Chapter 27 Part III Variation and Its Inheritance Chapter Fourteen Early Genetic Theory and Breeding Experiments

In sexually reproducing species each individual is unique and unique (except for identical twins).This degree of uniqueness is much higher than that of the non-biological world.While there are also unique "individual units" (such as planets or volcanoes) and unique systems (galaxies and meteorological systems) at the macroscopic level, the most abundant constituents of matter (molecules, atoms, and elementary particles) in all individuals are by no means unique .Most laws of physical science are fundamental and thus lack uniqueness (see Chapter 2). Concerned with the individuality of living beings is variation; and any group consisting of distinct individuals necessarily exhibits variation.The origin and nature of variation in living organisms were not clarified until the 20th century, and a major obstacle to the development of biology in the 19th century was the lack of an established theory of variability.This is the weakest link in the evidence link of Darwin's natural selection theory.Darwin himself was keenly aware of this, and troubled by it all his life.

Even primitive man must have been dimly aware that variation, or a part of variation, has something to do with heredity.The knowledge that offspring may resemble parents or ancestors in certain characteristics is of course ancient.All animal and plant breeding is based on the realization that certain traits are inherited.Any effort to improve a breed, whether through artificial selection or cross-breeding, is without exception based on genetic requirements.Even the role of sex in fertilization has been known for a long time in some cultures, such as the Mesopotamian Assyrians who used the pollen of male date tree flowers to make female date tree flowers at least as early as the second millennium BC. Fertilize.

However, the nature of inheritance and its mechanism have always been a mystery.The early observations of ancient naturalists and agronomists and the speculations of physicians and philosophers raised many questions, most of which were debated well into the early 20th century.Perhaps no other field in biology has had a more important role in its development than in genetics to disprove false ideas and beliefs.These false beliefs or beliefs are: (1) The spirit or temperament of the parents is the cause of inheritance, not the substance of the body; (2) Only one of the parents transmits the genetic elements (Buffon and Kolreuter have opposed it); (3) The contribution of the father is in the amount and It is qualitatively different from the female parent (from Aristotle to Linnaeus all hold this point of view); (4) environment and physical activity (use it or lose it) have a decisive influence on the genetic material (acquired trait inheritance); (5) There are two distinct types of heritability, one is discontinuous change (sudden change), the other is continuous infinitesimal change; (6) The characteristics (nature) themselves are directly inherited rather than forming traits probability (in the form of a genetic program); (7) the genetic contributions made by both parents are fused together in the offspring (fusion inheritance).

These are just a few examples of widespread misconceptions about heredity.Heredity is such an obvious phenomenon that it has become the subject of all sorts of folk "science," traces of which are still faintly visible even among ordinary people today.Animal breeders, for example, firmly believe that if a purebred female is inseminated by a male of another breed or a hybrid, the female's "blood" will become permanently impure and will no longer be useful for breeding.This belief is also often applied to humans, especially in racist literature.There are also many people who think that an offspring can have several male parents, so if the female parent mates with several male parents during pregnancy, the offspring will have the characteristics of these male parents.In addition, some people believe that genetic material has great plasticity. For example, some people think that any accidents encountered by the mother (such as being scared by a snake) may affect the fetus.

The most striking feature of the traditional view of heredity, viewed in retrospect, is that it is often incompatible with other simultaneously accepted views.Belief in an immaterial, unchanging nature is mixed with beliefs in various forms of environmental influence, or with differential views on parental contributions.Strictly quantitative concepts (such as the strength of paternal influence) and purely qualitative concepts (such as the inheritance of individual traits in Plato's eugenics) coexist.The heredity of constitutional impairment (disability) is also almost universally recognized, although everyone knows that the children of warriors who lost their arms are not without arms, not to mention that circumcision (circumcision) practiced among Jews for thousands of years is genetically is invalid.

14.1 Ancient Theory of Genetics Although many ancient Greek philosophers made original and critical analyzes of heredity or variation, there was no unified theory in ancient times, and the views of these philosophers were also very different from each other.However, in the tradition of Homer's Iliad or other epics in which the son inherits the heroic qualities of his father as a matter of course, the principle of heredity was generally accepted; Only a vague awareness is passed on to future generations.The ideas about reproduction and heredity had the most profound influence on later generations were Hippocrates and Aristotle.

Hippocrates (about 460-377 BC) was a famous doctor.He had spoken of the "seed matter" produced from various parts of the body being transported by the humors to the reproductive organs (see Aristotle, Procreation of Animals). Fertilization is the mixing of the seed substances of the parents.The involvement of various body parts in the formation of seed matter is evidenced by the fact that blue-eyed individuals produce blue-eyed offspring and the offspring of bald individuals also become bald.If some part of the body is unhealthy, the corresponding part of its offspring may also be unhealthy.

This Panspetmy or pangenesis view was apparently first proposed by anaxasoras (c. 500-428 BC) and was believed by others, including Darwin, at least until the end of the nineteenth century (see Chapter 16. ).Anyone who believes in the use-and-lose effect, or any other form of inheritance of merit (which was believed by almost everyone from the time of Hippocrates until the nineteenth century) is bound to accept this view.The alternation of the body (phenotype, constitution) forming and through it the seed material (sperm, genotype), which is then directly transformed into the body of the next generation again through growth and development is also a feature of pangenesis, and this concept has been basically maintained down, and it was not until the 1870s and 1880s that it was first opposed (Gallon, Weissmann).

Aristotle was the most interested of all the ancients in the question of reproduction, and he made use of one of his major works.Animal breeding and come devoted to this issue.He also discussed variation and heredity in another book, Anatomy of Animals.Aristotle categorically rejected Hippocrates and other pioneers' atomic explanations of heredity.He thought that this view could not explain the inheritance of some characters that could not produce seed matter, such as some dead tissues such as nails and hair, or behavioral characteristics such as voice and movement.And some traits can be inherited from parents before they reach the age at which they show, such as baldness or premature graying.Aristotle similarly objected to the view that the sperm of male animals is a rudimentary animal, a view held by some scholars in the 17th and 18th centuries.

Aristotle's theory of genetics is more or less holistic.He, like some pioneers, believed that male and female animals play different roles in heredity.Male sperm provide the body forming factors (eidos) and female menstruation (catamenia) is an amorphous substance shaped by sperm.He likened the role of the sperm to a carpenter's tool, while "the female always supplies the material, and the male the tools for shaping it into shape; and this, as we see it, is the characteristic of both sexes: the male is male and the female is female." The above statement seems to indicate that the functions of sperm and menstruation are obviously different, but in other places Aristotle also mentioned the competition, even struggle, between the female and male seed substances.Male offspring are produced when the male material prevails.

If only partial victories result in male offspring with the characteristics of the mother; if the parent is weaker than the ancestral parent, the offspring have ancestral characteristics, and so on. Most important in Aristotle's thought is the role played by each individual eidos.While each cub is characteristic of the species to which it belongs, it also has its own special personality.According to Aristotle, Socrates' children may have Socrates' characteristics. It has been mentioned, with good reason, that Aristotle's idea of ​​separating body-forming factors (eidos) from the material being molded is not too far removed from the modern notion of controlling genetic programs that shape phenotypes (Delbruck, 1971). However, this ignores that Aristotle's eidos is an immaterial factor; moreover, the fact that Aristotle's eidos is quite different from Plato's eidos has always confused later scholars, so Aristotle Aristotle's views were practically ignored until 1880 (Buffon's concept of "internal model" is superficially similar to Aristotle's eidos, but the two have no historical origin (Rogers, 1963); Buffon's model is purely physical substance.) It was not until 1970 that it was recognized that Aristotle's ideas were very similar to modern ideas. As in other areas of biology, the great contribution of the ancient Greeks was their radically new attitude to heredity.They no longer see heredity as a mysterious, God-given thing, but something that can be studied and contemplated.In other words, they demanded that heredity be recognized as a science.In fact they were the first to raise many questions that would later be the subject of famous genetics debates in the 19th and early 20th centuries.There was also a school of philosophy, the Epicureans, who proposed a new concept of the existence of very small invisible particles, which later became a basic concept of genetics. About two thousand years after Aristotle and the atomists of ancient Greece, nothing new was added to the problems of reproduction and heredity.Even the times of Alexandria and Rome were the same, and the Middle Ages were only discussed in terms of leftover ancient issues.Many of the questions posed by the ancient Greeks, which could not be answered with certainty, were also the main questions of the emerging science of the Renaissance.Some of these questions (not all of which were explicitly posed by ancient Greek scholars) can be listed as follows: (1) What is the essence of fertilization?What are the things that are passed on in mating that are related to conception? (2) Can organisms occur naturally?Or is the union of the sexes necessary to produce new individuals? (3) What did the father and mother contribute to the characteristics or traits of the offspring?Does the mother make a (we can now say genetic) contribution besides being the caretaker of the developing embryo? (4) Where is the male sperm formed, in a specific organ or in the whole body? (5) How is the sex of offspring determined? (6) To what extent are heritable traits affected by use or disuse (use it or lose it), environment, or other factors? Only when these questions, and others, are answered—indeed they must first be formulated properly and systematically—genetics can become a science. When interest in the natural world revived at the end of the Middle Ages, it confronted a spiritual and intellectual atmosphere quite different from that of the Greek age.God's will and creative power can be seen everywhere at any time.The emphasis was then on "origins," the generation of new individuals rather than the principle of continuity implied by heredity.This ethos or ethos (especially popular in the sixteenth century) has been brilliantly introduced by Jacob (1970:19-28).Occurring naturally, infusing life into inanimate matter is considered as natural as normal reproduction.Spawning monstrous creatures is no different than normal creatures.It is also common for the seeds or seedlings of one plant to transform into another (heterosony).New organisms originate from scratch (generatio ab initio).Because of the emphasis on post-reproductive development, the state of mind during this period is particularly important to the history of the field of study known as embryology around 1828. It must be noted that true biology did not exist from the 15th to the 18th centuries.At that time, the emphasis was on natural history and medicine (including physiology), and there was little connection between the two.Reproduction is primarily studied by anatomy professors, medical physiologists, who study recent causes and rarely ask questions involving genetics.Their interest is developmental biology. The naturalist, by contrast, is primarily interested in the diversity of nature, the effect of ultimate causes. Since all members of a species have a common essence, heredity is a necessity (phenomenon) and is not regarded as a matter of science.Since it needs to be considered, it is only within the scope of species issues.Variation, however, is a subject which is often thought of by everyone, especially by naturalists.Herbalists, botanists, hunters, and zookeepers all love unusual individuals.At first this involved only very different "mutations" (see below), but as botanical gardens and museums collected more and more specimens, normal individual variation was revealed and studied.This eventually became an important source of evidence against essentialism. From the Middle Ages to the 19th century, the thinking of Westerners was completely dominated by essentialism (see second child).According to this philosophy, all members of a species have a common essence (unaffected by external or accidental events), and the study of species is tantamount to the study of nature.Throughout the 16th, 17th, and 18th centuries, much of the essentialist thought prevailed, so that variation in individual traits does not seem to have been systematically studied.When naturalists encounter deviations from the typical manifestations of a species, they may admit that they are "variety" within the species (the schematist view), not deserving of special attention. It is precisely because of such an emphasis on species that it is not surprising that some of the earliest views on heredity (Linnaeus, Kolreuter, Unger, Mendel, etc.) were elicited by the species question. The study of genetic mechanism must be based on the crossing between individuals with certain traits and differences in appearance stable traits.Variation is thus the main problem to be explained by any theory of heredity.But essentialists don't know what to do with variation.For him, the conceptual conundrum is that all individuals in a species are "essentially" the same.Thus until the end of the nineteenth century and even the beginning of the twentieth century the different kinds of mutations were confused with each other.This confusion was not clarified until population thinking replaced essentialism in systematics and evolutionary biology.The nature of this difficulty is most easily explained only by examining it historically.It will show how the various variations came to be perceived, and what the differences between them are. As far as the essentialist is concerned, species, by definition, do not vary in nature.All variations are "accidental" and do not affect the nature of the species (see Chapter 6 for details).A variant is not a different species; it's a "variety".Although variants and varieties have long been known to naturalists and horticulturalists, it is generally accepted that Linnaeus first formulated the concept of varieties.He rather dismisses the variety and scoffs at flower lovers who are eager to name it.On the whole, he thinks that the variety is not important, and that the climate or soil conditions can make reversible changes.He knew about deformities, too, but he also dismissed them as irrelevant.He never asked about the biological significance of the variation. "A variety is a plant which has been altered by some accidental cause" (Philosophy of Botany, 1751). In his "Philosophy of Botany" (section 158), Linnaeus pointed out that the characteristics of varieties are as follows: "There are as many varieties as there are different plants produced by the seeds of the same species. Variations are changed by accidental causes. Plants; climate, soil, temperature, wind, etc., are accidental causes. When the soil is altered, the variety eventually reverts to its original state." A variant is defined here as a non-inherited change in what we can now call a phenotype.When referring to varieties in the animal kingdom (section 259) Lin Tiao points out that he includes not only non-hereditary climatic variants under "variety", but also genetic variants within breeds (races) and populations of domesticated animals.When we peruse Linnaeus' works carefully, we find that under the term "variety" at least four completely different groups of phenomena are listed: (1) Non-hereditary changes, which are caused by nutrition, climate, cultivation or other circumstances. Factors that influence phenotypes; (2) breeds of domesticated animals or cultivated plants; (3) genetic variation within populations; (4) geographic origin, such as race. With the passage of time and the discovery of very different phenomena listed under the heading "variety", new terms were developed for the various varieties, but the new terminology developed by this effort (Plate, 1914 ) doesn't solve the problem, because it doesn't clarify the conceptual confusion that underlies the noun.Many scholars are confused about (1) genetic variation and non-genetic variation; (2) continuous and discontinuous variation (see Chapter 16); (3) individual variation and geographical variation.In this way, when different scholars talk about "variation", they often present completely different phenomena in their minds.This situation was exacerbated by the shape of the tradition that began with Linnaeus and separated zoologists from botanists.When zoologists speak of variation, they generally mean geographic races, while botanists, when speaking of variety, usually mean cultivars or variations within populations.This traditional difference is the first indication that variation is divided into many kinds. 14.2 Mendel's Precursors It was the hesitant first steps taken in Linnaeus' time that eventually led to the founding of genetics.Methodologically, there are two approaches to studying heredity.One is to study genealogy (Pedigrees).It is fairly easy to trace the distinguishing features of a human race through several generations.Polydactyly (with a sixth finger and toe) was recorded according to this method in 1745 by Maupedet, and it is now clear that this was due to a dominant gene manifested through four generations. At about the same time (1751), Reaumur also discovered the dominant inheritance of polydactyly in humans (Glass, 1959).Hemophilia and color blindness were then studied in similar studies.Although this type of genealogy was familiar to biologists in the 19th century, it was not used as the basis for the theory of transmission genetics. Another way to study heredity is through breeding.There are two schools of thought that employ this method, the species hybridists and the plant and animal breeders.The aims and interests of the two schools are very different. Linnaeus is often portrayed as a pedant concerned only with artificial classifications.He is certainly quite eccentric or whimsical in his relentless efforts to classify anything in the world that shows variation.On the other hand, he often surprises readers of his articles with his various unorthodox ideas on various natural history issues.Like any scholar whose ideas are active but whose talents are not overflowing, he tends to present ideas that seem to contradict each other simultaneously, or at least successively.This is well illustrated by the example of Linnaeus's altered view of the nature of species.The immobility of species was the guiding principle of Linnaeus's early research work. He once said that "the number of species is always constant...", which is probably his most famous creed (see Chapter 6). In his later years, however, he "played" (and this is the only appropriate expression) the problem of free interbreeding of species in nature.In a compendium (Haartman, 1764) he listed no less than a hundred hybrids of hypothetical species, of which 59 were described in detail.In a prize-winning essay submitted to the St. Petersburg Academy of Sciences on the nature of sex in plants, Linnaeus described two hybrids obtained through artificial pollination.One is a jackfruit hybrid (TrasoposonPratensis x T. Porrifolius) and the other is a Clematis (herb) hybrid (Veronica maritirnaxVerbena officinalis). Whether the hybrid obtained by Linnaeus is a (doubtful) descendant of the above-mentioned parent species is irrelevant, what is important here is Lin Rong's assertion that through the hybridization of two species, a stable new species is produced, that is, a completely new essence . This statement completely contradicts previous views of Linnaeus and other essentialists.The hybrid would have an intermediate (transitional) essence if it did not contain the essences of the two parent species, and if the hybrid were crossed again with one of the parent species or with the other species, it would actually result in a continuity (sex) of essences, such The conclusions completely contradict the already established discontinuities between species in nature.Linnaeus himself, however, was so confident in the production of the new essence that he named the two hybrids a new species and included them in his definitive work Plant Species (1753). Linnaeus sent part of the seeds of his jackfruit hybrid to St. Petersburg, where it was cultivated by Kerr Luther, a German botanist keen on interspecific hybridization. The jackfruit hybrid he cultivated in 1761 (possibly F2 generation) showed a certain variability, completely negating Linnaeus' claim that a stable new species had been obtained. Joseph Gottlieb Kolreuter (Joseph Gottlieb Kolreuter 1733-1806), like almost all biologists in the 18th century, was also educated in a medical school (University of Tuppingen, Germany).Seven years later, he obtained his degree (1755) and went to St. Petersburg, Russia, to study natural history at the Academy of Sciences for 6 years.Among other things, he studies the fertilization (pollination) of flowering plants and breeds hybrids.Since Kerreuter is often later regarded as Mendel's forerunner, it must be emphasized that he did not approach plant breeding with purely genetic problems in mind.He was concerned with such questions as the biology of flowers and the nature of species. His first successful hybridization study was using two tobaccos, Nicotiana rustica and N. paniculata for hybridization.The resulting hybrids grew so robustly that even "the most critical eye could find no fault from embryos to more or less fully formed flowers." It seemed (like Linnaeus) that he had succeeded in obtaining a new species.All attempts to pollinate the flowers of the hybrids have failed, however.The hybrid does not produce even a single seed whereas a normal flower can produce fifty thousand seeds.This incident was regarded by Kerludr as "one of the strangest things that have happened in the vast world of nature." However, it also gave him a great relief, because it made him recover the concept of essential species. belief.In the ensuing years Kerrodt repeatedly experimented with hybridization of plants of many different genera.In fact, he conducted more than 500 different hybridization experiments on 138 plant species, and the results were all similar, with the hybrids greatly reduced in fertility (if not completely sterile).When Kerrod found some of the crosses of his "species" to be of normal fecundity, he eliminated them, thinking that these were obviously not good species.He is right to do so.He kept detailed records of all the cross-breeding experiments he made, which we can now agree with him in retrospect; what he got rid of was indeed the crossing between varieties within the species. When he examined the pollen of the hybrid plants under a microscope, he found that in almost all cases the pollen grains were shrunken and were actually just empty shells.It should come as no surprise that pollination was unsuccessful.Only in rare cases did he find intact pollen grains that produced several generations of plants.A reciprocal is more fertile, that is to say when he pollinates a hybrid plant with pollen from either of the two parent species.Repeating such backcrosses over many generations, he ended up with plants indistinguishable from the species backcrossed with the hybrid.In more or less quaint terms he describes the result of being able to restore the original species. In his other crossbreeding experiments, such as some species of Dianthus, sometimes the fecundity rarely drops sharply, and it is easy to get F2 and F3 generations, but in principle the results are always the same.Every species is evidently protected in varying degrees by some sterilitr barrier.Buffon, of course, had pointed this out long ago in his studies of mules and other animal hybrids, but hadn't been able to generalize or generalize it. Another important discovery of Kerr Luther involved first and second generation hybrids and backcrosses.He found that the F1 hybrids were more or less similar and that most of their traits were intermediate between the two parents.As is often said the traits of the parent species are fused in the F1 generation.In contrast, the F2 generation hybrids showed a great deal of variability, some being more like their ancestors than their parents.These findings, at least as far as interbreeding of species is concerned, were confirmed repeatedly in the more than one hundred years between Kerr Luther and Mendel. Kerr Luther belonged to the school that holds that scientific explanations in biology must be physical or chemical explanations in order to be convincing.That's why he explained the difference between generations and F2 with the help of chemical patterns.Kerr Luther said that, just as acids and bases form neutral salts, so in modern hybrids the female "seed matter" and the male "seed matter" combine to form a "compound matter."In several generations of hybrids they are not combined in equal amounts, producing a variety of offspring, some more like one progenitor, some more like another.He could not explain why this was so, but he clearly did not consider the union of the parental "seed matter" to be a fusion process.In fact, as far as I know, no other experienced plant breeder except Negri has insisted on the view that fusion inheritance is the only mechanism. Kerr Ludd observed that in certain hybrid situations several generations of hybrids were divided into three types with two types resembling the two progenitors and one type resembling F1 generation hybrids.However, since he was only concerned with individual characters in the matter of species, he found only a few cases of such definite segregation.His basic object was early to prove that the interbreeding of two species cannot produce a third, and this conclusion is as true now as it was two hundred years ago, with rare exceptions.The only exception was the allotetraploids discovered 150 years after Kerr Luther. Reading Kerr Luther's painstakingly written detailed records about his large number of hybridization experiments not only makes us admire his diligence and perseverance, but also express our admiration for his insight into the bottom line.He proved that if the pollen could not enter the pistil of a female flower, the flower would be sterile; thus conclusively demonstrating that the male seed substance is necessary for fertilization.Through the comparison of a large number of traits between the hybrid and the two parent species and the production of reciprocal hybrids, he first proved that the contributions of the two parents are equal (confirmed by the intermediate state of the F1 hybrid).He thus firmly established the importance of sex and fertilization, which were still debated in his day.In addition, he also completely denied the theory of pre-formation, whether it is the theory of ova or the theory of essence. It is self-evident to a modern man that both parents make genetic contributions to their children.Strangely enough this was not so obvious to previous generations.This reason can be traced back to ancient Greece, where the "sexist" (chauvinisticmale) ascribes to the father the principal character-forming temperament, while in Aristotle and others it is indicated that the father supplies the body and the mother supplies only the material from which the body is fashioned. In the 17th and 18th centuries, these problems were entangled with developmental problems.Is the embryo (bud) pre-formed (or even pre-existing) or is the unformed egg "epigenesis"?The preformationist necessarily had to make a choice as to whether the pre-existing embryo was located in the egg (oogonism) or in the sperm (spermiaism). The famous biologists of the 17th and 18th centuries (Malpighi, Spallanzani, Haller, Bonnet) were almost all oogonists, and thus attributed most of the genetic potential to females.Leeuwenhoek and Boerhave were among the essenceists, as the former, as the co-discoverer of sperm, certainly would have been. It is indeed difficult to explain why such learned and intelligent scholars put forward such a one-sided theory. All these scholars must have known long ago that in man each child displays a mixture of the characteristics of its parents.They also know that white and black mestizos have intermediate traits.They also certainly knew that hybrids between species (such as the mule from a horse and a donkey) were also intermediate.All these well-known facts and others amply disprove not only the naive illusion of emboitement, but also the notion of a one-sided female or male acting unilaterally.Observations of this kind, however, do not shake either the ovum or the essence, and it seems that these scholars keep these observations in two separate parts of their brains. Some of their contemporaries were more enlightened.Buffon was well aware that both parents made genetic contributions, but the theory of inheritance advanced by Maupedet (far ahead of others) could be considered predictive of later development (Glass, 1959; Stubbe, 1965).Mopedi advocates pangenesis, which is based on the ideas of Anaxagoras and Hippocrates, and holds that particles (elements, factors) from both parents are related to the characteristics of offspring.A great part of his theory can be found in the later theories of Naodin, Darwin, and Galton. Although Kerr Luther's discoveries were important for understanding plant sex and reproduction, it would be a mistake to regard him as a forerunner of Mendel.Kerr Luther always saw the essence of species as unity.The fact that the F1 hybrids he found in most cases were intermediate types seemed to confirm his holism to himself.He never divided phenotypes into individual traits and traced through generations the fate of a trait in different combinations.And these are exactly what are needed to establish the laws of genetics, as Mendel and de Vry first realized. Kerr Luther is respected not only for his important discoveries on the biology of flowers and the nature of hybrids, but also because his line of experiments showed a level of planning and execution first class than any of his contemporaries. unknown.It is a pity that, like many pioneers, he was too far ahead of the concerns of his day, and had to spend some of his best experiments in demonstrating the sex of plants, which for us Said again seems obvious. Kerr Luther's hybridization of species contradicted existing beliefs to such an extent, and his discoveries were so unexpected and revolutionary, that they were not accepted by his contemporaries.Academic works published as late as 1812-1820 still deny plant sex and cast doubt on the reliability of Kerr Luther's experiments.In view of this situation, the Prussian and Dutch academies in the 1820s and 1830s offered rewards for solving the problem of plant hybridization and its use in the formation of useful varieties and species.This bounty promoted the work of Wiegmann, Garrtner, Godron, Naudin, Wichura, and other hybrid workers whose work has been detailed by Roberts (1929), Stubbe (1965), Olby (1966), and others.All of these studies follow the Kerr-Lutheran tradition. They deal with plant sex and the nature of species. Only certain hybridization experiments were performed between Mendelian variants within a species, but in the case of Kerr Luther, his results were not pursued even after they were published.All these scholars have repeatedly proved Kerr Luther's results, such as the intermediate type and relative consistency of the F1 generation, the increased variability of the F2 generation (clearly showing a tendency to revert to the remaining species), the identity of reciprocal crosses, and both parents Contributions to hybrid characteristics (generally roughly equal), and occasionally somatic heterosis even in sterile hybrids.Clear Mendelian segregation is very rare (even in the F2 generation); this is not surprising since species differences are often (if not usually) highly polygenic.In addition, the Nicotiana species of Kell Lutheran and many species used by other hybrid workers are mostly polyploid, and the number of chromosomes in one parent is often more than that in the other parent, so the parent with more chromosome sets is in the Hybrids are visually dominant. It must be repeatedly emphasized that these scholars are not engaged in the study of the laws governing the inheritance of individual characters.They were concerned with the nature of the species as a whole, in a way they understood better than those who worked on beanbag genetics in the early days of the Mendelian school.The schism in evolutionary biology from 1900 to the 1930s of evolutionary synthesis can be traced in part to this hybridization craze by plant hybridists in the early 19th century. Gartner (Carl Friedrich von Gartner, 1772-1850) was the most knowledgeable and diligent species hybridist before Mendel.In his major work (1849) he summarized the results of nearly 10,000 hybridization experiments (involving 700 species, resulting in 250 hybrids).When Darwin talked about these works, he once commented that "the valuable things contained in them are more than all other scholars combined. If more people understand, they will make greater contributions." Summing up and summarizing the vast amount of data Gardenle had collected would have led to many general conclusions, but this did not happen.Not only Darwin, who had read his writings carefully, but also none of his contemporaries could draw general laws from the facts gathered by Gartenle.实际上伽登勒向自己提出的问题也就是克尔路德在几十年前提出的同样问题，从总体来看他也非常满足于只是描述他的杂交结果。 也许可以对伽登勒说几句口是心非的恭维活，说他如此肯定无疑地证明了对这些问题能够作出什么答案，不能够作出什么答案，从而为完全新的研究路线扫清了战场。我们知道孟德尔也有一本伽登勒的书并且非常仔细地阅读过，然而却没何帮助孟德尔提出新问题或为他在遗传学上的突破助一臂之力。在伽登勒进行的几千个杂交试验中有少数涉及碗豆和玉米的种内变种。就这方面来说，伽登勒确实是孟德尔的先驱，这在后面还要提到。 伽登勒并不是那个时代唯一的德国植物杂交工作者，但是其他的人（如Wiegmann，Wichura）也同样是在传统框架内进行研究，所以在丰富我们的遗传知识上并没有作出什么贡献。 法国杂交专家淖丁（Charles Naudin，1815－1899）和伽登勒所不同的是他有一个很明确的理论，但是在基本思想上两人则相差无几。淖丁认为在产生杂种中将两个物种的本质弄到一起根本就不是自然过程。这在杂种不育以及后代杂种回复到某个亲代物种就表现了出来。亲代本质并没有发生融合。此外淖丁将物种的本质看作是整体而不是独立性状的镶嵌（孟德尔在研究中也是如此）。淖丁的物种有一些显然只是孟德尔的变种（例如蔓陀罗），其中淖丁也得到了明确的孟德尔的比例关系，并且这种比例关系和淖丁对来代本质的完全分离的看法十分一致。虽然他的某些杂交试验结果完全是“孟德尔式”的，如第一代杂种的一致性和第二代杂种的变异性，但是无论在理论上还是方法上淖丁都不是孟德尔的先驱，这可以由他没有探索可重复的比例这点来说明。他的同胞D． A． Godron（1807-1880）也是如此，他只关心克尔路德几十年前提出的相同问题（杂种的繁殖力，它们回复到亲本型等等）。就像他的其他着作表明的那样，他的主要兴趣在于物种的本质。 和物种杂交家的活动同时并肩进行的是实际的植物育种家的工作并由之发展了一种完全不同于物种杂交的传统。他们的纯粹功利主义的目的是提高栽培植物的产量，提高它们的抗病力和抗寒力，以及培植新变种。虽然他们也运用物种杂交，但主要着眼于变种之间的杂交，很多变种只有一个或少数几个孟德尔性状（现代的说法）不同。这些植物育种家比植物杂交家更有理由被看作是孟德尔的直接先驱。 其中首先应当提到的是奈特（Thomas Andrew Knight，1759—1853），他专门研究果树的变种。对我们来说值得特别注意的是他认识到食用豌豆（Pisum sativum）作为遗传研究材料的优点，因为“豌豆具有十分稳定的生活习性，它的变种特别多，是一年生植物，很多变种在形状，大小，颜色上都有明显特征，因而在很多年以前就促使我选择它（通过长期试验）来确定将某一变种的花粉导入另一变种的花中所产生的效应” （1823）。食用豌豆的这种特别优点显然在植物育种家中（包括伽登勒）是早已清楚的，毫无疑问也正是由于这一原因孟德尔才终于倾全力来研究它。奈特是位很细心的实验工作者，他在引入不同植物的花粉前总要将花去雄，并采用未受粉的或真正受过粉的花作对照。他讲到过显性和分离（回交），但是他没有对他收集的种子计数，因而也没有计算比例。 奈特的两位同时代人Alexander Seton（1824）和JohnGoss（1820）证实了显性和分离以及我们现在称之为隐性的选育姓状。这三位育种家的某些试验结果并不一致，因为他们没有认识到F1代豌豆种皮的外观（透明或不透明）是由母本决定的，而豌豆颜色本身（子叶）则取决于双亲的遗传组成。伽登勒在较后时期的玉米杂交试验中由于种皮也遇到同样的困难致使他无法始终如一的取得孟德尔的比例关系。这种困难在很多年之后才解决。胚乳是通过两个母本核和一个花粉核的融合而形成的，因而可能显示父本的特征。这一现象（后来经由德弗里和柯仑斯研究）被植物遗传学家称为异粉性（xenia）（Dunn，1966）。 物种杂交家和许多植物育种家之间的根本区别（Roberts，1929）在于后者往往研究个别性状并通过几代探索它们的归宿。运用这一新方法取得特别成功的是法国的农学家Ausustin Sageret（1763—1851）。他在将西瓜（Cucumis melo）的两个变种进行杂交试验时将其性状分作五对： 变种1 变种2 黄色果肉白色果肉 黄色种子白色种子 网状果皮平滑果皮 棱线明显棱线依稀可见 味甜味甜／酸 他取得的杂种并不是父本和母本的中间型；而是每个性状更近于父本或者母本。他的结论是“杂种和它的两个亲本相似一般来说并不是由于父本或母本的某些特有性状的密切融合而是未经改变的性状的分布相等或不相等；我说相等或不相等是因为这种分布在来源相同的所有杂种个体中远不一样，在它们之中存在着明显的多样性（1826： 302）。 在描述他的杂交试验时他明确指出某一亲本或另一亲本的性状是“显性”。在他之前还没有人毫不含糊地用过这词。Sageret不仅证实了显性现象并发现不同性状的自由分离，而且充分认识到重组的重要性。“我们不能不赞扬自然赋予它本身以极其简单的方式就能够使它的产品无限制地多样化，避免单调划一。这些方式中有两个，即联合和分离性状（按多种不同方法组合）就能够产生无限量的变种。”Sageret还了解到某些祖先性状也偶尔会在这些杂交中出现，“这种可能性存在但是它的发育在以前并没有受到重视。”我们行将介绍达尔文后来对这类回复现象非常重视。遗憾的是Sageret后来并没有继续从事他的富于想像力的和创造性的研究。 近年来常常有人提出这样的问题，为什么这些植物育种家在看来即将作出关于遗传的学说时停步不前。有各式各样的答案，但大多都不合式。这和对细胞学的知识不足显然无关，团为孟德尔的解释就不是根据细胞学说，实际也是不必要的。 这些育种家之所以不能制订出遗传学说也不能诿之于技术上的错误，因为他们之中的某些人在防止不需要的受粉作用时非常仔细谨慎并进行对照试验。他们给人以这种印像就是十分满足于仅仅得到明确的结果。他们根本不过问机制问题；如果他们过问的话，就像孟德尔那样，他们就会在试验技术中增添上对后代仔细计数并计算各种比例的项目。 换句话说，他们的失败（如果这是我们所要用的字眼）归根到底是由于他们没有提问关键性的问题。他们之所以如此是因为他们不是按可变的种群概念来考虑问题。采用种群思想是研究遗传现象的新路线的必要前提。 然而到了19世纪50年代通过物种杂交家和植物育种家的共同努力广阔的基础毕竟已经奠定。他们已明确地提出了建立遗传学说所必需的绝大多数事实根据，诸如双亲的同等贡献，显性，F1代的相对一致性，分离（F2的变异性增高）以及一般的反交同一性。 舞台已经搭好，迟早只待一位特殊天才人物的出现，他将提问以前未曾问过的问题并用新的方法来解决它们。这个人就是孟德尔（Gregor Mendel）（见第十六章）。