dobzhansky - mendelism

8/13/2019 Dobzhansky - Mendelism

1/12

Mendelism, Darwinism, and EvolutionismAuthor(s): Theodosius DobzhanskySource: Proceedings of the American Philosophical Society, Vol. 109, No. 4, Commemorationof the Publication of Gregor Mendel's Pioneer Experiments in Genetics, (Aug. 18, 1965), pp.205-215Published by: American Philosophical SocietyStable URL: http://www.jstor.org/stable/985879Accessed: 12/04/2008 11:13

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at

http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless

you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you

may use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.jstor.org/action/showPublisher?publisherCode=amps.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed

page of such transmission.

JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We enable the

scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that

promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected].
http://www.jstor.org/stable/985879?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/action/showPublisher?publisherCode=ampshttp://www.jstor.org/action/showPublisher?publisherCode=ampshttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/stable/985879?origin=JSTOR-pdf


2/12

MENDELISM, DARWINISM, AND EVOLUTIONISMTHEODOSIUS DOBZHANSKY

The Rockefeller Institute(Read April 23, 1965, in the Symposium Commemorating the Publicationof Gregor Mendel's Pioneer Experiments in Genetics)

MENDEL is one of the tragic figures in thehistory of science. During the autumn of hislife he must have felt that his work as a scientistwas a dismal failure. It was overlooked andignored. He could hardly have foreseen that itwould be rediscovered and appreciated in 1900,i.e., sixteen years after his death. It was publishedin an obscure provincial journal, but this was onlya partial explanation of its having been disre-garded. The scientific literature was then notyet the flood that it has become now. Biologists of1865 were evidently less well prepared to under-stand Mendel's insights than were biologists of1900. There was, however, a biologist livingin 1865 to whom the above statement did notapply. This biologist was Darwin. Unluckily forboth parties, Darwin did not know about Mendel'sdiscovery. The library of Mendel's Brunn mon-astery had copies of some books by Darwin withmarginal notations in Mendel's handwriting.Mendel failed to send a copy of his publication toDarwin; perhaps by the time he became familiarwith Darwin's books he had given up hope ofhaving his contribution understood by anybody.Mendel was not a great generalizer of massesof heterogeneous data, as Darwin so pre-eminentlywas. Mendel's genius was in depth rather thanin breadth. His place among the greatest ofscientists is due to a single published work ofmodest size. In this work he was able, however, toanalyze fully his experimental results, and toapprehend with perfect clarity the causal nexuswhich these results revealed. A really new dis-covery in depth is apt to be less easily understoodthan a discovery in breadth. Darwin, not knowingof Mendel's work, was making some experimentsof his own, which led him within an ace of obtain-ing results paralleling Mendel's. Whether or nothe would have analyzed the results as masterfullyas Mendel did is a moot point. The late J. B. S.Haldane thought that Mendel's analysis wassomehow facilitated by his familiarity withThomist philosophy. This is an extraordinary

compliment for Thomistic philosophy, but I amnot convinced that it is warranted.II

Darwin was certainly aware of the importanceof understanding heredity for understandingevolution. In his books, especially in that dealingwith domesticated animals and plants, he pains-takingly collated every bit of information aboutheredity that he found in the literature. Evolutionis a complement of heredity, or rather a negationof heredity. Heredity tends to make the progenyresemble the parents and other ancestors. Evo-lution makes the descendants unlike the ancestors.If heredity were always exact, evolution couldnot happen. An offspring of a pair of parentsconsists, however, of individuals which differ tosome extent from the parents and from each other.This is variation.Variation is the fountainhead of evolution.

Taking variation for granted, Darwin proceededto describe how natural selection molds it intoshapes which make living beings adapted to theirenvironments. He was satisfied that the variationwas universal, observable in all organisms. Heacknowledged, however, that the origin of vari-ation was unknown. So long as this ignorance wasunbroken the theory of evolution was incomplete.One could even say that this theory was a colossuswith feet of clay. This was pointed out in 1867by Fleming Jenkin, an engineer rather than abiologist. Suppose that a light-skinned individualappears in a dark-skinned population. Could thelight-skinned variant eventually replace the origi-nal form? It seemed that it could not. Thenew variant, a mutant as we would call it now,is unlikely to arise in many individuals in anyone place and at any one time. The mutant willhave to mate with an individual of ordinary color.Their children will presumably be intermediatebetween the parents, and they too will have tomarry partners of the usual color. After a few

PROCEEDINGS OF THE AMERICAN PHILOSOPHICAL SOCIETY, VOL. 109, NO. 4, AUGUST, 1965205


3/12

THEODOSIUS DOBZHANSKYgenerations of intermarriage, the mutant will dis-appear like a drop of a soluble dye in a sea. Itwill be dissolved in the prevailing norm.Jenkin's argument tacitly assumes that theheredity of a child is a blend of the parentalheredities, and that the components of the blendnever regain their purity. This is the vernacularnotion, sometimes dignified by the name of bloodtheory of heredity, which Darwin and Jenkin,and everybody else in their day, assumed to becorrect. Everybody, that is, except Mendel, whoshowed it to be erroneous. Mendel's paper was,however, reposing on some library shelf whichDarwin did not reach. Darwin doubtless feltthat the blood theory contained some hidden fal-lacy, which he was, however, unable to pinpoint.Heslop-Harrison (1958) even argued that Darwindid not really accept the blood theory as valid,because he knew that some hybrids do not showblending but show instead what we at present callMendelian segregation, i.e., reappearance of in-dividuals resembling the parents of the cross.Darwin did know this, from the literature and fromhis own experiments. He also knew, however,that in many, in fact in most hybrids, segregationis not easily perceptible. This is called at presentpolygenic inheritance; it is basically Mendelianbut technically difficult to analyze. Darwin'sjudicious objectivity made in this case a dis-service. He drifted towards Lamarckian notions,which happened to be incorrect.The rediscovery in 1900 of Mendel's forgottenwork should have at once laid the ghost of theblood theory of heredity. It was not quite thatsimple. The early stages of the development ofgenetics are analyzed for you by Drs. Dunn andSturtevant. I had the opportunity to discussthe genetics of the orgin of variations before theAmerican Philosophical Society six years ago,when we commemorated the Darwin Centennial(Dobzhansky, 1959). I can limit myself hereto only a few remarks concerning these matters.Some log jams had to be cleared before Darwin-ism and Mendelism could join forces. For atime, it did not seem unreasonable to entertain akind of dualistic theory of heredity, assuming thatsome of the inheritance is transmitted from parentsto offspring by miscible bloods, and otherinheritanceby immisciblegenes. For example, thevariation of the eye color in man is nearly (thoughnot quite) discontinuous. Segregation of brown-eyed and blue-eyed types of progeny is observablein many families. On the other hand, the vari-ation of human stature seems to be continuous and

