s~;~ in the clear :: : : : : ~ : ::: : : :: : :: : : ~1~ ~ orange amber from a ~r~dwodd ~tree~th~at~grew~80 ~m~ill~n~years ~ ago in News Jersey. Use speci~mens~we~re~som~ing of a breakthrough in the~study~ of 1 insect ~evolution. ~I~Ubtil~the~n~ ~the~old~est~know~n~f~ssils twerp about~30 to ~140~1 : : ~ : : ::: : . · . , · ~ ~ malt Ion years ~ o o~ ~ ~ romp it Bell ~ ~gocene~ ~epoc ,, Cant :~ quite Amos em I n aspect. ~ ~ . . ., . : : i: ~ he m:aIn: Teatures o Wants egos upon dada ready been fleshed out. The only p~hytoge~n~etic~treeTtEhat~could~d~rawn~from~such~eYidb~n~ce~was the canopy with~the~tru~nk~and~roots~cut off~Th~e~Mesozoic ants provided what~appeared~i :: : ::: : it:: ::: : :: :: :~: : :~: ~ : Am: : :~ : :: ~ ~ ~to~be~a~pi~e~of~e~tr~unK~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: : :: ::: :::: : : : : : : ~ :: : :: : : ~ ~ ~ ~ ~Soon~alte~rward Soviet paleontologists~began~to describe ~ailong series ~ ~ ~ , ~ ~ , . :~::ot~ot~ ~ eran t Ilk e f ok; ~s,~aI=~u to O~million~:y Q arsol d,~givi n g::: ads Q karate I: : : : ::: ~ : I: :::: : ~ : ::: ::~: :~ ::: : Aim: ::: :: ~scie~ntdic~namQ~to~almost~eve~ry~specimen.~Wh~en~ia~llthe~se~:bits~and~pieCes 11~: :: ::: I: : ~ ~ : : ::: ~ i: w~re~fiffed~togeth~erland~ their New J~ersey~fossils~addedlin 1~986 ~a~remark~bi*~ picture ~ emerged: ~ Idle specimens fell Pinto three ~classes, ~representing~;~the fir worker caste,~the~que~en caster :~ Andre male ~d f the most~prim~itive~ants.~ ~ Lea ~workers~,lack~d~ wings And had~proportion~at~ely~small abdomens~,~th~e Shall :: : : : ~ : : : : marks of A Sterile caste. ~ ~:lTh~ese~los~sils~mad~e it~possible~to ~:conclud~e~ that ~1 social life ~ had been ~e~stab fished in Writhe ~ ants by ~ ~80~ ~ million Years Ago, A startling i ~conclus i on in view of ~the: earlie r Jack o f such ancient toss i Is. . :~: : ::: :::: :: ~ ::~ ~ :: :: : : I: : :: :~ ~ :: :: : :~: ::: ~ ::: ~:~ ~: ;~ ~ ~ : A close examination of the American and Soviet fossils~show~d them to ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ be~si~m~ilar to what had bee n~expectdd~ for ancestral ants. Their anatomy was : ~ : : : : :~: :: : : ::: ~ : :: : :::: ::: : :: ::: : : : :: :: ~ :: :: :: ~ a mosaic of traits, some untypical Of nonsocial wasps and some ~ more mod ::: :: ::~ : : : ::: :: :: ::: : : ::: ern~ut~i~ll typical Loft g~eneralized~ants.~l They provided the ~first~clue ~con- scorning the group of wasps tram which ants arose. ~ ~ ~ ~ ~;17~ ~ :~ ~ ~ ~ ~ The Harvard ~ collection recently ~ obtains Ethel first ~ ant fossils ~ of ~ mid : EoCene~age,~from~Arkansas~thistime.~Ghinese~a~ndSovi~etpaleontohgists~ were ~close~behind~,~ discovenng~;~E>oc~ene~ specimens from Manchuria and Sakhalin, respectively. ~1 hese~ants~are thought to best to ~60; millioni~years old~,~and almost of the~m~are very different from the Mesozoic fossils. ~ They are .. ~uive~rse~,~representi~ng~both~mod~ern taxonom~ic~groups and~(in~one case)~a Stock not too distant~from t~he~Mes~zaic ants. fit thusiseems that~the ants like . the mammals, crossed~a threshold around the~end of the Mesozoic era. For ~ ~ : : ~ : : : :: :~ some reason not yet understood, they expanded into~a~richly various' world cominant group. ~ :~ ~ ~ Entomologists and~paleontologists~cont nue to search avidly~for fossils from Mesozoic and ~eariv~lCenozoic ~denasits. The ~nuestic~nS:~wA~hocm In answer Include when: ~ where And from which wasplike insects the ants arose; exactly when they radiated into their modern aspect;~the directions they took when spreading around The world.; and, not least' what trays contributed to their spectacular sum so. ~ ~ ~
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EVOLllTION AND DIVERSITY 281 well documented in the fossil record, but it is not clear whether these grand adaptive radiations are analogous to the smaller-scale bursts of speciation ob- served, for example, among Hawaiian drosophilids and African cichlid fish. Extinction Has Been the Fate of Almost All Species That Have Ever Lived Extinction, as a biological process, is difficult to study in modern environ- ments. Although the background rate of extinction is low-estimated as about one global extinction per million species per year-extinction is not only frequent on a geological time scale but has been responsible for many complete turnovers in the biological composition of the earth. A proper understanding of the evolu- tionary process is impossible without knowledge of rates of extinction, quite apart from the importance of such knowledge in evaluating the magnitude of the increase in rates of extinction resulting from human activities in modern times. Understanding the environmental causes and evolutionary implications of the occasional, brief periods of mass extinction in earn history is a key problem in paleobiology. The most severe mass extinction occurred 250 million years ago and eliminated between 75 and 95 percent of the species then alive. In short, the global biota had a close call with total annihilation. Somewhat less severe mass extinctions are scattered throughout the fossil record. Recent work on the likeli- hood that some mass extinctions were caused by meteorite impact shows promise of establishing strong connections between biological evolution and the