Chapter 31 Chapter 17 The Growth of Mendelian Genetics-1

Darwin's "Variation of Animals and Plants Under Domestic Conditions" (1868), de Vry's "Pangenesis in the Cell" (1889), and Weismann's Theory of Germplasm (1892) promoted attention to genetic problems.De Vry and Collens carried out systematic hybridization experiments in 1892, and in 1899 both published the important results of their heteropowder (endosperm formed from pollen nuclei, see Dunn, 1966) experiments.Shortly thereafter, in the spring of 1900, a rare event in the history of biology occurred, which seemed very sudden but was actually the culmination of a long development of events.Three botanists, De Vry, Collens, and Chuchemark, published articles within a few months claiming that they had independently discovered important genetic laws, but they only found out about Mendel's theory when checking the literature. This law had been discovered 35 years ahead of them in the 1860s.Since then, there have been doubts about the veracity of the three botanists' claims.This question appears to be of crucial importance and deserves a closer analysis.

17.1 The biologist who rediscovered Mendel In his book "Intracellular Pangenesis", de Vry has clearly stated his point of view that heredity is divided into unit traits, and each unit trait is inherited independently.He also drew up a pilot program.Because he was engaged in physiological experimental research at the same time, it was not until 1892 that he seriously carried out hybridization experiments. At the beginning, he used plants such as Silene, poppy, and evening primrose: in 1894, he used 536 plants of F2 generation of Silene. 392 were found to be hairy and 144 were hairless (2.72:1). In 1895, he found 158 plants with black spots on the petals and 43 plants with white spots (3.67:

1); In 1896, he discovered that the white poppy was purely one-generation.His other experiments over the years confirmed these findings. In the autumn of 1899, de Vry observed a clear separation in species and varieties of more than 30 stalks.In the end he argued that segregation of corresponding traits obeyed some general law and felt that there were good reasons for publishing these results. In March 1900, he wrote three articles describing his discovery in a few weeks, two to the Paris Academy of Sciences (to be read at the meeting on March 26, 1900), and one to the German Botanical Society (received March 14) (see Krizenecky, 1965).The publication date of the article he sent to Paris (before April 21) was actually a few days earlier than the publication date of the article sent to Germany (April 25).In a footnote to the article sent to Germany, he wrote: "I first became aware of Mendel's article after I had completed most of the experiments on which I wrote this article." Olby (1966: 129) Judging by the overwhelming amount of circumstantial evidence, it is probable that de Vry had read Mendel's papers as early as 1896 or 1897.

Zirkle (1968) puts it at 1899, and Kottlet (1979) points to 1899 based on further evidence. During these years de Vry still used his own terms in his syllabus—active (A), latent (L)—instead of Mendel's explicit and implicit, In the wall chart, he expressed the separation with different percentages (77.5%: 22.5%, 75.5%: 24.5%), it seems that he did not understand the real reason for the separation.It is also worth mentioning that de Vry conducted a large number of hybridization experiments with evening primrose, and in his 1900 article he only selected this example (Lamarck evening primrose X short column evening primrose), which is in his The evening primrose test material was the only true genetic mutation he found.As he made clear in his correspondence with Bateson, he distinguished evolutionary traits from reproductive traits, only the latter obeying Mendel's laws.

De Vry has said that he found Mendel's list of articles in the references of an article published in 1892, and he apparently referred to the above-mentioned article in the years after 1892 and prompted him to read Mendel. Del's original text.Doubtless he knew about the segregation ratio (which we now interpret as the 3:1 ratio) and the recessive homogeneous passage at that time, but this does not necessarily mean that these discoveries would have prompted him to abandon his original error idea.Like all other researchers in the 1880s, de Vry originally thought that traits might be controlled by multiple particles (see Chapter 15).Ratios like 394:144, 158:34, or 77.5%:22.5% are meaningless to someone who believes that replicators determine traits.When using ratios, de Vry refers to 2:1 or 4:1 (Kottler, 1979).Did reading Mendel's article prompt him to abandon his original theory and accept the Mendelian theory that "one factor from each parent determines the individual's traits"?We will never know.That being the case, we must accept de Vry's claim that he "deduced" the law of separation from his own experiments, just as Mendel derived his theory from the results of similar experiments.De Vry devoted himself to the experimental analysis of unit traits, and was indeed very close to the solution of the problem.A small step further would have given up the last erroneous part of his original doctrine (often duplicating pans).However, Bateson did not make a Mendelian explanation even though there were a large number of Mendelian ratios before reading de Vry's article.

De Vry's apparent dismay at discovering that Mendel had gotten ahead might have been one reason why he moved away from exploring the deeper genetic consequences of his discoveries to an evolutionary interpretation of evolutionary mutation.Speciation seems to have been his main concern.De Vrij apparently believed that Mendelian inheritance was only one of many mechanisms of inheritance, otherwise it would be impossible to explain what he said in a letter to Bateson "It seems to me more and more clear that the Mendelian An exception to the rule." Thus, he somewhat abandoned Mendelianism to study other forms of inheritance that he believed to be more important for evolution.