the inheritance blending. It took close to thirtyyears to have almost everybody convinced thatcontinuous variability is fundamentally Mendelian.Mutation is the source of the hereditary vari-ation which Darwin was looking for. Howtantalizingly close he was to its discovery is evi-dent from his following statement:All the characters . . . which are transmitted n aperfect state to some of the offspring and not toothers, such as distinct colors, nakedness of skin,smoothnessof leaves, absenceof horns or tail, addi-tional toes, pelorism,dwarfedstructure,etc., have allbeen knownto appearsuddenly n individualanimalsand plants. From this fact, and from the severalslight, aggregated differenceswhich distinguish do-mestic races and species from each other, not beingliable to this peculiarform of transmission,we mayconcludethat it is in some way connected with thesuddenappearanceof the charactersin question.And yet, de Vries, the pioneer student of mutation,contrasted the mutational variability with theubiquitous continuous hereditary variation, whichDarwin believed to represent the raw materialswith which natural selection operates.De Vries dealt with mutations so sharply dis-tinct from the parental form that he believedthem to be new biological species. Mutationismwas construed not as an integral part of, but asan alternative to Darwinism and Mendelism.The work of T. H. Morgan and his school atlast resolved the puzzle. Mutations come, so tospeak, in all sizes, from so drastic ones that themutants are inviable, to so slight ones that a verykeen eye or a statistical refinement is needed todetect them at all. No matter how drastic amutation may be, it does not create a new species(except for the special case of doubling the chro-mosome complement in some otherwise sterilehybrids between distinct species). All the nu-merous mutants observed in Drosophila flies stillbelong to the same species in which they havearisen. Mutants do, however, possess all thecharacteristics which seem requisite in the ma-terials from which natural selection could com-pound species differences. They are hereditaryvariants that cannot be swamped by blendingwith the ancestral form, but they can become moreand more frequent if they are favored by naturalselection and can eventually replace the ancestralform. Hardy and Weinberg showed independ-ently, both in 1908, that the Mendelian mechanismtends to preserve the variant genes in the popu-lation, from one generation to the next, withconstant frequencies, unless they are either elimi-nated or multiplied by natural selection.

206 [PROC. AMER. PHIL. SOC.


4/12

VOL. 109, NO. 4, 1965] MENDELISM, DARWINISM AND EVOLUTIONISMThe stage was now set for further advances.Chetverikov in 1926 had sketched the outlinesof what has more recently come to be called thebiological, or synthetic, theory of evolution (anEnglish translation of Chetverikov's classic was

published in the Proceedings of the AmericanPhilosophical Society in 1961). Fisher (1930),Wright (1931), and Haldane (1932), largelyindependently of each other and unacquaintedwithChetverikov's contribution, gave more rigorousmathematical formulations of the basic tenets ofthe theory. This was unprecedented in biology-a theory was deduced mathematicallyfrom a singlefundamental premise-Mendel's law of segre-gation. Some additions and elaborations, but nobasic changes were made in this deductive theoryfor about thirty years. The mathematical theoryhas, however, far outstripped its biological foun-dation-again for the first time in the history ofbiology. Significant developments since Haldane-Wright-Fisher halcyon days were generalizingworks which examined the factual data accumu-lated in several biological disciplines, and foundthat those data make sense in the light of thedeductive theory. Mayr, Simpson, Rensch,Schmalhausen, Stebbins, Darlington, White, Ford,and Grant are the outstanding names among thefounders of the modern biological theory of evo-lution. This theory has also been named syn-thetic. It is synthetic, in the sense that it em-bodies a synthesis of data from biology as a whole.The word synthetic may, however, also meanartificial or factitious, as contrasted with genuine,and this makes the designation biological pre-ferable in my opinion.

IIIIn Mendelian terms, the process of organicevolution can be described as a sequence of sub-stitutions in consecutive generations of some genesfor others. Genes, let it be noted, are carried

mostly, though not exclusively, in the chro-mosomes, and a definition of evolution mustaccordingly be framed to include the chromosomaland the cytoplasmic heredity. This definition issatisfactory as far as it goes, but it does not gofar enough. It describes adequately only the ele-mentary components, and not the way the com-ponents compose the evolution. The definitionis reductionist, and it needs a compositionistcounterpart, to use the expression suggested bySimpson (1964a). For evolution is not onlysubstitution of independent components; it is

