Chapter 21 Chapter 11 Comprehensive Evolution-1

In the early 20th century, not many people supported Darwin's theory of natural selection.Field naturalists stick to Darwin's original emphasis on the role of geography in evolution, but are also interested in other adaptive mechanisms, such as Lamarckianism.Paleontologists are convinced that evolution is a directed and linear process, and its mechanism is Lamarckian or Orthogenetic.A new generation of experimental biologists went to the other extreme, using genetics to attack Lamarckism but refusing to acknowledge the role of adaptation and selection in controlling the spread of new traits (through mutation). In the 1920s, the first attempts to unite the different branches of biology were made, and Darwinism, recovering from the eclipse situation, became the key to a new line of thought that was solving some famous problems. played an important role in the problem.It was realized that the more sophisticated understanding of heredity afforded by Mendelianism could be used to explain populations with a great deal of variation, and it was also recognized that selection affects the relative frequencies of genes.By 1940, many naturalists began to realize that their work could fit into this new form of selection theory, and abandoned less solid ones like Lamarckism.The result was "synthetic evolution" or "modern synthesis" that brought Darwinism back into the mainstream of biology.No one doubts the significance of synthetic evolution, but there is controversy over how it should be put together.It was once thought that the pivotal breakthrough was due to the newly created population genetics as the new basis for natural selection.This modified version of Darwinism was then exploited by field naturalists and paleontologists who were trying to escape their ambiguities about the mechanisms of evolution.William B. Provine's book "The Origin of Theoretical Population Genetics" (Provine, 1971) discusses this synthesis in detail.However, Ernst Mayr (1959c) suggested that the real situation was much more complicated.In its original form, population genetics was a highly abstract science, often unable to accommodate the geographic considerations that field naturalists value.Synthetic evolution emerges only when the studies of these two pathways are integrated and each makes a significant contribution to the final result.Provine (1978) has always emphasized the role of population genetics, and now a more moderate view has emerged (Mayr and Provine, 1980; Grene, 1983; for a more critical evaluation, see Eldredge, 1985).Population genetics is important not so much because it offers new concepts, but because it destroys legacy anti-Darwinist sentiment and focuses attention on new research.The study of microevolution (intraspecific evolution) was revolutionized by showing that geographical effects, previously explained by Lamarckianism, were in fact governed by genetic mechanisms.However, there is no evidence that natural selection can cause macro-evolution on a large scale, but the comprehensive theory of evolution is supported by the paleontological community mainly because it is consistent with existing knowledge.

population genetics Even though many early forms of population genetics ignored crucial Darwinian factors, such as geographic isolation, they were still able to rediscover natural selection as a plausible mechanism for adaptive evolution.In the period before 1920, geneticists had become convinced that mutations were the only source of new traits in evolution; they did not believe that differences in the fitness of new genes alone could control the extent to which new genes spread through wild populations .Experimental biologists are too subjective, ignoring the stresses an organism might face in the wild.They downplay the role of adaptation in evolution and exalt the role of processes that can be studied in artificial settings, such as mutation.The divide between [geneticists] and field naturalists was exacerbated by the rivalry between early Mendelians, such as William Bateson, and those biostatisticians who sought to inherit Darwin's approach to variation in wild populations.A crucial step in bringing Darwinism back to its glory was the realization that the genetic makeup of a population is far more complex than originally thought.Only then would it occur to the mind that selection-induced fitness advantages might increase the frequency of some genes and decrease the frequency of others in a population.This development was largely due to the work of three individuals: R.A. Fisher and J.B.S. Haldane in Great Britain, and Seval Wright in the United States.Mathematically matured in the work of Fisher and Haldane, and from their work gave rise to what Mayr (1959c) called the "beanbag" direction of genetics: assuming that selection acts on individual genes, Each gene has its fixed fitness value.Evolution is simply the addition and subtraction of genes in the "gene pool" of a population.Wright's research highlights the interaction of genes as a source of additional variation in small, interbreeding populations.This refined view, advanced by geneticists, is easily combined with the view of field naturalists that geographic isolation is crucial for speciation.A disagreement between Bateson and the biostatistician festered into a personal grudge, and any hope of union was soon wiped out.However, with the intervention of young scientists, it was finally recognized that it was possible to combine the research of the two paths (Provine, 1971).As early as 1902, G. Udney Ull pointed out that Mendel's laws do not necessarily conflict with the variation view obtained through measurement by the biostatistical school.Mendelians emphasized discontinuous variation because it could be demonstrated through their experimental techniques, and they therefore believed that continuity had no genetic