There are three reasons why de Vry will always be remembered as a great scholar in the history of genetics: (1) Independently of Mendel, he put forward the idea of ​​dividing the differences between individuals into unit traits; (2) He first confirmed the existence of Mendelian segregation in a large group of various plants; (3 ) He developed the concept of mutationality of genetic units. So he was by no means just one of Mendel's discoverers.Of course de Vrij had an advantage over Mendel.He was able to use the new results of cytology research at that time to develop his theory.When Mendel wisely eschewed the question of genetic "factors"

(Elemente) essence is the exploration of its material basis, but de Vry connects it with the redefined Darwinian pan.As far as genetic phenomena are concerned, de Vry synthesized Darwin and Mendel. The case of the second rediscoverer of Mendelian inheritance, Carl Correns (1864-1933), was much simpler.He once said that Mendel's dissociation theory came to his mind "like a lightning bolt" while he was lying awake in bed waiting for daylight (one October 1899).He was busy with other research at the time and only read Mendel's paper a few weeks later (pointed out in his December 1899 article on Heterogeneity).When he received de Vry's copy of the French (Paris) Academy of Sciences article (April 21, 1900) he did not (within a day) write up the results of his experiments and present them at the German Botanical Society on April 27. Presented at the conference, followed by publication on or about May 25.From the beginning, Colrens did not think that he played an important role in the rediscovery of Mendel.In the title of one of his bulletins is "Mendel's Law".He believes that "as far as I am concerned, the intellectual labor required to rediscover these laws (due to the extensive research in biology in the past _ years, especially the work of Weissmann) is greatly reduced compared to Mendel." Lens independently rediscovered Mendel's problems, and the only doubt is that he was a student of Negri (his wife was Negri's niece) and may have known about Mendel's work.This possibility is hard to accept, however, and it would be very strange if Currens had known about Mendel's work 20 years earlier and he hadn't followed this lead sooner.

The third person who has been credited with independently rediscovering Mendel's laws is the Austrian plant breeder Erich Tschermak, and as Stern (1966) points out, there is little reason to list Tschermak as a rediscoverer.He did see Mendel's papers, but in his 1900 paper he showed that he did not understand the fundamentals of Mendelian inheritance.Yet he played an active role in drawing the attention of plant breeders to important aspects of Mendelian genetics. Why many of the early Mendelians happened to be botanists (Mendel, de Vry, Currens, Churchmark, Johnson) is never explained.Perhaps there is a greater tradition of selective breeding in horticultural plants and other cultivated plants, since plants are easier to cultivate and breed than animals.Leaves and flowers may have more discrete traits than livestock such as sheep, cattle, and pigs.Most traits studied by animal breeders are highly polygenic and simply not amenable to simple Mendelian analysis.However, in early 1900, Bateson began to study poultry, Cuenot in France and Kessel (19o2) in the United States began to study rodents, and in 1905 Kessel adopted fruit flies as experimental animals.

The study of animal genetics soon caught up with plant genetics, and when the Morgan school and the Chetvinikov school carried out their research work, it surpassed plant genetics.By 1914, A. Lang's monograph, which only reported the research results of mammalian genetics after 1900, took up 890 pages. The diversity of genetic systems in plants (even higher plants) is much higher than in animals.This is easily misleading for those who want to establish general laws.For example, the apomixis (apomixis) system of the mountain willow chrysanthemum (Hieracium) frustrated Mendel's research, and the balanced heterozygous chromosome ring of the evening primrose led de Vry to propose a false speciation theory , self-pollinating nearly homozygous bean (Phaseolus) led Johnson to downplay the role of natural selection.Cytoplasmic effects are more common in plants than in animals, so many plant geneticists (especially in Germany) have focused so much on this aspect that (before molecular genetics) no particularly important research results have been made.On the other hand, the plant kingdom provides not only peas but also cereals (especially wheat, barley, maize), cotton, tobacco, and many other genetically interesting species.No one has yet undertaken a comparative study of the advantages and disadvantages of the various animal and plant species employed in genetic research.It must be admitted that most of the work just confirms the facts already established by fruit flies or maize.Prior to molecular genetics, most genetic research was conducted separately in departments of botany or zoology, and scholarly exchanges between plant geneticists and animal geneticists were not always as active as might be hoped. After the 1930s, lower plants (algae, fungi, yeast) and protozoa (bacteria, viruses) increasingly became the experimental materials that geneticists paid attention to.Due to the recognition of the major differences in genetic systems between eukaryotes and prokaryotes, interest in the study of eukaryote genetics was rekindled after the 1960s.