also integration of the components to form adapt-ively coherent systems. My favorite analogy isthat genes act not like solo players but more likemembers of a symphony orchestra.The origin of strains of bacteria resistant toantibiotics can serve as a paradigm of elementaryevolutionary events which are experimentally re-producible. The origin of insect populations re-sistant to pesticides is less easily reproducible butequally clear. What is involved is adaptation oforganisms to man-made environmental factors.Antibiotic-resistant bacteria and insecticide-re-sistant insects can live in environments in whichbacteria and insects ordinarily do not live. Theadaptation occurs through a mutation-selectionmechanism. Mutation is a change in a gene, orin a chromosome which carries the genes. Itis adaptively ambiguous; i.e., mutations ariseregardless of whether they will be useful or harm-ful to their carriers, and a great majority ofmutations are in fact harmful. Mutation is notevolution, but, as pointed out above, it suppliesthe raw materials from which evolution can bebuilt in response to challenges of the environment.The builder is natural selection.Mutant bacteria resistant to antibiotics, andinsects resistant to insecticides, arise in the specieswhose genes are capable of producing these mu-tants, irrespective of whether antibiotics or in-secticides are present or absent in the environ-ment. A bacterial culture containing many mil-lions or billions of cells, or an insect populationof many millions of individuals, would usuallyinclude a few resistant variants. Resistant vari-ants have no advantage, and they are likely tobe at a disadvantage in survival and reproduction,in environments free of antibiotics or insecticides.Natural selection not only does not increase theirfrequencies in the bacterial cultures of the insectpopulations, but keeps the frequencies down. Thesituation changes when the antibiotics or insec-ticides arrive on the scene. What was disadvan-tageous becomes advantageous, and it may be theonly form able to survive. The nonresistant formsfail to be perpetuated, and the resistant ones taketheir place. The speed of the replacement de-pends, of course, on how great the respective ad-vantages and disadvantages are. It may be that,at high concentrations of an antibiotic or aninsecticide, only the resistants survive and all thesensitives are killed. The change is accomplishedin one generation. On the other hand, one formmay produce 100 offspring in an environment inwhich the other produces only 99. The replace-

207


5/12

THEODOSIUS DOBZHANSKYment by natural selection will then take manygenerations. It is nevertheless important to knowthat, given enough time, natural selection willbe effective even when operating with small fitnessdifferences.

Another kind of experimentally reproduciblegenetic changes should be mentioned. A formof natural selection, called balancing selection isparticularly important in higher organisms. Itleads not to replacement of one gene by anotherbut rather to maintenance of both. Contrary towhat some geneticists liked to think, naturalselection does not usually establish some kind ofan optimum genetic endowment shared by allmembers of a species, but rather sustains agenetic diversity. The population becomes poly-morphic, consisting of two or more geneticallydistinct kinds of individuals. Human populations,like those of most sexually reproducing organ-isms, are highly polymorphic; so much so, thatno two individuals, identical twins excepted, areat all likely to have the same genotypes, comple-ments of genes.The two most interesting kinds of balancingselection are the heterotic and the diversifying.Heterotic selection occurs when the heterozygote,the genotype having two variants of the samegene or gene complex, enjoys hybrid vigor, hetero-sis, compared to the homozygotes, carrying thesame gene in double dose. Diversifying selectiondepends on the complexity of the environment.Suppose, for example, that there are two kindsof food available, and two genotypes, one ofwhich thrives better on one and the other on theother food. In human societies there may bedifferent occupations or professions which aremost congenial to, or which can be performedmost successfully by, carriers of different geno-types. Natural selection will, then, tend to makeeach kind of genotype reach a frequency in thepopulation conforming to the prevalence of therespective foods or opportunities.In many species of Drosophila flies, the popu-lations in their natural habitats are polymorphicfor the structure of their chromosomes. Someindividuals have chromosomes differing fromothers by inversions of blocks of genes. Thechromosomal variants interbreed freely. Thechromosomal polymorphism is maintained byheterotic balancing selection. The highest fitnessis found in flies which have the two chromosomesof a pair different in structure (heterozygotes),while flies with pairs of similar chromosomes(homozygotes) are inferior in fitness. The ex-

citing thing is that the selection pressures actingon these naturally occurring variants are so greatthat the natural selection can be reproduced andmeasured in the laboratory. We can make ex-perimental populations, maintained in special cagesmade of wood or of plastic, and observe thechromosomal variants change in frequencies fromgeneration to generation, until they reach stableequilibria. The selection is not only strong, butexquisitely responsive to environmental changes.Two chromosomes found in the populations ofDrosophila pseudoobscura in the western UnitedStates give a heterozygote which has a fitnessmore than twice that of one of the homozygotes,in the experimental populations kept at 25?C andfed on a certain food. Lowering the temperatureby only 9 degrees, to 16?, makes these hetero-zygotes and homozygotes identical in fitness,within the limits of precision of the experimentaltechnique. Altering the food on which the popu-lations are kept also produces considerablechangesin fitness relationships.

IVNatural selection is often compared, especiallyin popular writings, to a sieve. It retains theuseful genetic variants, and lets the harmful onesbecome lost. So crude a mechanical analogy isof limited usefulness. It does fit the simplest

situations, like the selection of antibiotic-resistantand insecticide-resistant strains, or the eliminationof hereditary diseases and malformations whichmany mutations produce. These elementary proc-esses are repeatable, predictable, and reversible,at least in principle. Provided that mutations re-sistant to streptomycin are produced in a speciesof bacteria, exposing to streptomycin a number ofbacterial cells large enough to contain at leastone mutant makes it very probable that a strepto-mycin-resistant strain will be obtained. Con-versely, placing a streptomycin-resistant strainon a nutrient medium without streptomycin en-courages the selection of mutants reverting to theoriginal form.The sieve analogy is less appropriateto describebalancing natural selection. Here the sievewould have to be so contrived that it will retaingenetic variants when they are rare and removethem when they become frequent. Adaptation toheterogeneous environments is most readilyachieved by genetic diversity. Genetic diversity,polymorphism, raises a new problem, to whichthe sieve analogy is irrelevant. This is mutual