or evolutionary significance.However, Ure proposed that if more than one pair of genetic factors can affect a trait, then the continuous variation of traits can be explained by genetic factors that conform to Mendel's laws.Simple Mendelian proportions disappear in the constant recombination of a series of genetic factors that cause the same trait to vary only to a small degree.As a result, the interaction of multiple factors leads to a continuous distribution of variation.Ure's point has deep implications.If biostatisticians could recognize Mendelian inheritance, they would refute Fleming Jenkin's classic "drowning argument" against including mutations in selection.The new factor does not exhibit only half the effect in each generation as a result of fusion; rather, the new factor diffuses through the population intact, especially if the factor has a fitness advantage.By adopting Ure's ideas, biostatisticians would gain a huge advantage and still retain their techniques for studying continuous variation.Unfortunately, the feud between the two factions has reached such a point that neither faction can understand Ull's point of view.It will be a while before the two sides can combine along this route. In 1909, Swedish biologist H. Nils-Ayer conducted a series of breeding experiments using grains, which verified Ull's ideas.He showed that some traits are affected by 3-4 genetic factors, each of which segregates independently following Mendel's laws.With 10 such factors, he calculated that there would be close to 60,000 different phenotypes.Because the differences between these phenotypes may be small, they will exhibit continuous variation.Edward East of the United States also put forward the same point of view (East, 1910).Another decade later, a growing number of geneticists began to endorse continuous variation in terms of multiple factors, including Niels-Ayer himself, when he proposed that selection might act on such a large range of genetic Mutations.Although selection can cause increases in the gene frequencies of genes controlling useful traits, most biologists still accept Johansen's view that the range of variation existing in a population sets the limits of this process.In this case, the new genetic elements produced by mutations will be the only source of new mutations in the long-term evolutionary process.This is at the heart of what Meier calls "bean bag" genetics: mutations introduce new genes into the population, and selection increases the frequency of the new genes, or, if the new genes are harmful, eliminates them.In fact, this is an extremely narrow view that ignores the complex effects of gene-gene interactions, but it was the framework for early theories of genetic selection, where experiments in the laboratory showed that mutations consistently produce small changes , these changes passed into the population undisturbed.Selection can affect how quickly new genes spread through a population, and evolution can be seen as the long-term accumulation of such small changes.Early traces of this line of thinking can be seen in the thinking of Thomas Hunt Morgan (Allen, 1968, 1978; Bowler, 1978. 1983).At first Morgan agreed with de Vries that mutation was the immediate basis for the production of new species; but he soon began to realize that new traits must be preserved in existing breeding populations.At first, he still dismissed the role of selection, insisting that any new trait, whether or not it confers a fitness advantage, can diffuse through a population. In 1910, Morgan converted to Mendelism through experiments with fruit flies and began to discover that naturally occurring variation can be small.He gradually came to admit that new traits created by mutations cannot spread without the aid of selection.By 1916, he proposed a theory of genetic selection that was not yet mature in form: harmful or neutral mutations cannot spread, while beneficial mutations can gradually occupy a dominant position in the entire population because they reproduce faster than the original genes.Morgan was never willing to admit that there could be any number of harmful genes in a population, perhaps because he found the idea of ​​weeding out unfit individuals morally repugnant.His theory is "an extreme form of "bean bag" genetics, the idea that deleterious new traits are eliminated as soon as they are created by mutation. The "classical" hypothesis of population genetics maintained by Morgan's student Herman J. Muller (Muller, 1949) remained alive until the mid-20th century. By this time this view had reduced genetic variation in populations Extent of: It is thought that only a few genes in most individuals differ from those of the species' normal or "wild-type" trait. Through mutation, such abnormal genes are produced from time to time, but are continually selected for their deleterious effects Weeding out. Only a very small number of beneficial mutations can spread and become the new wild type of the species. This view is supported by some experimental geneticists who study the more obvious and certainly more harmful mutations, and believe that mutations are almost always disrupting the function of a gene. Most field naturalists doubt this classic hypothesis, believing that natural selection is acting rather actively by maintaining a high level of genetic diversity in virtually all populations. It is only with molecular inheritance The development of science made it possible to prove that traditional methods greatly underestimated genetic variability. However, as Lewontin (1974) pointed out, this view does not automatically confirm that selection plays a more active role in evolution. The "neutral" school of population genetics emerged, which asserted that most of the observed genetic variability is neutral in fitness and thus their accumulation is not affected by natural selection (Kimura and Ohta, 1971; Kimura, 1983).