17.2 The Golden Age of Mendelian Genetics The early history of genetics can be divided into two phases, the first from 1900 to about 1909 and the second from 1910 onwards.The first, often called the Mendelian period, focused on controversies about evolution and whether Mendelian inheritance was universally valid.The main exponents of this period (phase) were de Vrye, Bateson, and Johnson, often referred to as "early Mendelians." Different people have different opinions on "Mendelism" The term has different meanings, depending on which aspect of Mendelianism he is emphasizing.For scholars who established genetics, it refers to the period when granular inheritance was definitely settled, and the emphasis was on hard inheritance.For evolutionists, "Mendelianism" refers to the period during which certain prominent geneticists propagated completely erroneous views on the problem of evolution and speciation, during which Neutral mutation pressure is thought to be far more important than natural selection, and such views are just as alien to naturalists.Thus, the same word "Mendelian" is sometimes used in a favorable or supportive sense, and sometimes in a pejorative sense. The second stage began in 1910, mainly represented by the Morgan school; it mainly focused on the research of pure genetics issues, such as the nature of genes, the arrangement of genes on chromosomes, and so on.The word "genetics" suggested by Bateson in 1906 was later generally accepted as a broad concept of the science of genetic phenomena. It took 34 years for Mendel's publication to be rediscovered, but once rediscovered it was widely disseminated at an unprecedented rate.Neither Colrens nor Churchmark had seen de Vry's article in April 1900.And published their own related articles in May and June respectively.Bateson presented Mendel's experiments at a meeting of the Royal Horticultural Society on May 8, and Mendel's work was soon presented in Cuenot, France. As with many important scientific events, the momentum that followed varied from country to country.There is no doubt that Britain is far ahead in the progress of Mendelian genetics, but it was soon overtaken by the United States (representatives of the United States are Castle, East, Morgan and other scholars).German genetics still carried on the tradition of the 1880s, focusing on developmental genetics and some uncommon genetic phenomena (real or apparent cytoplasmic inheritance, protozoan inheritance, etc.).In France, Cuenot didn't do much until the 1930s after a good start.In the Soviet Union, as Gaissinovich (1971) pointed out, "genetics did not develop as a science until the Soviet period".In north-western Europe no genetic science was born.Where genetics flourishes and in which direction it develops depends entirely on the leaders in the field.Curiously, however, neither Currens nor de Vry played a major role in the subsequent development of Mendelian genetics.This credit, at least early on, must go to Bateson (1861-1926), who appreciated the significance of Mendelian genetics far more than the so-called rediscoverers. Bateson has been studying at Johns Hopkins W. K． During his stay in Professor Brooks's laboratory (1883-1884), he was very interested in discontinuous variation (see Part II) and carried out breeding experiments since the 1980s, but he really concentrated on this research around 1897 . On 11 July 1899 he read a paper to the Royal Horticultural Society entitled "Hybridization and cross-breeding as a method of scientific investigation".It can be seen from this paper that he had not proposed the theory of genetics at that time, although there were many experimental results that were easy to explain according to Mendel's point of view.It was not until he read Mendel's original work on the train from Cambridge to London on May 8, 1900 that he was deeply inspired.He soon became an ardent Mendelian and published a translation of Mendel's text with footnotes in the Journal of the Royal Horticultural Society (1900). Part of Bateson's enthusiasm stemmed from his belief that Mendel's segregation theory was an affirmation of his (erroneous) thesis that "speciation is the result of discrete variation".De Vry also proposed a similar theory of evolution and also believed that the discontinuity of Mendelian genetic factors was an important evidence for his theory of catastrophe formation.Thus, it turns out that Mendel's teachings