6/12

VOL. 109, NO. 4, 1965] MENDELISM, DARWINISM AND EVOLUTIONISMadjustment, coadaptation, of constituents of agenetic system. Let me reiterate that the analogymost appropriate to describe the gene action inontogeny, the development of an individual, isa symphony orchestra. The ontogeny, from fer-ilization to birth, adulthood, and death, is nota sequence of independent gene effects followingeach other, but a marvelously well-integrated sys-tem of feedbacks. To be adapted to an externalenvironment, the components of a genotype mustbe internally coadapted, i.e., must fit harmoniouslytogether. A gene, A, may interact favorably withB but not with C; natural selection will favor Aif it arises in a genotype containing B, and willdiscriminate against A if it arises with C.This has important consequences. Evolutionarychanges depend on the changes that precededthem, and condition the changes that follow them.The role of the environment in evolution is nowseen in a new light. In the origin of antibiotic-resistant strains the environment is the determin-ing factor. In the presence of an antibiotic, thebacteria must either become resistant or be de-stroyed. Even here, it appears that the organismhas a certain amount of freedom -there areseveral genes, any one of which may mutate toproduce a resistance. With genetically morecomplex changes, the environment can only bedescribed as presenting challenges, to which theorganism may respond by any one of the manypossible adaptive reconstructions. Which responsewill actually be given depends on the geneticmaterials which will happen to be available whenand where the challenge is to be met.Another consequence is the so-called law ofirreversibility of evolution. As pointed out above,the elementary evolutionary events, sometimescalled microevolutionary, such as the mutation-selection episodes yielding the antibiotic-resistantbacterial strains, are reversible. Not so withmacroevolutionary processes. The changes whichled to the origin of mankind from its pre-humanancestors are irreversible. The reason is that theseries of consecutive changes which took placein, presumably, thousands of genes are infinitelyunlikely to be retraced in the same sequence inwhich they occurred before. By the same token,they are unlikely to be re-enacted. Microevolu-tion is repeatable; macroevolution is unrepeatable.In recent years this matter has been debated inconnection with the speculations concerning thelikelihood of existence elsewhere in the universeof living beings, including humanoids resemblingthose on earth. Dobzhansky (1960) and Simpson

(1964a) have discussed the problem in moredetail. Very briefly, even assuming that somesort of life arose in many places in the cosmos,it seems highly improbable that it would evolveinto anything resembling the creatures met withon earth. Those who hold the contrary opinion,usually argue that the adaptive features of theliving beings fit remarkably the demands of theenvironment. This is true, but beside the point.The problem of becoming adapted to a givenenvironment can be solved in evolution usuallyin many different ways. It cannot be lightly as-sumed that whenever a solution is possible itwill in fact be achieved. Microevolution is deter-ministic, macroevolution is creative. The resultsof a creative process are uncertain-it may succeedor fail.

Experimental evidence bearing on macroevo-lution must, of necessity, be indirect. We cannotre-enact the evolution of the horses during theTertiary, or the emergence of the land-dwellingfrom the water-dwelling vertebrate animals. Atbest, experiments can be made on complex kindsof microevolutionary changes, for which I havesuggested the designation mesoevolution(Dobzhansky, 1954). Two examples of such ex-periments can be briefly reviewed here. In bothof them the chromosomal variants of Drosophilapseudoobscura are utilized as materials. Asmentioned above, these variants are maintained innatural populations of this fly by the heteroticbalancing selection. Now, the cultures in whichthe flies are kept in laboratories have environmentsobviously not identical with the natural ones. Thelaboratory flies are maintained either in culturebottles, or in the population cages mentionedabove. Natural selection taking place in thesehighly artificial, or if you wish unnatural, con-ditions makes the flies progressively more fit tolive in the respective laboratory environments, theculture bottles or the population cages.

Suppose, then, that one has strains of twochromosomal variants, A and B, which have livedfor a series of generations in culture bottles, andother strains which lived in population cages.Strickberger (1963) made two kinds of experi-mental population cages; in the first kind, the Aparents were from bottles and B from cages, andin the second A from cages and B from bottles.The equilibrium frequencies which the chromo-somes A and B attained in the experimentalpopulations were different; A chromosomes wereless frequent, and B more frequent, in the firstthan in the second kind of population. The

209


7/12

THEODOSIUS DOBZHANSKYdifference persisted generation after generation.The chromosomes had their histories, as it were,inscribed in their genes.The evolutionary histories of natural popu-lations which live in territories with differentclimatic and other conditions are also inscribedin their genes, in the sense that such populationsbecome different races, each adaptedto its environ-ment. The chromosomal types, which we havedenoted above as A and B, often occur in thepopulations of different territories. Experimentallaboratory populations containing A and B may bearranged in two ways. In experimental popu-lations of geographically uniform origin thechromosomes A and B are descended from wildancestors collected in the same locality; in popu-lations of geographically mixed origins the chro-mosomes A come from one locality and B fromanother. Dobzhansky and Pavlovsky (1953) andDobzhansky and Spassky (1962) found an inter-esting difference between the behavior of thepopulations of uniform and of mixed origins. Theresults obtained in populations of geographicallyuniform origin are repeatable and predictable; ifone arranges several replicatepopulationswith fliesfrom the same cultures, and keeps them in thesame controlled environment, all the populationsreach, within the limits of experimental errors, thesame equilibrium frequencies of the chromosomalforms. Scientists take it almost for granted thatwell-executed experiments should be repeatable;if a repetition fails to yield the same result asobtained formerly, one looks for undetected flawsin the experimental procedure. And yet, replicateexperimental populations of geographically mixedorigins often reach quite diverse equilibrium fre-quencies of the chromosomal forms.This, at first sight, complex and confusingsituation has a simple explanation. Assume thattwo geographic areas are inhabited by populationsdiffering in n genes. Mendelian segregation andrecombinationin the progenies of hybrids betweensuch populations may produce as many as 3different genotypes. If n is in tens, not to speakof hundreds, the numbers of potentially possiblegenotypes become vastly greater than the numbersof individuals in any experimental or naturalpopulations. In other words, many potentiallypossible genotypes will not in fact be formed.Consider now the situation presented by severalreplicate experimental populations. The genotypeswhich will arise will usually not be the same inany two populations. How will natural selection

act in these circumstances? It will encourage thepropagation of whatever favorable genotypes willhappen to present in any given population. Rep-licate populations give therefore dissimilar anddiverging results. We observe, in miniature, whatwe called above the creativity of the evolutionaryprocess. The problem of becoming adapted toa given environment may be solved in a variety ofways. V