cosmic environment. When combined with the more speculative possibility that impact- induced extinctions are regularly periodic, this hypothesis opens the possibility for major shifts in the way the evolution of the global biota is interpreted. Within the past 2 million years of the fossil record, constituting the Pleisto- cene epoch, are special opportunities for studies of biological diversity. During this period, the blots was essentially modern but subjected to the effects of well- documented major changes in climate and geography that set the stage for the present distribution of plant and animal species. Modern tropical rain forests, to pick just one example, can be understood only by knowing the historical under- pinnings that led to their present distribution and composition. This understand- ing is critical in developing a strategy for dealing with the effects of human activities, especially in the moist tropics. Paleebiologists Have Made Major Progress in the Past Two Decades Research results have been astounding, at the other end of the time scale, in deciphering the oldest records of life on earth. Not only did life begin far earlier than biologists had previously envisaged, but, perhaps even more surprising, the earth's biota was composed solely of bacteria over such an extended period. These fossil discoveries have recast concepts of evolution and diversity and have reemphasized the fact that, when viewed over the long sweep of geological time, a
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282 OPPORTUNITIES IN BIOLOGY significant part of evolutionary progress has resulted from changes in the intracel- lular biochemistry of bacteria. Paleobiology includes several research areas that have special promise of making significant contributions to evolutionary biology and to other fields of the natural sciences. Among these are the origin of life itself, including not only when life began, by what processes and in what types of environments, but also whether life might exist elsewhere in our solar system or in the universe. Such issues are ripe for exploration during the coming decade because recent progress in studies of ancient Precambrian fossils has extended the known record of life on earth to more than 3.5 billion years. Studies of even more ancient deposits, coupled with laboratory studies of chemical reactions that can occur in a lifeless environment and biochemical studies of existing microbial organisms, promise to provide new evidence of the beginnings of life and of the environment in which life originated. Organisms Alive Today Are Well Adapted to the Vagaries of Their Present-Day Environment Environmental conditions such as atmospheric composition, day-night light regime, and temperature conditions have changed markedly over the course of geological time. Until about 1.7 billion years ago, well after the origin of living systems, the atmosphere contained too little oxygen to sustain obligately air- breathing forms of life. Day length has progressively lengthened as the distance between the earth and moon has gradually increased, and there is good evidence that the earth's average surface temperature has changed markedly. Each evolv- ing species became adapted to the environment in which it originated, and as the environment changed, life evolved and built on foundations that had become established under earlier regimes. Therefore, recorded in the genetics, biochemis- try, cellular structure, and gross anatomy of living organisms may be a coded history of their evolution. For example, analyses of growth bands in fossil corals and mollusks have made it possible to track the changes in day length caused by tidal friction. Even more spectacular has been the recent recognition of Mi- lankovich cycles of climatic change over the past 700,000 years, which almost certainly were responsible for the pulses of continental glaciation during the Pleistocene epoch. Deep-sea drilling during the past two decades has provided continuous sec- tions in which important population-level analyses of evolutionary changes are feasible. This increased resolution in the fossil record introduces a time scale comparable to that of microevolutionary change in population genetics, and it opens a more complete synthesis of these two disciplines. The oceanic fossil record is excellent for the last 160 million years, and the dee~sea cores provide a rich source of information on the evolution of single-species lineages. Statistical analyses have already documented important patterns of morphological change