Another basis for modern synthesis theory is the "balanced" hypothesis of group structure.Morgan and Müller disagreed with what would become one of the most important insights in the new population genetics: that a large number of genes affecting each trait are already present in any natural population, open to selection.Even if the genetic traits produced by mutations do not have any advantages, they will flow in the population in the form of low frequency, and the species thus reserves variability. When conditions change, these specific variations will be subject to selection.Even in a stable environment, "balancing selection" can effectively help maintain the range of genetic variation for future adaptive evolution.

The idea that there is a large amount of heritable variation was once a central tenet of biostatistical Darwinism.At this point it is necessary to explain from a Mendelian perspective that multiple factors cause continuous variation and to show that selection has the power to alter the frequency of beneficial genes.The latter view was confirmed in 1915, when R. C. Pennet published a study of butterfly mimicry, which was accompanied by a table by the mathematician H. T. J. Norton showing how selection resulted in beneficial The spread of genes within a population.Ponnet himself believed in discontinuous evolution, but Norton's calculations showed that even a slight advantage could raise the frequency of a gene shortly thereafter.At this time, the way was opened for a new theory of gradual evolution based on natural selection acting on populations of genetic variation.

Ronald Aylmer Fisher, who studied mathematics at Cambridge University, became interested in Pearson's biometric techniques (Norton, 1975b; Box, 1978; Bennett, 1983).He soon disagreed with Pearson over his views on Mendelism, but Mackenzie (1982) argues that he has always been in favor of the biometricist program.Fisher realized that many of Pearson's problems arising from his reliance on fusion inheritance could be solved by using Mendel's laws.Unit traits can be maintained without fusion, thus maintaining population variability.His first paper on this subject was rejected by the Royal Society of London, mainly because they were too sensitive to the emotions caused by the dispute between biostatisticians and Mendelianism (Norton and Pearson, 1976), Fei Scheer's article was later published in the Proceedings of the Royal Society of Edinburgh (Fisher, 1918).Another 10 years later, Fisher used his techniques to study the effects of selection on populations of genetic variation, culminating in his famous work, A Genetic Theory of Natural Selection (Fisher, 1930).

Fisher built a convincing mathematical model based on a set of assumptions.According to this model, selection acts uniformly across large populations, and recombination maximizes variability.If a particular gene carrying a useful trait reproduces rapidly, it is possible to calculate how fast its frequency will increase.In Fisher's formula, selection is the decisive process that works slowly but does increase the frequency of individual genes.Because the model is based on individual genes, it is still based on the beanbag approach, although this model shows that selection can only reduce the frequency of unfavorable genes, not completely eliminate them if they are protected by recessiveness.Fisher demonstrated that selection works to keep the two alleles in balance when heterozygotes are more fit than homozygotes.Most of the mutations found are deleterious, but because these mutations generally arise at a fixed rate, this counteracts the effect of selection to reduce their frequency.In a small population, a rare gene may go extinct by chance.For this reason, Fisher believed that large populations are good for evolution because variability can be preserved in large populations.As soon as a gene is beneficial to the species in a certain situation, its frequency will increase immediately.According to Fisher, relatively few new factors are generally supplied by mutation into the gene pool, and these factors then become part of the regular variability of the species.So even though selection takes advantage of different mutations, evolution remains a relatively continuous process without sudden jumps.

J. B. S. Haldane published his first paper on population genetics in 1924 and an important treatise in the field in 1932 (Clark, 1969).Haldane, like Fisher, made certain assumptions to simplify the math: randomly fertile, infinite populations, fully exhibiting Mendelian dominance and segregation.He also emphasized the selection of individual genes, but Haldane showed by practical examples that this process is much faster than Fisher imagined.The best-known example is the industrial melanization of the birch-foot moth (Amphidasys betularia, now Biston betularia). The dark or melanized form of this moth was first mentioned in 1948, and later the moth began to distribute in some industrial areas of England, where its color and soot-covered backgrounds could make it possible to Avoid predators.By 1900, the melanized types had almost completely replaced the normal gray types in the region.Haldane showed that the melanized type diffuses so quickly that 50% of the offspring must be melanized, a much greater selective advantage than Fisher imagined.