According to Wald (1963), living organismsare the greatly magnified expressions of themolecules that compose them. This trenchantaphorism is, of course, a restatement of theorganism-the-machinetheory of Descartes. But asWald himself said on another occasion (Wald,1958),Confrontingany phenomenonn living organisms, hebiologist has always to ask three kinds of questions,each independent f the others: the questionof mech-anism (how does it work?), the questionof adapta-tion (what does it do for the organism?), the twinquestionsof embryogenyand evolution (how did itcomeabout?).The first kind of questions call for Cartesian,reductionist; the other kinds for Darwinian,compositionist, answers (Dobzhansky, 1964;Simpson, 1964b). Organisms do not arise byaccidental conflux of molecules. The creaturesthat are alive today are the products of unbrokensequences of patternings of molecular components;these sequences extend back to the origin of life,two or more billion years ago. Every generationinvolves formation and dissolution of a pattern, butthe consecutive patterns are not independent. Theyare products of accumulation and storage ofgenetic information. Natural selection is a cy-bernetic process which transfers the informationconcerning the state of the environment to thegenotype.Already Darwin grappled with the difficultythatthe formation in evolution of complex organs, suchas the vertebrate eye, seems an improbable event.A few years ago, one of the outstanding livingmathematicians sent me a long and closely arguedprivate letter, in which he urged that a combinationof many gene mutations adding up to such anorgan is so absurdly improbable that we have tosuppose that organic evolution is guided by adeity. I cannot gainsay his mathematics, butbiological mathematics is at best only as valid asthe biological assumptions on which they rest.The assumption implied in his argument was



8/12

VOL. 109, NO. 4, 1965] MENDELISM, DARWINISM AND EVOLUTIONISMthat, in order that an organ be formed, numerousmutants must arise and all come together in oneplace at the same time. This is, indeed, toofar-fetched to credit. But it is the assumption thatis at fault. Natural selection was working in along succession of generations; it was not aimingto build the organ or the body which we nowobserve in a state of relative perfection; it wasacting to modify the structures and the functionsof a succession of ancestral organs and bodies inaccord with the challenges coming from theancestral environments.The argument is out of focus also in anotherway. It tries to envisage the evolutionary develop-ment, the phylogeny, as though it were an indi-vidual development, an ontogeny. An individualbegins as a single cell, a fertilized ovum, andproceeds to develop through a complex series ofmaneuvers. Body structures and functions thatare formed fit together as if planned by someforesight for the purpose of making a body whichcan live in a certain environment. Ontogenyseems to be attracted by its end rather thanimpelled by its beginning. This is an astoundingthing for a pile of molecules to do, and Sinnott(1950-1957) sees himself forced to assume thatthe development is governed by a psyche, a newname for the old vital force. This misrepresentsboth the ontogeny and the phylogeny.

Individual development is understandable onlyas part of the phylogenetic development of thespecies, not the other way around. The ontogenyfollows a certain course, because it is a part ofa cyclic (more precisely, a spiral) sequence of thedevelopments of the ancestors. Organs in adeveloping individual are formed for future uses,because in evolution they were formed for con-temporaneous utility. The development of anindividual may be said to end in death; a betterway of understanding it is to say that it continuesin the progeny. It is a part of the process ofthe storage of genetic information which continuesthrough time. Ontogeny may be likened to build-ing an automobile or some other complex machineon an assembly line. The automobile is not beingused while on the assembly line, it is being pre-pared for future uses. Phylogeny is more likethe gradual derivation of the present automobilemodels from the primitive ones, and eventuallyfrom coaches, chariots, and pushcarts. Naturalselection performs the role of the engineer-itdevises both the ways to improve the models andthe techniques of manufacturingthem.

VIIn discovering the genes, Mendel has, withoutknowing this, furnished the keystone of the archwhich Darwin was building. With bloodheredity, biological evolution would, at best, beexceedingly slow and inefficient; with gene hered-ity, evolutionary mechanisms are comprehensible.In turn, the theory of biological evolution is thekeystone of the evolutionary world view. How-ever, it is useful to be reminded that the cosmicand the cultural evolution theories were arrivedat before the biological one. The nebular hy-pothesis of Kant (1755), Herschel (1791) andLaplace (1796) antedates the biological theory ofLamarck (1809), and the uniformitarian geologyof Hutton (1795) and Lyell (1830) comes beforeThe Origin of Species of Darwin (1859). Therise of the evolutionary view of man is less easyto date. Condorcet's (1793) inspired vision ofthe ten periods of historical development ofmankind was clearly evolutionistic. Herder'sIdeas of the Philosophy of the History of Hu-manity (1784) leads the way to the evolutionaryspeculations of Fichte (1806) and to Hegel'sPhilosophy of History (1837).Evolutionist world views consider the inorganic,the organic or biological, and the human or cul-tural evolutions as integral parts of a universalevolutionary development. On the other hand,some people have objected that biological evo-lution is an extension of the inorganic, andhuman evolution an extension of the biological,only in a chronological sense. Is it legitimate touse the word evolution for such disparateprocesses? I believe that it is legitimate, andyet the objection contains a kernel of truth anddeserves consideration. The elementary compo-nents of the biological evolution are mutations,changes in the hereditary materials. Mutationpresupposes heredity, and heredity is self-repro-duction, or self-copying, of certain molecularpatterns, which exist only in living systems, andwhich are, in fact, the chief characteristics oflife. These carriers of genetic information arethe nucleic acids and, secondarily, proteins.Furthermore, the process of mutation suppliesonly the genetic raw materials, from which evo-lutionary developments may or may not be con-structed by natural selection, Mendelian recom-bination, and other processes.Natural selection is predicated on mutation andself-reproduction, and hence on life. To applythe term mutation on the human level to novel

211


9/12

THEODOSIUS DOBZHANSKYideas and inventions, is to use a vivid but rathermisleading analogy. The same must be saidconcerning natural selection of physico-chemicalprocesses in the inorganic nature, which sup-posedly led to the origin of life on earth. Tohave natural selection, life must already be present,because natural selection is differential repro-duction, and reproduction is a basic characteristicof life. Culture is learned behavior which isshared by members of a human group. In non-human animals only barest traces of such be-havior can be found. Culture is not inheritedbiologically through some special genes; it islearned, i.e., acquired, by every individual inevery generation. Acquired biological traits arenot inherited; all the so-called cultural inheritanceis, on the contrary, acquired.