have attracted widespread attention due to a true rather than a superficial (if not erroneous) understanding.The objections to the Bateson-De Vry argument have been introduced in Chapter 12, and I shall confine myself to Bateson's contribution to transmission genetics. Most of the important terms in genetics were coined by Bateson.He coined "genetics" for the new discipline This new word, and in 1901 created the first "allele" (allele, originally allelomorPh, later simplified), "homozygous", "heterozygous".Having these semantically clear terms greatly facilitated scholarly communication during this period. Of course, Bateson and his colleagues also made substantial contributions to our understanding of genetic phenomena.They were the first to discover situations that did not correspond to simple phenomena observed by Mendel (such as polygenicity, incomplete linkage).It was through Bateson that genetics gained in England a momentum or momentum that it had absolutely nowhere else in Europe. Bateson was a complex figure, pugnacious and borderline brutish in debate, but at the same time totally dedicated to his cause.He was a strange mixture of conservative and revolutionary.During the first decade after 1900 he was the leading activist in genetics, and Caslle (1951) is actually quite plausible when he says that Bateson "is the true founder of genetics." However, in 1910 His subsequent opposition to the chromosome theory and his continued insistence on the sudden formation of species could no longer be said to be constructive.As a revolutionary he left immortal words (1908:22): "Treasure the exceptions you find; without exceptions the work would be so dull that no one would bother to push it further .to keep these exceptions in full view forever. Exceptions are like the rough stones of a mansion under construction, which tell how to proceed and indicate where the next component should be placed.” In his own research work, He paid great attention to actual or apparent exceptions, and some of his important discoveries were the result of following this precept. The speed with which new discoveries in genetics have been made after 1900 is almost without precedent in the history of science.Whether we consult Lock's textbook of genetics (1906, especially pages 163-275) or Bateson's textbook (1909), we can wonder how mature the understanding of Mendelian genetics was in the immediate post-1900s. rather surprised. What is the reason for such rapid progress?One reason, of course, is that the beauty and simplicity of the new doctrine itself is enough to tempt anyone to carry out genetic experiments to see if it is generally valid.Because this is new frontier, almost anyone has the opportunity to make new discoveries.Mendel's laws make predictions about patterns of inheritance and immediately test those predictions. Another reason is still inconclusive, that is, it is believed that the brilliant achievements of cytological research in the 35 years before 1900 have laid a solid foundation. It should be possible to explain almost all purely theoretical heredity from the perspective of cytology, especially from the perspective of chromosomes. Learn to discover.Chromosome cytology has become a bridge to other fields of biology that was built before it could be used.However, it is strange that even when it can be used, it has been almost completely ignored by geneticists, such as Bateson, Kessel, East, etc. before Morgan. Knowledge of the mechanisms involved in genetic phenomena is applied in various fields of biology, such as evolutionary biology (see Chapters 12 and 13), and developmental physiology.The following will focus on the transmission genetics aspect. Of the seven pairs of traits Mendel analyzed, he identified only two variants for each pair of traits, dominant and recessive.But as Mendel himself discovered, this is not true for all pairs of traits (paired traits).He once said that the flowering period is "almost exactly between the parent plants." Kollens (1900) also found that some factors were not fully dominant but "semi-dominant", so the F1 phenotype produced was more or less between between parents. Two years later, Bateson discovered the semi-dominance of the blue Andusia (Spain) chicken when crossing white and black chickens. This not only confirms the existence of semi-dominance but also shows that Mendel's laws apply to both animals and plants.This was also demonstrated by Cuenot around the same time, based on studies of the coat-color gene of the house mouse.Given the fact that animal and plant cells and nuclei show exactly the same phenomenon, this finding may not be entirely unexpected.Yet the discovery that Mendel's laws of heredity applied equally to animals and plants further destroyed the ancient boundary that existed between zoology and botany. Before 1909 there was no accepted term for the genetic factors underlying visible traits.Spencer, Haeckel, Darwin, de Vry, Weissmann, and others who