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EVOLUTION AND DIVERSITY 283 and the not-uncommon lack of such change known as stasis. But the surface of this field has only been scratched by the investigations camed out thus far, and we have much more to learn about the tempo and mode of evolution. Much also remains to be learned regarding the timing and nature of major evolutionary events. Some of them, such as the origin of invertebrates, verte- brates, flowering plants, angiosperms, and humans, have been recognized as important research problems since the mid-nineteenth century. Other events, such as the advent of photosynthesis, oxygen-dependent respiration, the anaero- bic-aerobic global ecosystem, nucleated cells, and eukaryotic sexual reproduc- tion, have been addressed only recently with the upsurge of interest in the Precambrian fossil record. Future studies will promote a better understanding of the timing and context of major evolutionary events in the history of life on earth. CURRENT STATUS OF RESEARCH Contemporary Research in Evolution anal Diversity Features Several Exciting Growing Points A particularly promising field spanning the synthesis of molecular biology and evolutionary biology is expected to reveal new evolutionary principles even as it resolves some longstanding issues. Important as this new synthesis is, it must be emphasized that not everything in evolution and diversity can be reduced to molecular biology. Many central issues of evolution at the organismic level require different kinds of approaches. These include the study of the evolution of complex multifactorial traits within populations and the evolutionary role of selection among populations. Innovative approaches to uniting physiology, behavior, and development should also be encouraged. Those who set research priorities in evolution and diversity must also recog- nize the continuing importance of cataloguing the diversity of life on earth and understanding its origins through speciation and its disappearance through extinc- tion. Apart from the scientific value of such research are the many potential practical applications of He findings in medicine, agriculture, and biotechnology. Groups of organisms that are already relatively well known, such as vertebrates, plants, and butterflies, are important to study because of the light further informa- tion about them would shed on overall biogeographic problems. In addition, economically important groups of organisms, such as legumes and mosquitoes, should be emphasized in choosing priorities for study. Areas of vegetation that are already decimated and those that are being destroyed rapidly but that contain large numbers of endemic species should also receive special emphasis. Con- certed efforts to survey more or less completely the blots of selected places, especially in the disappearing forests of the tropics, would be much more reward- ing than miscellaneous sampling of poorly known groups over wide areas. Greater
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284 OPPORTUNITIES IN BIOLOGY attention should also be given to groups that are especially tractable for the solution of basic problems in ecology, population biology, and evolution. To accomplish this, additional systematic biologists must be trained and employed, since the current world supply is much too limited to attack the millions of species of unknown or poorly known organisms profitably. Paleobiology also presents significant new opportunities for breakthroughs in understanding the history of life on earth, including its earliest history in the Precambrian, its diversification and geographical distribution, and its extinction through, in some cases, global processes. Collections and Special Facilities Museums Are One Logical Place to Concentrate the Effort to Encompass Diversity These institutions are already the repositories of vast numbers of priceless specimens, often representing species that are endangered or recently extinct. Yet most of the collections are fallow, and the halls of some of the leading research museums are largely empty of qualified researchers. The same is true of zoos and botanical gardens, which are in effect museums fUled with living specimens. One of the premier tropical botanical gardens in the continental United States has purchased no major items of research equipment in 20 years. Although it averages only one postdoctoral fellow per year, it could easily accommodate six. An additional need exists for regional or international centers for the storage and analysis of fossil pollen and other microfossils, which are vital in the recon- struction of evolutionary histories and past environmental change. For example, we are only now becoming aware of the considerable extinction of species that has been caused by human disturbances, especially on islands, lakes, and other geographically restricted habitats. One of the most promising domains of re- search is the detailed analysis of this impoverishment during the past several thousand years, with an emphasis on the factors that make certain species more vulnerable than others. The future of systematics and its contribution to evolutionary studies depend on collaboration among workers in different fields, funding of interdisciplinary studies, and mutual education. Museums, the traditional home of systematics, will find it necessary to expand their facilities and personnel to encompass statistical, molecular, and experimental approaches. The traditionally modest sums granted to systematists will not support molecular investigations, and it will be useful to set up facilities for molecular systematics that can be used by multiple workers. Above all, university biology departments, in their staffing and curricu- lar decisions, must take into account the growing impact of the new systematics on the study of evolution and its implications throughout biology, from molecular biology to ecology.