Inorganic, organic, and human evolutions occurin different dimensions, or on different levels, ofthe evolutionary development of the universe.The changes in the organic evolution are morerapid than in the inorganic. Nevertheless, theinorganic evolution did not come to a halt with theappearance of life; organic evolution is super-imposed on the inorganic. Biological evolutionof mankind is slower than the cultural evolution;nevertheless, biological changes did not ceasewhen culture emerged; cultural evolution is super-imposed on the biological and the inorganic. Theevolutionary changes in the different dimensionsare connected by feedback relationships.The attainment of a new level or dimension is,however, a critical event in the evolutionaryhistory. I propose to call it evolutionary tran-scendence. The word transcendence is obvi-ously not used here in the sense of philosophicaltranscendentalism. I am using it in the samesense as Hallowell (1960): The psychologicalbasis of culture lies not only in a capacity for highlycomplex forms of learning but in a capacity fortranscending what is learned, a potentiality forinnovation, creativity, reorganization and change.Erich Fromm (1959) wrote that man is drivenby the urge to transcend the role of the creature,and that he transcends the separateness of hisindividual existence by becoming part of some-body or something bigger than himself.Dubos (1962) said that what is still so com-pletely mysterious as to acquire for many humanbeings a mystical quality, is that life should haveemerged from matter, and that mankind shouldhave ever started on the road which so clearly istaking it farther and farther away from its

brutish origin. This is just as mysterious, butI hope no more so, as is the ability of life tocontinue amidst hostile environments. Cosmicevolution went beyond the range of inorganicprocesses when it produced life. The origin of manwas a transcendence of biological evolution, be-cause it opened up a new range of potentialities,of processes and events, which occur exclusivelyin man or under the influence of man. Thesefateful transcendences are not, however, beyondhope of understanding. They may be envisagedas extreme cases of evolutionary innovation, lesserexamples of which are also known. A quantitativedifference may, to be sure, be large enough toappear as a qualitative one. The origin ofterrestrial vertebrates from fishlike ancestorsopened up a new realm of adaptive radiations inthe terrestrial environments, which was closed towater-dwelling creatures. The result was whatSimpson (1953) has called quantum evolution,an abrupt change in the ways of life as well as inthe body structures. Domestication of fire andthe invention of agriculture were among themomentous happenings which opened new pathsfor human evolution. In a still more limitedcompass, the highest fulfillment of an individualhuman life is self-transcendence.

Rough stone tools have been found in associationwith australopithecineremains both in east-centraland in South Africa. Homo erectus in China isthe oldest known user of fire. The Neandertha-lians were burying their dead. These are evi-dences of humanization. Some animals, birds, andeven insects are known occasionally to use objectsas tools, but intentional manufacture of a toolis a sign of a psychic organization known toexist in man alone. All animals die, but manalone knows that he will die; a burial is a signof a death awareness, and probably of the ex-istence of ultimate concern. The ancestors ofman began to transcend their animality perhaps asearly as 1,700,000 years ago. The process isunder way in ourselves. Nobody has charac-terized this process more clearly than Bidney(1953):Man is a self-reflectinganimal in that he alone hasthe ability to objectify himself, to stand apart fromhimself,as it were, and to considerthe kind of beinghe is and what it is that he wants to do and to be-come. Other animals may be conscious of their af-fects and the objects perceived; man alone is capableof reflection, of self-consciousness, of thinking of him-self as an object.



10/12

VOL. 109, NO. 4, 1965] MENDELISM, DARWINISM AND EVOLUTIONISMAnd according to Hallowell (1959):The great novelty, then, in the behavioralevolutionof the primates,was not simply the developmentofa culturalmodeof adaptationas such. It was, rather,the psychological restructuralization hat not onlymade this new mode of existence possible but pro-vided the psychologicalbasis for cultural re-adapta-tion andchange.

To an orthodox reductionist, the concept ofevolution transcendence may sound faintly vital-istic. A similar view has, however, been arrived atby the simon-pure dialectical materialists inRussia. Despite his Marxist jargon, Present(1964) states it fairly clearly as follows:Wherever t arose,the humansocietymusthave comefrom the zoological world, and it was work, theprocessof production, hat mademan human. How-ever, what has removedpeoplefrom the animalwayof life and gave a specificity to their (new) life,became the essence and the basis of the history thatensued. . . . Likewise, in the realm of living nature,what removed the novel form of material motionfrom its nonliving prehistory, necessarilybecameitsessence, ts fundamental asis.Reductionism is not wrong, but it tells only apart of the story. Where man is concerned, itis only a small part. Reductionism must go handin hand with compositionism, Cartesian withDarwinian inquiry and discovery.

VIIMendel, a peasant's son, found an opportunityfor his intellectual pursuits only behind a mon-astery's walls; Darwin, a wealthy English countrysquire, made the study room in his house hislaboratory. Neither of them was a professionalscientist, and unknown to each other (Mendelread some of Darwin's books probably after hisown biological work was finished), they collabo-rated to lay the foundations for an evolutionaryworld view. The universe, life, a man, areevolving products of evolutionary developments.It is often alleged that Darwin's evolution theoryhas rendered complete man's alienation from theworld which he inhabits. Copernicus and Galileoshowed that man is not the center of the world,and that the earth is but a speck of dust in thecosmic spaces. Before Darwin, man was believedto be only slightly lower than the angels,Darwin showed that he is only slightly higherthan brute animals. And animals are, to con-sistently reductionist biologists, automata onlyslightly more complicated than watches, and per-haps less complicated than some electronic com-