considered the phenomena of heredity postulated the existence of some particulate matter with different properties, but the names they used were not widely adopted (see Chapter XVI). In his work Mendel kept inferences about the nature of hereditary material to a strict minimum, which was wise in view of the limited knowledge of the nucleus and chromosomes in 1865.The traits ("MerKmale") and traits ("Charaktere") he refers to in experiments are basically limited to the phenotype level, although the symbols A, Aa, a he used are generally accepted to refer to the structure of the genotype.In the conclusion of his thesis (1866:41-42), he used the word "Elemente" as many as 10 times, several of which have very similar meanings to the word "gene" we use now, but he There is no clear concept of genetic material.Regardless of what Mendel really thought, to the early Mendelians he was describing what we now know as Mendelian inheritance. Although Weissmann once hinted at a distinction between germplasm and constitution, there was no "phenotype" until 1900 and "genetic type" these two academic terms.As far as de Vrij is concerned, there is no substantial difference between genetic material and body (phenotype), because the ubicon he envisions can move freely from the nucleus to the cytoplasm.He thinks that Panzi is a unit trait or a basic trait.He argued for a separate genetic basis for each independently inherited trait.De Vry sometimes referred to the genetic elements as "factors," a term initially adopted by Bateson and the Morgan school. Like de Vrij, Bateson could not tell the difference between genetic factors as the basis and phenotypic traits as appearance.He sees "unit traits" as "alternative (traits) in gamete structure" (1902).In order to be able to indicate such alternatives, such as pea or pea among peas, Bateson quotes (the Greek word for each other and transforms it) "relative shape" (all-elomorph, later simplified For allele; now translated as alleles).Yet he could not distinguish between a somatic (constitution) trait and its determinants (genes) in gametes.For various reasons, there was almost universal agreement before 1910 that there was a 1:1 relationship between genetic factors (genes) and traits.It therefore does not matter whether one refers to a genetic basis or to its phenotypic manifestations when referring to a unit trait.It is precisely because of this automatic tacit understanding that Kessel put forward his "pollution theory" (contsmination theory). With the rapid increase in genetics activity after 1900, it became necessary to develop a term for the material basis of independently inheritable traits.The Danish geneticist WL Johannsen (WL Johannsen, 1857-1927) found that Mendel’s factors were very similar to the pangen proposed by de Vry, so in 1909 he suggested that the word pangen be simplified to gene to indicate heredity The material basis of traits.Johnson was a physicalist whose last thought was to color the definition of the word gene with preformation.He denounced "the conception of the gene as a physical, morphologically expressive structural feature, which is so harmful to the steady development of genetics that it must be guarded against immediately" (1909:375). Thus he does not define the gene but simply says that "the gene can be used as a unit of measurement (Rechnungseinheit). We have no right to define the gene in terms of Darwin's microbud or Weissmann's biogenic body or stator or other similar inferential concepts to a certain morphological structure. Nor are we entitled to conceive that each particular gene corresponds to a particular phenotypic trait or (as morphologists often say) a characteristic of a developing organism" (1909). This definition reflected the differences of opinion throughout the field of biology at that time.Physicalists (including Johnson, who was greatly influenced by his education) explain everything in terms of mechanics.Embryologists are also reluctant to accept granular genes in the epigenetic (epigenetic) tradition because it reminds them of preformation. Morgan's initial reluctance to acknowledge genes, or at least granular ones, stemmed from such considerations.Finally, there is some influence of essentialism, which opposes the division of the essence of species. In 1917 Goldschmidt severely criticized the overly cautious attitude of geneticists towards genes: "We believe that this mental attitude towards the problem is the result of Johnson's agnosticism about the nature of genes, which produces a certain mystical reverence for the geneticists. The idea of ​​the secular properties of the gene is abhorrent." Of course, when the time came later, the gene was shown to have exactly those (structural) characteristics that Johnson had so carefully excluded from his definition.In fact, from Morgan through Muller to Watson and Crick has been getting closer to the concept of gene structure.Johnson's coinage of the term "gene" was quickly