Because both Fisher and Haldane postulated that selection is most effective when it acts on a large population with wide variability, their theory therefore concerned only a continuous, unbranched lineage of evolution.They ignore the speciation problem of interest to field naturalists, namely the divergence of a population into distinct clades, and they are reluctant to admit that geographically isolated groups may have important evolutionary significance.Alternatively, they used a beanbag approach, treating each gene as an independent unit with a specific fitness value.This method does not take into account the possible interactions between genes. According to this new concept of interaction, the variability of the population can be expanded without the need to produce new genes through mutation, and it can also solve Johansen's claim The number of genetic variations in question is strictly limited.William E. Caster (Castle, 1911) in the United States took the first step to break this limitation. He showed through the breeding experiments of crested rats that in some environments, additional mutations may occur.Continuous selection acting on small populations facilitates the formation of unusual genetic combinations that miraculously produce variations beyond the normal range of variation in large interbreeding populations.Castells' experiments also showed that certain "modifier genes" can affect the genetic effects that produce specific traits.At this point, one has to think of the breeding population as a complex system, capable of a great deal of variation under the influence of selection and/or inbreeding.

Custer's student Serval Wright developed a new form of population genetics based on Custer's view (Provine, 1986).Through experiments on body color in guinea pigs, Wright was convinced that the system of interactions between genes was important. He also found that inbreeding in small groups stimulated variation through experiments on crested mice he participated in.By 1920, he had developed a powerful mathematical technique to analyze inbreeding effects, which led him to discover that systems of genetic interactions could be fixed in this way and then subjected to selection.He set out to apply this idea to natural populations, arguing that in small natural populations, inbreeding occurs more readily, and this inbreeding is strong enough that by a random effect known as "genetic drift," the natural generate new interactive systems.The most favorable situation for evolution is when a large population is not evenly divided into isolated regional strains.Natural selection then begins to act on the new interacting system, causing rapid evolution.The random effect of inbreeding will cause a small population to move away from the "adaptation peak" of the species, so that the small population will pass through a relatively unadapted intermediate zone until a new fitness peak is established.Wright heavily criticized Fisher's ideas (Wright, 1930) and then discussed his own theory of evolution in more detail (Wright, 1931).By this time the bean bag approach had been abandoned in favor of a more sophisticated view in which gene pools act as intermediaries between mutation and selection in the creation of new interacting systems.

modern synthesis Few field naturalists understand the complex mathematics employed by the creators of population genetics.They can only translate the mathematical conclusions into common sense language, and then see if the insights can be used in their own research.Among naturalists who study geographic variation within and between species, a type of groupthink has arisen spontaneously.They have realized that the complex patterns of variation do not fit the typological view of species, according to which regional conditions merely alter the superficial forms of the basic internal composition.Each landrace or subspecies should be considered as a distinct breeding population with its own characteristics, and its breeding should not intermingle with adjacent populations even when geographical isolation ceases to exist.These naturalists believed that geographical isolation was crucial to the first separation of once homologous groups, and they were convinced that the environment in which each subspecies lived made each subspecies unique.Only when this group thinking was fused with mathematical population genetics did the general framework of modern synthesis emerge (Mayr and Provine, 1980).The methods used by Fisher and Haldane are less readily adopted in naturalist geographical insights, but the work of Fisher and Haldane has been sufficient to convince most biologists at this time that selection is An important mechanism for adaptive evolution.As field naturalists gradually learned some of the conclusions of mathematicians, they also began to abandon some unproven theories that they had relied on earlier, such as Lamarckism (Rensch, 1983).It is not surprising that Wright's approach proved the easiest to apply to field work, largely because his emphasis on the role of small inbred populations lends itself well to geographical research.Dobzansky adopted Wright's conclusions in his influential book Genetics and the Origin of Species (Dobzansky, 1937), thus actively promoting the modern synthesis of .