puters. All this misses the main point. Evolutionmeans that, whether one considers the presentstate of the world and of man satisfactory orotherwise, it is not necessarily fixed and unchange-able forever. It is at least thinkable that manmay recast the whole situation in a directionwhich he believes to be good, even though a longtime and much effort may be needed to accomplishthe reform. Evolutionist world views range fromdeeply pessimistic to brightly optimistic ones.To Sir Julian Huxley, H. J. Muller, Sir CharlesGalton Darwin, and others, mankind is headedfor biological twilight, unless something is veryquickly done to rescue it. And what will a worldwithout man be worth? The development ofculture and civilization has brought about anunpremeditated reversal of the trend of the bio-logical evolution from beneficial to nefarious.Mankind evolved as it did because natural select-ion fostered improvements of the genetic basisfor intelligence, group solidarity, cooperation, and,so it is believed, for human ethical values. Civi-lization has tended increasingly to frustrate andpervert the action of natural selection. Manykinds of hereditary infirmities and weaknesses arecured or relieved by ministrations of the medicalarts; the carriers of genetic defects are helpedto survive and to reproduce, thus increasing theincidence of the same defects in future generations.Living in dense populations, particularly incrowded cities, may have also more subtle butsinister effects. When nuclear families and evenindividuals must be sufficient unto themselves,instead of mutual help being enjoined on all bycustom, natural selection which in the past favoredaltruism may now favor selfishness.The way out is an eugenic selection of desirabletypes. One must begin, with all deliberate haste,to collect and preserve in deep-frozen conditionthe semen of eugenically approved donors, partic-ularly of great and illustrious men. This willbe utilized for artificial insemination of numerouswomen. Eventually techniques should be de-veloped to obtain and preserve also the egg cellsof superior women. Even more ambitious meth-ods may be possible in the future. Sir CharlesGalton Darwin thinks, however, that the willing-ness of people to regulate their procreative activi-ties taking in consideration the common good is it-self a genetic trait. If so, those who fail to heedsuchconsiderationswill outbreed those who do, andtheir uncooperativeness will grow more and morefrequent in future generations. A human flood,

213


11/12

THEODOSIUS DOBZHANSKYrising higher and higher, will overwhelm a multi-tudinous but degenerate mankind. The nextmillion years will see the eclipse of the humanspecies.The evolutionary world view of Teilhard deChardin is in a different key.1 Its considerationmust, unfortunately, begin with a refutation ofthe author's statement in the first paragraphof thePreface to his most widely read book (1959: p.29): If this book is to be properly understood,it must be read not as a work on metaphysics, stillless as a sort of theological essay, but purely andsimply as a scientific treatise. Read as a scientifictreatise, it is equivocal, as has been pointed outby scientific reviewers, sometimes in needlesslyscathing ways. Teilhard was a Christian mystic,who happened to be also a scientist, a meta-physician, and a poet. This can be seen in hisother books (e.g., Teilhard de Chardin, 1960,1964), which expound the same evolutionaryworld view as The Phenomenon of Man, withoutclaiming to be purely and simply scientific trea-tises. However, it is sheer misunderstanding tosee in Teilhard's writings attempts to derive hisreligious beliefs from, or to prove them by, hisscience. What he is trying to do is rather toinclude his science in his total world view, whichis basically a religious one. Such an attempt isof interest to scientists. We have heard a greatdeal in recent years about the divorce of the two,or several, cultures, about science being a glori-ous entertainment, etc. Teilhard attempts toeffect a reunion.Teilhard's basic insight is that the cosmic, bio-logical, and human evolutions are not only com-ponents but are developmental stages of a singleprocess of universal evolution. This single processhas a discernible direction. It has advanced frommatter, to life, to thought. Teilhard's extrapola-tion anticipates further advances, to the comingmega-synthesis and to the Omega point. Adifficulty arises because of his unfortunate use ofthe word orthogenesis to describe the direction-ality of the organic evolution. The direction-ality is indisputable. We do not know what theprimordial life was like, but it must assuredly havebeen represented by some very simple forms.More complex organisms developed later. Land

1 In the Introduction to the English translation of Teil-hard's The Phenomenon of Man (1959), Sir Julian Hux-ley claims that Teilhard's ideas are mostly similar tothose published earlier by himself. This is true only inso far as both authors are, of course, evolutionists. Be-yond this, their ways of thinking are almost at polaropposites.

plants appeared in the Silurian period, landanimals in the Devonian, first mammals in lateTriassic and early Jurassic, first primates inPaleocene, hominids in late Pliocene or earlyPleistocene. However, orthogenesis was notsimply a word describing the fact of directionality,but a now almost defunct hypothesis pretendingto explain the causation of this directionality.It postulated that evolutionary changes are theunfolding or manifestation of preexisting rudi-ments. Evolutionary changes are predetermined,in the same way that ontogenetic changes, fromembryo to adult to death, are predetermined.The comparison between ontogeny and phylogenyis, to believers in orthogenesis, more than a simpleanalogy. It is envisaged as a causal similarity.This is inconsistent with Teilhard's basic viewthat the organic as well as human evolution pro-ceeds by groping (tdtonnement). Gropingis pervading everything so as to try everything,and trying everything so as to find everything.Ontogeny and orthogenesis do not try anything,because they move in a straight line toward apredetermined end result. The grouping leadsto a succession of layers (nappes), of progres-sively more complex levels of organization ofmatter, of life, and of thought. This is neitherorthogenesis nor vitalism. Mendelian recombi-nation of genes is the way, on the biological level,of pervading everything so as to try everything,i.e., to try out as many genotypes as can beformed. Teilhard did not know that the numbersof potentially possible genotypes are far greaterthan the numbers of individuals in which they canbe realized and exposed to natural selection.Trying everything so as to find everything isa splendid metaphorical description of the oper-ation of natural selection.Teilhard was sceptical concerning the compe-tence of natural selection to arrive at evolutionaryinventions. This seemed to him relying toomuch on chance. He did not realize thatnatural selection is not building perfect organismsout of piles of unrelated genes; selection acts ona succession of parental and descendant gener-ations modifying the organisms to fit their environ-ments. Any orthogenetic theory of evolutionpostulates preformation; all that happens wasbound to happen; man and animal and tree wereequally present in the primordial life, and itjust took time to have them gradually emergefrom their hidden to their manifest state. Thisis completely contrary to Teilhard's basic philos-ophy of universal evolution being a creative