adopted generally because it filled an urgent need for a term for a unit of inheritance.However, the lack of a strict genetic definition was partly the cause of some controversy later on.Another source of confusion is that, almost until now, scholars have disagreed about what genes mean.For example, when it comes to the white eye gene in Drosophila, some scholars think it refers to the white eye allele, while others think it refers to the locus (site) where the white eye mutation occurs, that is, the gene of all white eye alleles seat. From coining the word "gene" for the invisible, submicroscopic unit of heredity to fully understanding its nature, the road has been long and arduous.Numerous geneticists (the foremost of which is Muller - H.J. Muller) have actually devoted their entire scientific career to this quest.As we shall see, it was finally discovered (in the 1950s) that macromolecules functioning as genes do possess the structural complexity and specificity that Johnson refused to acknowledge.How to discover the secret of genes is indeed a very tormenting problem at first.Morgan and colleagues quite rightly decided to start by studying altered genes, or "mutations," which they thought could be a promising insertion wedge. 17.3 Origin of New Variations (Mutations) With the rediscovery of Mendel's law of segregation, the question of the origin of hereditary variation became prominent.The presence of alleles is required to be elucidated.Darwin postulated that variation is constantly being replenished so that natural selection has sufficient selection objects to act upon.However, he could not tell the source of the mutation.The time had come to solve Darwin's enigma, but the Mendelians made little progress at first in their work on the problem.In fact they had to overcome obstacles. The main obstacle was that most scholars who studied variation at the time still believed that there were two types of variation.Darwin, for example, discovered that there are "many small differences that may be called individual differences" (), later called individual variation, continuous variation, or fluctuating (fluctuating) variation.His recognition of the importance of this type of variation was one of the pillars of his theory of evolution. Darwin, however, admitted that "some mutations ... may have occurred suddenly, or in one step" (p. 30) and cited the turnspit dos and the sheep as examples of such mutations.Bateson called such mutations discontinuous mutations.And admit that these two types of variations have a long history and are closely related to Plato's concept of essence.Essences are subject to small chance variations, and any major deviations can only occur suddenly through new essences (that is, new types or patterns).At the time it was thought that the causes of these two types of variation were quite different, and that they played very different roles in evolution.This was especially the point of contention in the debate between the biostatisticians and the Mendelians (see Chapter 12), which actually continued from the time of Lamarck until the time of evolutionary synthesis in the 1940s. .De Vry's Treatise on Variation (1909) reflects the depth of disagreement on this issue. (See also Mayr and Prvoine, 1980). If you admit that there is soft inheritance, it is not difficult to explain individual variation.Any change in internal conditions or environmental influences, such as nutrition or climate, can affect and alter an individual's traits.As Darwin put it, "in cases where the structure of the body is altered by a change of condition, by increasing or decreasing the activity of the part used or not, respectively, or by other causes, the microbes which fall off the part of the body which has undergone an altered structure The buds themselves also change, developing into new, altered structures when they have multiplied sufficiently."Other scholars who believe in soft inheritance have adopted a similar explanation.Old traits will be graded into new traits, and the differences between these graded traits are small, showing continuous variation.If the new genetic variation arises from some unknown process, it will also undergo soft inheritance and be classified as the pre-existing variation.This admits of the nature of the species as capable of producing successive individual variations, and leaves no problem of interpretation.Animal and plant breeders generally hold the view that the environment can affect genetic variability (Pritchard, 1813; Roberts, 1929). The situation changed radically in 1883 when Weissmann abandoned soft inheritance.If "conditions of life" do not give rise to new variations, or even increase them, what are the causes of individual variations?Neither Weissmann nor de Vrye came up with a well-founded theory for this, and the early Mendelians were concentrating on the problem of discrete variation, with little or no emphasis on individual variation.How to reconcile the contradiction