In Russia, work directed by Sergei S. Chetvelikov played a very important role in paving the way for this synthesis (Adams, 1968, 1970; see also Mayr and Provine, 1980 ).Around 1900, Russian naturalists were unaffected by the anti-Darwinist trends prevailing in the West, so Chitvelikov was in an excellent position to consider the possibility of synthesis with genetics.The Russian school arose out of the belief that there are many unseen variations in the form of recessive genes in natural populations, which Chitvelikov was able to test by crossing wild populations of fruit flies with pure-line flies brought from the United States (Chetverikov, 1926, English translation, 1961).His students D. D. Romasov and N. P. Dubinin proposed methods for studying statistical effects in groups of different sizes, which proved what Chitvelikov believed to be more likely to produce essential effects in small groups. Variant point of view.They introduced the concept of a gene pool, representing potential genetic combinations; and have recognized that such combinations obey the laws of probability (Adams, 1979).One of the results of the Lysenko affair was the disappearance of the Chitvelikov school (see Chapter 9); however, Chitvelikov's work influenced N.W. Ski, who went to Germany in 1925, and it was largely he who developed the more sophisticated ideas about the role of mutation in establishing genetic variation.Although Dubzhansky did not belong to the Chitvelikov school before emigrating to the United States in 1927, he was also influenced by Chitvelikov. In England, E. B. Ford explained the results of Fisher's research in Mendelianism and Evolution (Ford, 1931).Ford was interested in ecological problems, and his subsequent work further showed that selection acts more rapidly than Fisher had expected.This proves that, in fact, the industrial melanization of the birch moth analyzed by Haldane was not an exception.Gavin De Beer's Embryology and Evolution (De Beer, 1930) shook the evidence in favor of reenactmentism, on which the Lamarckians had once relied heavily.Because the effects of genetic mutations do not add to the existing growth landscape, there is little reason to think that individual organisms must grow through the adult stages of their evolutionary ancestors.In Britain, the most influential was J. Huxley, grandson of T. H. Huxley, who taught at Oxford University from 1920 (Huxley, 1970).Naturally, Huxley took Darwinism from previous generations of biologists and set out to revive the theory.He co-authored the book "Life Science" (Wells and Huxley, 1930) with H. G. Wells, which comprehensively and popularly explained Darwin's view of evolution.He himself strictly followed the tradition of natural history in the study of animal behavior, but he was also interested in the growth of embryos and followed the latest advances in genetics. In 1940, he edited The New Systematics, a collection of contributions from all aspects of biology, and a comprehensive treatise, Evolution: A Modern Synthesis, was published in 1942. In the United States, Francis B. Sumner did pioneering work (Provine, 1979), including anticipating many aspects of later synthesis.Sumner had a Lamarckian bias when he began his study of geographically isolated variations in white-footed mice, but he soon proposed (Sumner, 1924) that morphological variation in populations had a genetic basis. Ernst Mayer came to the United States from Germany in 1930. After several years of field research on birds in the islands of New Guinea and the Solomon Islands, Mayer became influenced by Bernhard Lensch, who revived the A belief that there is a clear correlation between geographic variation and climate.At first Lensch and Mayer explained the phenomenon in terms of Lamarckism, but in the 1930s both of them realized that it was possible to explain it in terms of Darwinism.At the time, Meier had not read the literature on mathematical genetics, so he based his own experience on proposing a group view of species that had developed independently on the basis of field studies.In the eyes of Mayer and many other naturalists, it was Dubzhansky's book Genetics and the Origin of Species, which presented the mathematician's conclusions in a concise manner, that pointed the way to the union.Mayr's own Systematics and the Origin of Species (Mayr, 1942) is