12/12

VOL. 109, NO. 4, 1965] MENDELISM, DARWINISM AND EVOLUTIONISMprocess, not just an unveiling of what was thereall the time in a concealed state. Creation impliesthe risk of miscreation, and Teilhard envisagedthe possibility of the evolution being a failure:There is a danger that the elements of theworld might refuse to serve the world-becausethey think; or more precisely that the world shouldrefuse itself when perceiving itself through re-flection. Having been a palenotologist, Teilhardwas familiar with the phenomenon of extinction ofphyletic lines. Believers in orthogenesis assumethat the cause of extinction is a senescence ofthe phyletic line, predetermined by the organiza-tion of the latter in much the same way as thesenescence and death of an individual organism.Predetermination is foreign to Teilhard's think-ing. If all that happens in evolution is a longstrip-tease act, all evolution becomes meaningless.Why should there be such a delay in reaching thestate of final perfection? This, together with theproblem of the existence of evil in the world, wouldvitiate any attempt to build a theodicy, an under-standing of the meaning of God's creative activity,which is in the center of Teilhard's whole thought.Evolution is meaningful only if it involves cre-ativity and freedom. Extinction is comprehensiblebecause evolution is, to use Teilhard's metaphor,groping in the dark, among dangers and pit-falls. Extinction is a consequence of becomingirrevocably adapted to environments which donot last.

Despite the dangers and pitfalls, evolution hasbeen, on the whole, a success rather than a failure.It has achieved the two great transcendences,the origin of life and the origin of man. In thisarticle, which has attempted to trace the directionsin which Mendel's work has led evolutionary bio-logy, it would be out of place to discuss Teilhard'sextrapolations that the evolution will eventuallyreach the transcendences of the mega-synthesisand the Omega. It is perhaps appropriate toconclude in Teilhard's words:The outcome of the world, the gates of the future, theentry into the super-human-these are not thrownopen to a few of the privileged, nor to one chosenpeople to the exclusion of all others. They will opento an advance of all together, in a direction in whichall together can join and find fulfillment in a spiritualrenovation of the earth....2

2Somewhat similar ideas were a part of the creed ofthe Tientai (Tendai) sect of Buddhism, which arose inChina in the sixth centuryA.D. One of the tenets of thissect was that all human souls, and even all that exists,will eventually rise to the dignity of Buddha himself(cf. Anesaki,1963).

REFERENCESANESAKI, M. 1963. History of Japanese Religion (Rut-land & Tokyo, Ch. Tuttle Co.).BIDNEY, D. 1953. Theoretical Anthropology (NewYork, Columbia Univ. Press).CHETVERIKOV,. S. 1926 (1961). On Certain Aspects

of the Evolutionary Process from the Standpoint ofModern Genetics. Translated by Malina Baker.Proc. Amer. Philos. Soc. 105: pp. 167-195.DOBZHANSKY,TH. 1954. Evolution as a CreativeProcess. Proc. 9th International Congress Genetics,Caryologia, Suppl. 6: pp. 435-449..1959. Variation and Evolution. Proc. Amer.Philos. Soc. 103: pp. 252-263.1960. Evolution and Environment. In: S. Tax(ed.), Evolution after Darwin 1: pp. 403-428.1964. Biology, Molecular and Organismic.Amer. Zoologist 4: pp. 443-452.DOBZHANSKY,TH., and 0. PAVLOVSKY. 1953. Inde-terminate Outcome of Certain Experiments of Droso-phila Populations. Evolution 7: pp. 198-210.DOBZHANSKY, TH., and N. SPASSKY. 1962. GeneticDrift and Natural Selection in Experimental Popula-tions of Drosophila pseudoobscura. Proc. Nat.Acad. Sci. 48: pp. 148-156.DUBos, R. 1962. The Torch of Life (New York, Tri-dent Press, Simon & Schuster).FISHER, R. A. 1930. The Genetical Theory of NaturalSelection (Oxford, Clarendon).FROMM,E. 1959. Value, Psychology, and Human Ex-istence. In: A. H. Maslow (ed.), New Knowledgein Human Values (New York, Harper).HALDANE, J. B. S. 1932. The Causes of Evolution(London, Longmans Green).HALLOWELL,. I. 1959. Behavioral Evolution and theEmergence of the Self. In: Evolution and An-

thropology (Washington, Anthrop. Soc.), pp. 36-60.HESLOP-HARRISON,. 1958. Darwin as a Botanist.In: S. A. Barnett (ed.), A Century of Darwin, pp.267-295.PRESENT, I. I. 1964. On the Essence of Life in Con-nection with Its Origin. (In Russian.) In: O su-shchnosti zhizni (Moscow, Nauka).SIMPSON, G. G. 1953. The Major Features of Evolution.(New York, Columbia Univ. Press)..1964a. This View of Life (New York, HarcourtBrace).. 1964b. Organisms and Molecules in Evolution.Science 146: pp. 1535-1538.SINNOTT,E. W. 1950. Cell and Psyche (Chapel Hill,Univ. North Carolina Press).

. 1957. Matter, Mind and Man (New York,Harper).STRICKBERGER,. W. 1963. Evolution of Fitness inExperimental Populations of Drosophila pseudoob-scura. Evolution 17: pp. 40-55.TEILHARD DE CHARDIN, P. 1959. The Phenomenon ofMan (New York, Harper).. 1960. The Divine Milieu (New York, Harper).. 1964. The Future of Man (New York, Harper).WALD, G. 1958. Innovation in Biology. Scient. Amer.199: pp. 100-113.. 1963. Phylogeny and Ontogeny at the MolecularLevel. In: A. I. Oparin (ed.), Evolutionary Bio-chemistry (London, Pergamon).WRIGHT, S. 1931. Evolution in Mendelian Popula-tions. Genetics 16: pp. 97-159.

215

dobzhansky - mendelism

Documents