between discontinuous Mendelian factors and continuous variation is a headache for them. Not only is the lack of proper data hindering the resolution of the problem, but also the tacit acceptance of many misconceptions.These misconceptions, in addition to the two types of variation, include soft inheritance (despite Weismann's objections), fusion inheritance (despite Mendel's laws), pattern thinking, and confusion about hereditary and phenotype. Faced with the aforementioned difficulties and misconceptions, it was not possible at the time to directly address the genetics of continuous variation and the origin of new variations.The solution to the problem is actually a roundabout way of studying discontinuous variation, although the premise of this method is that discontinuous variation is completely unrelated to continuous variation. Ancient peoples have long known that in a group (population) occasionally there will be individual individuals who are different, that is, beyond the normal standard of population variation.This has been found in wild animals, domestic animals and cultivated plants, and even in humans.Any variant that exceeds the normal variation of a population is an example of discontinuous variation.Albino bodies, six-fingered people and various deformities have been vividly described in folk literature. In the 15th and early 16th centuries, when nature was endowed with enormous "reproductivity," the ability to produce novelty, monsters of all kinds, most of which were truly deformed animals (such as two-headed cows), others are purely fantastical creatures, such as chimeras with the lion's head and the body of a man. In 1590, Sprenger, a pharmacist in Heidelberg, found a May celandine (Chelidonium majus) with completely different leaf shapes in his medicine garden.He bred it, sent the seeds around, and after a while had specimens of it in every major European herbarium, and it was described in most botanical books of the 17th century.This new variant is generally regarded as a new species of Celandine. 310 years later, an unusual plant of the genus Oenothera inspired de Vrij to propose a new and important theory of evolution. Apparently abnormal variants in cultivated plants are relatively common, and indeed many of the famous horticultural varieties (especially in flower color or shape) have arisen from these variants.It is also found in domestic animals, hornless individuals in cattle herds, or sheep characterized by short legs, such as the once common ancon sheep, which were notoriously short for jumping fences or low walls.In all these cases, breeders were able to produce pure lines by backcrossing with the parents followed by inbreeding, which we can now call true Mendelian inheritance.Contrary to Kerrrud's findings in hybridization of species, there is no "fusion" phenomenon, nor is there a gradual reversion to the parental pattern.Oddly enough, both Jenkins and Darwin completely ignored this fact in their famous fusion genetics debate (see Chapter 11). 最有名的异常变异体的例子是所谓的反常整齐花（Peloria）。1741年林奈的一个学生从瑞典乌普沙拉带回了一株植物样品送给他，这植物在外观，特殊气味，花、花萼、花粉与种子的特殊颜色上乍一看和普通的蛋黄草（Linaria）完全相同。然而普通蛋黄革具有和金鱼草相似的典型不对称花，而反常整齐花则是具有5个突起的辐射对称花。 林奈得出的结论是“这种新植物由本身种子繁殖，因而是一个新物种，并不是一开始就有的。”更有甚者，按林奈的分类方法，反常整齐花不仅是一个新种或新属，而且还是一个完全新的纲。这不仅动摇了林奈的物种固定不变的概念而且还似乎否定了他的分类原则（Larson，1971：99－104）。起初林奈以为这和杂交有关，但他很快就放弃了这种观点。最后证明反常整齐花并不像原先所认为的那样是固定不变的，林奈后来决定不再理会这讨厌的“物种”，甚至在他的《植物种志》（1755）中也没有提到它。 林奈之后的一百多年中这类异常个体或新变种发现的越来越多且越频繁，但这并没有提供什么新见识，但是在这一时期中着重点却发生了微妙变化。就杯茶及其同时代人来说，这样的一些变异体只是和物种概念有关，但是随着进化思想逐渐发展，变种及其起源方式便具有了新的意义。正如前面指出，温格对这个问题的关注为孟德尔的试验起了促进作用。出版以后，变异体的问题就越来越和进化有关。 对信仰一次性创造的原教旨主义者来说这一类表面上是新种的突然出现完全是一场麻烦，而对那些相信在地质年代中不断发生灭绝并主张以新的创造来弥补空白的人来说这倒是值得宽慰的事。在达尔文以后时期就那些基本上是本质论者的进化主义者来看也很有吸引力，因为他们由此可以把物种形成看成是骤然的新起源过程（见第十二章）。 达尔文特别强调进化的渐进性，也就是说，连续变异在进化上的重要性，正这并没有使他的所有同时代人信服。赫胥黎，高尔敦，Kolliker及其他人，偏重通过不连续变异的骤变式新物种和模式起源。然而再也没有别的人比贝特森更加清楚地认识不连续变异的重要意义，他曾经收集了大量材料来证明他的论点（1894）（见第十二章）。 一直到重新发现孟德尔定律以后，不连续变异的观点才充分发展成为一种重要的进化学说，即德弗里的《突变学说》（1901；1903。这学说在进化生物学中的作用见第十二章）。德弗里在提出并发展他的新遗传学说时，他不仅开展了栽培植物变种的杂交而且也研究了自然种群中的变异。1886年他在拉马克月见草（夜来香）的大种群（生长在荷兰一片荒芜的马铃薯地里）中发现了两个植株，他认为这两个植株与所有其它个体极不相同可以看作是新产生的物种。它们在德弗里的试验园中经过自花授粉仍然极端稳定。 从马铃薯地里移植到试验园的拉马克月见草的个体中也还有其他的新模式产生。后来除了许多次要的变异体而外德弗里还发现了20株以上的个体可以认为是新种，在自花授粉后确实稳定不变。 德弗里为这类新“物种”产生的过程引用了“突变”这个词。考虑到这词在遗传学说中十分重要因而不妨多说几句。“突变”这个词早在17世纪中叶就被用来表示形体的剧烈变化（Mayr，1963：168）。从一开始它就既用于不连续变异又用于化石的变化。 1867年这词被Waagen正式引用于古生物学，指种系系列中可以分辨的最小变化而言。德弗里很了解这种用法因为他曾特地提到过（1901：37）Waagen。就像我们的语言中很多词（例如“适应”）一样，“突变”这词既用干过程又用干过程的结果。但是比这更复杂更容易混淆的是“突变”有时用来指遗传型的变化，有时却指表现型变化。更糟的是，在德弗里心目中，突变是一种进化现象而在以后的遗传学史中它越来越成为专门的遗传现象。关于突变概念的这种混乱情况必须有所了解才能懂得为什么突变在进化中的作用一直长期争论不休。 虽然德弗里用“突变”这个词表达新种的突然出现，但是他当然不了解这类变化的物理本质，而且事实上，他是将之用来表示表现型的突然变化。这已经被后来研究月见草的学者们证实，他们证明了德弗里所说的突变几乎全都是染色体重排（包括多倍性）的表现，其中很少是现代所指的基因突变（见下文）。 经过几十年的遗传学研究才使“突变”这词摆脱了它原来的含义不清，和德弗里所断言的，它是产生新种的过程的羁绊。德弗里明确的将这个词限于用在不连续变异的单位上：“突变……构成了变异性科学的一个特别分支。突变无需过渡即行发生而且极为罕见，而正常的变异则是连续的并且一直出现……如果假定生物的特性是由彼此截然不同的一定数目的单位组成，这两个主要分支（狭义的变异性和可突变性）之间的差别就很明显。一个新单位的出现就标志着一个突变；然而这新单位的表达按照物种的其他原已存在的遗传因子的相同规律也是可变的”（1901：iv--v）虽然德弗里对他的突变所作的进化意义上的解释是错误的，但比起在他以前的任何人，他更强调新遗传性状的真正来源，在这一点上理应归功于他。后来，孟德尔以及研究遗传现象的其他学者就一直探索原已存在的遗传因子和性状的传递。德弗里促使人们注意遗传性新事物的来源问题。不管“突变”这个词的涵义从1901年以来发生了多么大的变化，从那时起突变一直是遗传学的一个重要问题。 德弗里叙述了他是多么勤奋刻苦地寻求一种理想的植物来明确论证通过突变的物种骤然形成。他研究了一百多种植物，但是除了一种以外他将其余的全都放弃，因为它们的变异都不能像他所预期的那样保持下去。他曾强调指出月见草是多么特殊，然而他显然从来没有意识到将一个新学说奠基于从单个特殊物种所观察到的现象上是多么危险。 正如Renner，Cleland，S．Emerson以及其他遗传学者的出色研究所论证的那样（Cleland，1972），月见草有一套特殊的易位染色体系统。这系统由于纯合子的致死现象因而在杂合性上是永远平衡的。德弗里看作是突变的现象实际是这类染色体环的分离产物。这种情况在其他植物物种和动物中（除了某些罕见的、具有同样的平衡系统的以外）并不存在。德弗里的突变既不是正常变异的来源也不是物种形成的正常过程。然而他的“突变”这一术语却在遗传学中保留了下来，这是因为摩根保留了它，尽管摩根是将之转用于十分不同的遗传现象。 17．4现代遗传学的兴起 1910年在遗传学史上几乎和1900年同样重要，在这一年摩根发表了他研究果蝇的第一篇论文。重新发现孟德尔后的头一个10年贝特森对遗传学的发展影响极大。他和他的同事不仅充分论证了孟德尔定律，而且还发现和解释了许多看来是例外的特殊问题。贝特森在遗传学的词汇方面也作出了重要贡献。在这10年中波弗利（Boveri）也证实了染色体的连续性和独立性（个体性，individuality）而深受欢迎。 胚胎学家摩根是全然不相信瑟顿-波弗利（Sutton－Boveri）染色体学说的学者之一，他和威尔逊（E．B．Wilson）是在纽约哥伦比亚大学的同事。他们彼此之间虽然友谊深厚，然而那时两人对染色体与遗传之间的关系的解释却完全不同。 1908年摩根开始进行遗传学实验，起初用的实验动物是大鼠和小鼠。他的最具有决定意义的决定可能是放弃了用哺乳类动物作实验，因为它们的世代时间长。管理费用高，而且容易生病。当时有另外两位美国遗传学家，W． E．凯塞尔和Frank Lutz，已经采用普通果蝇（Drosophila melanoggaster）进行实验多年；这种果蝇每二、三个星期就繁殖一代，用扔掉的废牛奶瓶就可以培养而且几乎完全不受病害侵袭。 “普通果蝇还有一个重要特点是只有4对染色体，而大多数哺乳动物的染色体数目变化幅度是土24。因此果蝇特别适合于研究交换现象，而这正是最后证实染色体学说所必需的。 19世纪9O年代中期以后有一股思潮反对魏斯曼时期的恣意于推论。在这种新的严肃学风影响下，德弗里、柯仑斯和贝特森对孟德尔定律的阐释在相当大的程度上是描叙性的，强调比值和分离现象。但是，几乎就在同时，有一些研究遗传现象的学者，尤其是那些具有细胞学基础的人，认为必须对孟德尔现象作出解释，更确切地说必须探索孟德尔分离现象的物质基础。就这些学者看来在染色体与遗传现象之间虽然应当有某种关系，但这种关系并不是所有的人都能接受。为了了解这种对立局面必须再一次指出遗传学这门新学科是从发育生物学派生的。魏斯曼、贝特森和摩根的原来概念框架都是胚胎学的。 虽然先成论与后生论之间的争论似乎在一百多年前就以后生论的决定性胜利而告终，胚胎学家却仍然对哪怕是一丝一缕的先成论思想特别敏感。只要读到摩根在早期（1903）对孟德尔学说的议论或约翰逊对基因的议论就能感觉到他们对孟德尔的颗粒遗传学说（在他们看来就是先成论者的学说）的厌弃心情。 将他们的遗传学说奠基于物理力之上的学者们，例如贝特森的动态涡流学说（theory of dynamic vortices，Coleman，1970），认为遗传型体现了整体性与后生论的统一，和颗粒学说看来根本不相容。在孟德尔遗传学已经确立了很久之后还有一些遗传学家坚持这类“动态”学说。例如R．Goldschmidt直到这个世纪的50年代仍然相信遗传力的“力场”和整个遗传型有规律的系统性变异的可能性，这也是一种整体性概念（holistiC concept）。约翰逊反对将基因定义为“形态结构”似乎也出自同一背景。 他们的对立面则赞成形态性颗粒遗传学说，但是对遗传物质是怎样在染色体中组织起来的却全然不清楚。在1890年代中期，建立遗传现象的染色体学说的事实根据已经具备，然而当时并不能由之建立起一个健全的学说。原因是多方面的： （1）顾虑可能被看成是先成论者的学说。 （2）没有按个别因子来分析遗传现象。 （3）从1885年到1900年特别强调细胞分裂的纯粹机械作甩方面。 （4）对纯粹的发育现象特别关注（尤其是波弗利）。传递遗传学涉及种群现象，而这是细胞学中的功能分析方法所无法处理的。 1900年以后，遗传学的发展受到一件偶然巧合事态的影响。年轻的美国胚胎学家威尔逊（E．B．Wilson）在欧洲的几次逗留期中对细胞生物学发生了极大兴趣，特别是受到他的朋友波弗利的影响。虽然当时他本人做过一些十分专门性的具有独创意义的细胞学研究（细胞谱系），但更重要的是他将当时对细胞、特别是对染色体的知识进行了出色的综合，撰写了专着《细胞和发育与遗传》（The Cell in Development andInheritance），（1896；第二版，1900），这一专着在后来细胞学与孟德尔学说的综合上所起的积极作用比什么都重要。后来他的八篇经典性系列文章（1905－1912）大大推进了对染色体的研究和理解，这些都对摩根的所有助手起了启迪作用；作为摩根的同事和至交，他对摩根本人也产生了深远影响。有充分的理由将威尔逊列为遗传学这门新科学的创始人之一。 虽然有不少学者在1890年代就表示他们认为染色体的染色质或核素（nuclein）就是遗传物质，但是单凭这一点还并不足以构成有实质性内容的遗传学说。只是到了1900年以后的10年才一点一滴地确立了孟德尔学说与细胞学2间的密切关系。推测和假定才被确凿的证据与无可挑剔的实验证明所代替。 要阐述这些证据或证明逐步集成的步骤很困难，因为染色体学说的历史和基因学说的历史交错在一起。只有主观地将这种连续性切断才有可能分别介绍这两者的历史。但是，应当强调的是这里不只是为了教学的原因而且也是从知识发展的历史角度的理由才将两者分开介绍；因为如果没有染色体学说在先，将很难（如果不是不可能）发展健全的基因学说。 1900年重新发现孟德尔定律，使情况发生了急剧变化。不仅由于这重新发现所激起的极大热情产生了非常多的研究成果或新发现，而且1880年代和1890年代的细胞学发现突然也显示了新的意义。孟德尔定律是遗传物质染色体结构的逻辑结果，这一构想多少是独立地由Montgomery（1901），柯仑斯（1992），瑟顿（1902），威尔逊（1902），波弗利（1902）几乎同时提出。尤其是瑟顿和波弗利为他们的结论提供了详细证据。这些学者有意识地将细胞学证据和遗传学论点结合起来的结果是形成了生物学的一门新学科，细胞遗传学，威尔逊及其学生是这门新学科的创始人。值得注意的是斯特体范特（Sturtevant），布里奇斯（Bridges），穆勒（Muller）在加入摩根研究小组之前都是威尔逊的学生。 17．5瑟顿一波弗利（Sutton－Boveri） 在遗传学历史上细胞学在1900年前后的进展中再也没有什么比论证了染色体的个体性和连续性更重要。染色体在细胞分裂之间是见不到的；静止（细胞）核仅仅呈现为轻微染色的颗粒或由细丝组成的网络。染色体在有丝分裂结束时完全溶解，并在下一轮有丝分裂周期开始时重新形成的论点似乎得到了显微镜观察的支持。这也正是一些有经验的细胞学家如赫特维克与R．Fick（19O5；1907）一直到孟德尔时期仍然持有这一论点的原因。在细胞核静止期每个染色体保持其个体性和连续性的论点确实只是根据推论，不能直接观察到。Rabl（1885）首先明确地提出染色体的个体性和连续性的假说。他认为染色体溶解而成的染色质丝当细胞核进入静止期在下一轮有丝分裂开始时又重新合并成原来的染色体。这只是一个推论，所依据的资料很少，其中大多数是根据染色体的数目固定不变作出的推论。Van Beneden（见第十五章）和波弗利随后都声称这一推论的优先权属干他们。毫无疑问波弗利为染色体个体性学说比其他人提供了更具决定意义的证据。早在1891年他就讲过，“我们可能通过组成细胞核的某一指定染色体去鉴别由静止核产生的每个染色体。”他由之便作出了着名的结论：“从受精卵的正常分裂过程中所有细胞的染色体一半必定来自父本，另一半来自母本”（1891；410）。 经由细胞核静止期的连续性以及每个染色体的个体性，在今天看来不过是一件事物的正反两个方面，然而在1890年代却并不如此。魏期曼及其他人以为每个染色体含有一个物种的全部遗传特性，也就是说他们不承认孟德尔意义上的染色体的个体性。但是，如果一个染色体只含有个体的一部分遗传物质，每个染色体就会和其他染色体不同，也就是说它必定具有个体性。换句话说，如果每一染色体和其他的不同，就必须论证其连续性和个体性。 关于染色体的连续性问题Montsomery（1901）和瑟顿（Sutton，1902）都提供了肯定证明。他们指陈在有丝分裂和减数分裂中有些染色体是可以个别分辨的，具有同一特征的染色体在每次细胞分裂中都一再出现。此外，他们还指出在第一前期中两个相同的染色体配对（联会）但在减数分裂时彼此又分开（见下文）。这样一来就得出了这样的结论，每个物种的染色体组含有成对的同源染色体，其中一个来自雌配子（卵细胞），另一个来自雄配子（精子），这已由van Beneden于1883年观察到。从受精（形成合子）开始经过无数细胞分裂直到形成新配子以前的减数分裂，这些染色体显然保持着它们本身的同一性（完全相同）。瑟顿在他的文章结尾的结论是：“父本和母本染色体结合成对以及随后在减数分裂时分开……可能构成孟德尔遗传定律的物质基础”。第二年他又将这一思想加以展开（McKusick，1960）。 上述这些观察并不能完全排除形态上不相似的染色体也具有相似遗传性质的可能性。 波弗利（Boveri，1902；19O4）通过独出心裁的实验否定了这种可能性。在一种具有36个染色体的海胆中波弗利通过适当处理（如多重授精等）在头四个子细胞中能够得到会有各种不同数目染色体的胚胎。然而在所有这些胚胎中只有子细胞含36个染色体的能够正常发育。波弗利从这一事实得出的结论是，每个染色体具有“不同性质”，只有当所有这些性质恰当组合时才能正常发育。 现在已很清楚地证实染色体与遗传性状都遵从同一规律，即它们也显示分离与自由组合现象。瑟顿和波弗利公开地或含蓄地提到基因位于染色体上，每个染色体有其特殊的基因组。很明显，尤其是瑟额（1903）和波弗利（1904）所阐明的，这就是一个全面的染色体遗传学说，是从细胞学证据和孟德尔性状的自由组合现象推论得出的。它似乎能够解释孟德尔遗传的全部现象。 奇怪的是，“瑟顿-波弗里染色体遗传学说”（这是由瑟顿的老师威尔逊于1928年命名的）的重要意义和普遍适用性起初完全没有被承认。不仅贝特森和Goldschmidt拒不接受而且其他一些知名的生物学家（如E．S．Russell）也迟至1930年才承认。一部分原由是由于这学说是根据观察作出的推断。摩根就曾说过他不接受“不是依据实验” 作出的结论，约翰森也曾讲过类似的话。事实上瑟顿-波弗利学说大部分是根据实验得出的，摩根对这学说的抵制显然还有更深层的原因。 染色体经由静止期的连续性到了1910年已有大量证据证明；它们的个体性的证据主要是波弗利的实验。起初并没有明确的证据证明某个特殊性状和一个特定的染色体有关。 性别决定是首先提出这种证据的遗传性状。最彻底的证据最后来自连锁图。