a foundational work of modern synthesis, emphasizing the role of geographical factors in the process of speciation. When Dubzhansky joined T. H. Morgan's research group at Columbia University in 1927, he brought experience with the group methods of the Russian school.He was thus able to understand what the field naturalist wanted from the geneticist, and the result was Genetics and the Origin of Species (Dobzansky, 1937), which combined the naturalist's practical experience with experimental and mathematical Biologists build bridges between abstract expressions.In the book, Dubzhansky outlines the experimental evidence for the true nature of mutations, emphasizing the small effects of mutations and how they cause natural variation in populations.He generalized the conclusions of mathematicians, especially Wright's work.He also discusses his own work on the genetic basis of geographic variation in insects and other work demonstrating the same effect.Beginning in 1938, Dobzhansky, in collaboration with Wright, conducted a series of studies in the genetics of natural populations (collected in Dobzhansky, 1981).One aim of these studies was to show that selection is not only a mechanism of change but also maintains stability through balancing mechanisms, such as those based on adaptation in superheterozygotes.This widespread homeostasis is important to modern Darwinism, because it suggests that populations do contain large reservoirs of genetic variation that are capable of expressing variation in new environments. Dubzhansky coined the term "isolation mechanism" to refer to non-genetic characteristics, such as differences in behavior, that prevent mating between two related groups when the two groups occupy the same area.The founders of modern synthesis believed that speciation would only occur if an initial geographic isolation had been experienced, and for this reason Meyer introduced the term "allopatric speciation".Without interbreeding, different populations can develop distinct traits.Even if geographic isolation later disappears, isolation mechanisms can prevent interbreeding between different populations; this allows subspecies to remain distinct, and under the influence of selection, subspecies further diverge from the original species.Neo-Darwinism is based on the assumption that speciation does not require special genetic mechanisms; natural selection alone can diverge from one species into multiple species if geographical isolation exists. The extension of the comprehensive theory of evolution into the field of paleontology was largely by George Gaylord Simpson, in his The Rhythm and Way of Evolution (Simpson, 1944).Simpson's work sheds light on the rationale for the idea that macroevolution revealed in the fossil record arose through the cumulative effects of microevolution, which was being studied in modern populations at the time.No evidence could be presented; despite the objections of an earlier generation of anti-Darwinian biologists, it was possible to show that evidence from paleontology was at least consistent with the new theory (Gould, 1980a).Simpson showed quantitatively that apparent evolution proceeds in the irregular and non-directional manner that Darwinism had predicted.He exposes the fragility of the evidence in favor of linear development, which is more in line with Lamarckism and Orthogenesis.For example, the evolution of horses was not a simple progression toward modern specialized structures. The evolutionary tree of horses was very irregular, with many branches, some of which were extinct.The vexing problem of discontinuity in fossil sequences, Simpson recognized, was more than an incomplete fossil record.Substantial change can occur according to non-adaptive "quantum evolution", the mechanism being what Wright describes as genetic drift.Because these transitions occur relatively quickly and in small populations, such changes are unlikely to leave fossil evidence.In later writings (eg Simpson, 1953b), Simpson adopted a more strictly Darwinian approach, paying more attention to all the adaptive features of evolution. Typical of Dubzhansky's genetic studies of natural populations, modern synthesis has entered the era of collaborative research.Different mathematical models are tested one by one by direct application to nature (Provine, 1978).Fisher had to admit that selection was acting faster than he had thought, but Wright's idea of ​​nonadaptive evolution through genetic drift in small populations remains controversial.At present, synthesis theory has become "ossified," even abandoning the limited non-Darwinian mechanisms acknowledged by its creators (Gould, 1983).At the same time that it became clear that speciation easily occurs when small groups are isolated, many naturalists felt that differences between groups were due to natural selection acting on adaptive traits. Further research on the phenomenon of industrial melanization by H.B.D. Kettlewell (1955) illustrated the effect of camouflage as a method of combating predators, thereby emphasizing the importance of adaptation.Recently, molecular biology has been used to confirm the fact that there is a large amount of genetic variability in populations (eg Lewontin, 1974).Efforts have also been made to suggest that many of these variations do not translate into phenotypic differences on which selection can act (Kimura and Ohta, 1971), but this view has not been widely accepted.Despite a great deal of controversy in recent years, extensive research continues within the theoretical framework established by the synthetic theory of evolution (see Chapter 12; for modern Darwinism, see Simpson, 1953b; Mayr, 1963; J. Maynard Smith, 1976; Dobzhansky et al., 1977; Ayala and Valentine, 1979; Futuyma, 1979; Ruse, 1982). Origin of Life A corresponding development that contributed to the revival of Darwinism was the creation of the first modern theory of the origin of life on Earth.In the late 19th century, the age-old idea that life arose from nonliving matter through "spontaneous generation" finally fell into disrepute, yet evolutionists were unable to come up with a sound theory of how the initial life process began. In 1936, the Russian biologist Alexander Obalin answered this question from a new perspective. He proposed to replace the original view of spontaneous generation with the chemical evolution process at the level of ever-increasingly complex tissues.This not only broadened the scope of evolutionary thinking, but also reaffirmed Darwinism, since O'Barin posited that natural selection was the mechanism for the improvement of pre-living structures before they arose.When, in the 1950s, Stanley Miller and others used experiments to confirm certain stages in this process, O'Balin's view of the origin of life was accepted as an integral part of modern evolution (Farley, 1977). In one, Darwin wrote that the first forms of life appeared to have been created by gods, although privately he admitted that saying that was no more than a way of dodging the problem.In the early 19th century, the concept of spontaneous generation was not in vogue, and Darwin was reluctant to see his theory associated with any wild guesses about entirely unknown causes.尽管如此，他的追随者意识到，为了保持一致，进化理论应该象解释生命的发展一样，从自然原因的角度解释生命的起源。海克尔假定存在一种生命的原始形式，“原核生物”，它是没有结构的原生质，但是已经具备了生命的基本特性。这就形成了非生命物质与含细胞的更高级生物之间的关键联系。 1868年，赫胥黎在从海底挖掘出的泥土表面发现一层胶状物。他认为这是进化第一阶段的活例证，并将其称之为海克尔原肠虫(Bathybius haeckelii)（Rehbock，1975）。有人认为海底仍然覆盖着原始泥状物(Urschleim)，生命的高级形式正是由它演变而来。然而很快就发现，当把酒精作为防腐剂加到海水中后，便沉淀形成这种胶状物。因此赫胥黎被迫放弃今天仍然可以观察到生命起源第一阶段的论断。 因为原肠虫没有细胞结构，所以它极大地支持了下述看法，即物质一经达到适当的化学组织水平，就会出现生命的特性。这使得化学合成作用——在今天的世界仍然可以重复这种作用——是生命起源根由的观点更加说的过去。19世纪80年代，一时兴起的对自然发生的热情很快就退去了，因为大多数科学家都相信路易斯·巴斯德详细证实的不存在从非生物到生物的证据。细胞理论和种质概念的发展也有助于人们相信新的生命结构只能来源于原先存在的生命。核物质从一个细胞到另一个细胞的连续性，被看作是生殖必不可缺的基础，意味着生命的特性是由于细胞的组织化。因为仅仅是纯化学活动不可能产生一个细胞的复杂结构，所以，在任何自然条件下生命都不可能自然发生。为了避开地球上的生命最初是如何出现的这个问题，有人甚至认为最初的生命是来自太空的孢子(其来源也不清楚)“播种”到地球上的（Arrhenius，1908）。 在20世纪早期，许多生物学家开始对胶状的悬浮液感兴趣，而且一些生物学家认为这些悬浮粒子的特性多少预示了生命自身的活动。奥巴林首先把生命的起源看作是一个独特的事件，认为当第一个细胞从胶状的悬浮液中沉淀下来时，便发生了生命的起源。这仍然强调的是经过一个阶段的自然发生，这样便跨越了生命与非生命之间的界线；关键的进展来自于偶然的化学合成。J·B·S·霍尔丹1923年发表了一篇论文，他在这篇论文中提出，通过有机化学的自然作用，形成了类似病毒样结构的原始形态。奥巴林后来工作的创新是放弃了建立在偶然合成基础上的单阶段生命形成的思想。相反，他提出了更加成熟的图景，其中生命的出现过程中不存在明确的界线。化学进化的过程逐渐提高组织化的水平，导致从非有机物到第一个活细胞之间的完整连续性。 法利(Farley，1977）认为，奥巴林观点的改变是受了辨证唯物主义的影响，而且法利指出，奥巴林著作的成功加重了李森科主义的灾难。如果说李森科是个机会主义者，其他人则发现，比起一些老式的唯物论，辩证唯物主义具有明显的优点。奥巴林支持李森科，他自己也遭到一些遗传学家的批评，他们宣称他的理论也想要破坏基因作为生命组织化基本源的地位。然而，在奥巴林的案例中，唯物主义逻辑在生物学上的应用所带来的并非科学上的滑稽，而是真正的进步。辩证唯物主义既不是机械论哲学，也不是还原论哲学，因为辩证唯物主义认为，当组织化达到一个新的水平，便出现全新的自然规律。在较低水平上量变造成质变的辩证规律，促使奥巴林提出生命的出现是物理复杂性程度逐渐提高的结果。另外，辩证法认为，新特性的出现否定了前一状态的特性。于是奥巴林认为，生命一旦在地球上出现，这一过程将绝不会再次重复。生命有机体在其自身的形成过程中便将更早的状态破坏。因此正如巴斯德所表明的那样，现在不可能创造出生命。