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EVOLUTION AND DIVERS17.Y 285 Museums are also vital to the continued health of research on the fossil record by maintaining and developing systematic collections. These collections are the lifeblood of research progress. Research questions change continually, and it is important that museum collections remain an effective source of empirical data and that the data be actively studied and described by competent specialists. The paleontological collections of the United States are in reasonably good shape, thanks to many years of financial support from the Biological Research Resources program of the National Science Foundation. Continued support is critical to sustain active research programs of relevance to broader problems of evolutionary biology. Museum collections are becoming especially critical in some areas because of the phasing out of support for collections by many major research universities. Collection and Conservation of Germplasm Is Crucialfor Improved Agricultural Production Human activities associated with modern civilizations are causing a loss of diversity at all levels of biological organization. Once lost, this store of genetic diversity can never be recovered. A case in point is the loss of genetic diversity associated with the primitive land races (plants that are adapted to a region in which they evolved) and wild relatives of our crop plants. These genetic re- sources have repeatedly provided genes for disease resistance when agriculture has been challenged by serious disease epidemics, and these resources constitute an important source of novel phenotypes in conventional plant improvement. They must be found and conserved for our common good. Modern agriculture is characterized by extensive plantings of genetically uniform monocultures. For example, genetically uniform hybrid corn is widely grown in the United States. Genetically uniform populations have the advantages of high yields, uniform size, and uniform dates of maturity, and these features have played a major role in the great increase in productivity of agriculture in the United States during the past 50 years. Uniformity of size and maturity are also required by highly mechanized agricultural practices. On the downside, genetically uniform crops are vulnerable to large losses from pest or disease outbreaks because monocultures may lack the genetic vari- ability for resistance to the pathogens. In 1970, a fungal pathogen raced through the U.S. corn crop in the corn leaf-blight epidemic. It was quickly discovered that susceptibility to the fungal pathogen was associated with a particular mitochon- drial genotype that had been widely incorporated into breeding stocks. Luckily, other mitochondrial genotypes conferred resistance, and the resistant genotypes were introduced into commercial lines of corn. By 1971 corn varieties resistant to the leaf blight had largely replaced the susceptible type in agricultural production, and the corn crop was protected. The resistant types were available because an international effort had been made to conserve plant genetic resources for just such contingencies.
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286 OPPORTUNITIES IN BIOLOGY A second major disadvantage associated with the wide, and in some cases nearly global, adoption of monocultures is that these plantings supplant and drive to extinction the wild relatives and primitive cultivated forms of crop plants, which provide a source of genetic variants for future breeding efforts. Genetic conservation is faced with two problems-how to save and maintain useful plant germplasm and how to evaluate plant gene pools in order to preserve as wide a sample of potentially useful genetic variants as possible. The problem of evalu- ation is particularly difficult because we have no way of predicting which kinds of novel genetic variants the future may require. At present, the best that can be done is to evaluate the plants of interest for a wide range of genetic traits and select a sample for conservation that includes as much diversity as possible. Little is known about the adequacy and scope of contemporary germ-plasm collections. Genetic screening procedures and statistical sampling plans need to be developed for this task. Finally, what little effort is expended to protect plants is almost entirely devoted to crop plants and their wild relatives about 150 species out of the more than 260,000 kinds of plants known. Botanists estimate that tens of thousands of kinds of plants could probably be developed into useful crops~ot only for food but also as sources of medicines, oils, waxes, and other chemicals of industrial importance-if we would carry out the appropriate investigations, identify them, and develop them according to their cultural requirements. Virtually no effort is being expended in such investigations; yet fully a quarter of all plant species, along with a similar proportion of animals and microorganisms, are in danger of extinction. Even if the techniques of genetic engineering are fully applied to the development of new kinds of crops, there will need to be a source of appropriate genes; the plants that we are passively allowing to become extinct could well provide such genes, and we should find and conserve them while they still exist.
Representative terms from entire chapter: