Chapter 26 Chapter Thirteen Development After Synthesis-2
Some modern scholars have suspected that there might be a conflict between Weismann's theory of sex and the principles of reproductive success.A parthenogenetic species produces twice as many offspring as a sexual species, which "wastes" half the zygotes on the male side.One might therefore think that natural selection would select parthenogenesis between the two (Williams, 1975; Maynard Smith, 1978).Parthenogenesis is indeed common in both the plant and animal kingdoms, but is much less frequent than sexual reproduction, a mystery for which there is as yet no satisfactory explanation.There is no doubt that sexual reproduction is superior in the long run because it provides a fire escape when major changes in circumstances occur.
But in a more stable environment in the short term one would think that diploid fecundity of parthenogenesis would prevail.Perhaps people will turn to the "expendable surplus" (expendable surplus) principle: even in sexual reproduction organisms, there is already a large enough expendable surplus; Will there be any special benefits.And giving up sex will undoubtedly greatly reduce the freedom of choice for future evolution.The evolutionary line that transitioned to parthenogenesis is likely to become extinct sooner or later, along with any mechanisms that made the transition possible.What will remain are sexual lineages that cannot transition to parthenogenesis but are able to occupy the habitat vacated by the extinct parthenogenetic lineages.
Of course, sexual reproduction is mandatory wherever there is the possibility of a second parent participating in childcare.There are many other interrelationships between sex, behavior and habitat use (Ghiselin, 1974a).It has long been known that in many organisms (certain parasites, freshwater plankton, aphids) there is a normal alternation of generations between sexual and parthenogenetic reproduction, and that the transition from one state to the other is closely related to environmental changes.
Natural selection is indeed often bewildering, and modern evolutionists are still as confused as Darwin and Wallace about the selection of certain natural phenomena.Considering what a useful organ the human brain is, the question is sometimes asked, why didn't natural selection produce equally large brains in all living things?Indeed, why?Or to turn the question around, what selection pressures gave Neanderthals brains as big as Darwin, Einstein, and Freud?It was the inability to account for the large brains of our primitive ancestors that led Wallace to doubt that natural selection could account for the derivation (origin) of humans as humans.What Wallace ignores is that the critical moment of all choices is an emergency or an extraordinary disaster.A certain organ or function is generally not changed by natural selection in normal times, but is selected at that time at the end of the variation curve, and the carrier of this organ or function is not changed by other thousands of species in an emergency. Thousands of individuals can survive when they die. "Catastrophic selection," as Lewis (1962) rightly emphasizes, is a very important evolutionary process.
13.4 Modes of Speciation
Darwin is recognized as the representative of population thought, and he emphasized the gradual nature of the process of geographic speciation (speciation) (see Chapter 11).The Mendelians categorically rejected this conclusion of Darwin's, postulating, as de Vry put it, "that new species and varieties arose by some kind of leap from existing forms." One of the points of contention among schools of thought (Mayr and Provine, 1980).Comparative anatomists, scholars who study phylogeny, and even experimental geneticists consider evolution strictly from a "vertical" point of view, considering the Phyletic line to be the unit of evolution.An important contribution of the new systematics is the adoption of the population as the unit of evolution and the interpretation of speciation in terms of this concept.New systemists claim that new species form when populations are isolated, a contention that Mayr (1942) has demonstrated and supported in rich detail.The size of the isolated population was not mentioned at first, only Wright (1932) pointed out the possibility of genetic drift due to sampling error in small and very small populations.
Geographic speciation is based primarily on studies of birds, butterflies, other wide-ranging insects, certain groups of snails, and other groups of animals with well-defined patterns of geographic variation.Geographic speciation is so certain in the above-mentioned groups, and the divergence steps of isolated populations so extensively documented, that geographical speciation, an important and probably common mode of animal speciation, has been re-established after 1942. Nor can there be any doubt.
Since the number of insurmountable geographic barriers (mountains, bodies of water, etc.) on the continents is limited, other types of barriers must be associated with very active speciation processes on the continents.Some scholars (cf. Mayr, 1942) suggest that vegetative barriers or other inhospitable areas may be such barriers. Keast (1961) amply demonstrates this view with the examples of Australian birds. Haffer (1974) found that the alternation of drought and flood in the Pleistocene of the Amazon River Basin was related to the unusually active speciation of local birds. Williams, Vanzolini and Turner respectively pointed out that the same is true for reptiles and butterflies.The barrier effect of the vegetation zone depends on the dispersal ability of the species.Even narrow inhospitable ecological zones in the case of flightless or subterranean mammals can be barriers to dispersal.Some scholars proposed a non-geographic speciation mechanism because they did not understand vegetation barriers (White, 1978).
Until recently, some textbook illustrations have shown geographic species as geographical barriers cutting widespread species in half.That is to say, the two halves will become so different after a period of time after being isolated from each other that they interact as different species identities when contact is later restored.But detailed studies of the distribution patterns of species-forming taxa, especially those that have recently formed, have led to a different view.When Meyer was studying the geographic variation of South Pacific island birds in the 1940s and 1950s, he unexpectedly found that the outermost populations were often the most divergent, often so different that they could be placed in different species or even in different genus .Meyer in 1942 had documented numerous cases of highly variable exotic "genera" that were merely distant subspecies in distribution.At that time, although his attention was only on the classification problem (how to divide such populations), he continued to explore the reasons for these phenomena.
Knowing that founder populations were often at the outskirts of the ranges of true species, he finally thought that such a population could exist in the absence of any perceptible flow of genes and with somewhat different physical and biological environments. The founder population would be the ideal base for genetic reorsanization of the gene pool (Mayr, 1954).
There are two reasons why Meier places particular emphasis on the importance of the founder population.The first reason is the observation that the most unusual populations of species are always isolated at the fringes of their ranges, and that these most unusual populations are often the most distant.For example, Dicaeum tristrami (Sancristobal) in the genus Dicaeum, Ducula galeata (Marquesas) in the genus Ducula, and Meyer (1942; 1954) listed many similar examples.In contrast, the amount of geographic variation in species ranges that are close to each other is generally small.Another reason pointed out by Haldane (1937; 1957) is that large widely distributed populations (indeed all dense species) are evolutionarily retarded because new alleles, even beneficial ones , it will take a long time to spread across the entire range of the species.Genetic homeostasis (Lefner, 1954) strongly prevents large, undivided gene pools from changing.The facts of geographical isolation do not seem to support Wright's model, according to which the most rapid evolution occurs within large species consisting of only partially isolated small populations (demes).In fact dense, widespread species tend to show little change throughout the entire period of the fossil record from their first appearance until their extinction.In contrast, the evolutionary trends of marginally isolated populations are often very different.These populations are usually established from a small number of individuals (indeed, often from a single fertilized female) and contain only a small fraction of the total genetic variability of the parental species.Meyer thinks this will greatly increase heterozygosity and alter the fitness of many genes on a drastically altered genetic background.Many epistatic interactions will be quite different from what they were in the parental population.Mayr therefore argues that such founder populations should be particularly susceptible to genetic modification, sometimes capable of true 'genetic revolutions' (Mayr, 1954).The possibility of drastic genetic changes in these founder populations is beyond doubt.
The excellent study of Drosophila speciation in the Hawaiian Islands by Hampton Carson (1975) fully supports Meyer's contention.The conclusion drawn from experimental observations that speciation is most easily and rapidly achieved in very small populations is perfectly valid.
The possible important role of chromosomes in speciation was recognized during the first 20 years of this century.Almost one-third of the first edition (1937) of Dubzhansky's Genetics and the Origin of Species was devoted to phenomena related to chromosomes, and the role of chromosomes was even more important in the botanical literature.In fact, de Vry's evening primrose "mutation" turned out to be mostly a chromosomal rearrangement, not a formal speciation mechanism.However, polyploidy was soon discovered, a process in which new species can be formed in one step due to genome multiplication (Stebbins, 1950; Grant, 1971).Yet the discussion of the role of chromosomes in speciation has been hampered by two misconceptions.
The first misconception is that experts of one class of organisms assume that their findings apply to all organisms; many researchers point out that this is not true.For example, the idea that all speciation is due to chromosomal modification was rejected by Carson (1975), who pointed out that the very active formation of new species in Drosophila hawaii can proceed without any visible change in chromosomes.Because Drosophila hawaii possesses large salivary gland chromosomes for detailed analysis, structural chromosomal changes that may have occurred when speciation occurred must have been very subtle.In other types of organisms, closely related species often diverge widely due to karyotype inversions (intra- or inter-arm inversions), translocations, Robertsonian fusions or splits, or other changes in chromosome structure.Different groups of organisms have different mechanisms of chromosome change. (Mayr, 1970:310-319; White, 1974).
Another misconception is that chromosomal speciation and geographic speciation are incompatible, either one or the other.In fact, the scale of the two speciation modes is completely different.Chromosomal differences of the kind that distinguish closely related species (as opposed to polymorphisms) almost always reduce the fitness of heterozygotes due to various perturbations during meiosis.Such chromosomal rearrangements have little chance of occurring in a large population, where they go through many generations of heterozygosity.Only in the small founder population, due to the high degree of inbreeding, do they have the opportunity to quickly pass through the heterozygous stage and reach the homozygous state with higher fitness for the new chromosome type.This situation of chromosomal rearrangement is also fully applicable to the new epistatic balance of genes, to the acquisition of new isolation mechanisms and new attempts at habitat utilization.These are easier to achieve through the bottleneck of the founder population than through the slow selection process in a dense and large species.There is no contradiction in saying that a new species arose through geographic speciation and chromosomal speciation. The term "marginal isolation" is somewhat ambiguous when applied to species with low population densities and greatly reduced dispersal capabilities.In this case the species may consist of many more or less isolated colonies, and a new isolated colony may be established in the formerly vacant part of the species' range.
Even such a founder population would go through the same stages of inbreeding and homozygosity as if it had been isolated outside the periphery of the species' range.
Some evidence has been found that the ease of speciation is primarily (inversely) related to population size and that rapid speciation is not necessarily limited to founder populations.Rapid reductions in populations (such as some remnants of the Pleistocene) can also accelerate speciation, as has been confirmed by Haffer et al. (1974) on remnants of Amazonian forests.Such species, however, are evidently less anomalous than some species which have formed in peripheral isolation.
Another important difference of opinion on speciation remains the old issue of the debate between Darwin and Wagler in the 1860s and 1870s as to whether geographical isolation was at all necessary (Mayr, 1963; Sulloway, 1979).Some scholars have successively proposed some sympatric speciation mechanisms that can separate a single cell population (deme) into two reproductive isolations without external barriers that prevent gene flow. Among them, three mechanisms are often mentioned:
(1) disproportionation selection (split selection), which can tear apart the bimodal gene distribution (1bimodal gene distribution), (2) allochronic speciation, which is the result of staggered reproductive seasons, (3) in Colonization of a new host in the case of a host-specific (specific) species.The concept of homogeneous speciation via host specialization, popular from Darwin to evolutionary synthesis, is now gaining widespread attention again (Bush, 1974).However, as I pointed out in 1942, although adaptation to new hosts may be an important mode of synaptic speciation in monophagous species, especially plant-eating species, such species There are many constraints on the occurrence of formation; whether it occurs with high frequency due to geographic speciation remains an unresolved question (White, 1978).I think drawing a strict line between geographic speciation and new host transplantation speciation is unscientific and misinterprets the issue again.
It is obviously much easier to switch to a new host in a small founder population (whether in nature or in the laboratory) than in the contiguous range of a large, dense population.
The most important unresolved question in the study of speciation remains the genetic basis of speciation.In order to describe the speciation process, inferences from distribution patterns are still mainly relied on.The debate over the frequency and validity of the various possible modes of speciation can only be resolved with a better understanding of the underlying genetic processes.As late as 1974, Lewontin said, "We know practically nothing about the genetic changes that occur in speciation"
(Lewontin, 1974; 159).Sadly, this is still the case today.Due to the discovery of DNA inhomogeneity (heterogeneity) the earlier literature is effectively obsolete (Jameson, 1977).
It was initially thought that comparing the frequency of the acid allele in populations before and after speciation could provide conclusive answers.This kind of research followed the tradition of beanbag genetics, trying to "establish a quantitative theory of speciation according to genotype frequency" (rewontin, 1974: 159).Yet all the evidence accumulating along this line of research indicates that the switching frequency of the chelate allele is not a major agent in speciation.For example, the degree of difference between isozymes in closely related species varies greatly in different genera.Crossing species boundaries does not appear to coincide with dramatic shifts in gene frequencies.Some scholars have interpreted this as a refutation of Meier's theory of genetic upheaval in founder populations.This view would be correct if enzyme genes were the main genetic mechanism of reproductive isolation.
It now seems very likely that the degree of reproductive isolation is controlled by specific genetic mechanisms or regulatory systems.Such mechanisms may be limited to a relatively small number of genes or a limited part of the karyotype (Carson, 1976), and they may exist on some new DNA (such as intermediate repeat DNA) identified in recent years.Rapid and unexpected discoveries in molecular genetics are likely to lead to important revisions in the genetic explanation of speciation in the near future.
If only a limited portion of DNA controls reproductive isolation between species, it is possible that only a few mutational steps or limited karyotypic shuffling are required to initiate the speciation process.This is obviously much easier in a founder population consisting of only a few individuals than in a dense species with a wide distribution.On the other hand, the numerous isolation mechanisms separating most species suggest that in most cases complete species hierarchies can only be formed by some long-term process.Since speciation is asymptotic, meaning that even founder populations continue for many generations, it cannot be expected to be complete with just one mutation.All indications point to Chen being otherwise.But exactly what's going on during speciation remains a mystery. Carson (1976: 220) conceived of it as "an internal balance in a certain flux of gene interactions in which regulatory genes play an important role."
What remains in Meier's theory entirely unexplained is the irregularity of genetic upheaval.They occur in some (not all) peripherally isolated founder populations.Why?The understanding of the genome has come a long way since 1954.It has now been shown that certain parts of DNA, the isozyme genes, are less susceptible to genetic upheaval than other parts (and possibly certain regulatory systems). Templeton (1980) has considered some of these factors, especially why genetic upheaval occurs only under certain conditions.Given the current limited knowledge of the roles of the various types of repetitive DNA and aspects of the newly identified genotypes, it is too early to provide a definitive explanation.Yet all studies in recent years have provided further evidence in support of Meier's theory, showing that decisive evolutionary events occur most often in peripherally isolated founder populations via genetic upheaval.
13.5 Macroevolution
After evolutionary synthesis, besides natural selection and speciation, the third major active area of evolutionary biology is macroevolution.Macroevolution has several different definitions: evolution above the species level; evolution of higher taxa; or evolution studied by paleontologists and comparative anatomists. Around 1910, palaeontologists, especially invertebrate paleontologists, lost interest in evolutionary history by paying special attention to geological problems because of their achievements in determining geological layers.Before evolutionary synthesis macroevolution was studied by paleontologists who had no substantial connection to genetics.Only a handful of paleontologists are true Darwinists, acknowledging that natural selection is the main agent of evolution.Most paleontologists believe in catastrophism or some form of teleology (spontaneous occurrence).Macroevolutionary processes and their causes are generally considered to be of a special type, quite distinct from the population phenomena studied by geneticists and speciation scholars.
All of the above changed dramatically with the development of evolutionary synthesis.The main result is to cast doubt on certain beliefs widely held by previous scholars of macroevolution.Some important hypotheses that have now been rejected include the following:
(1) Sudden changes must be used to explain the origin of new species and advanced taxa; (2) The continuous improvement of evolutionary trends and adaptations must have a natural process; (3) Inheritance is soft.
The important contribution of Renxi and Simpson was to be able to show that the explanation of macroevolution does not need to admit any of the above-mentioned theories, and that the phenomenon of evolution above the species level is in fact consistent with the new findings of genetics and microsystematics (microsystematics).This conclusion must of course be inferred from morphological, taxonomic and distributional evidence, since genetic analysis was inaccessible to higher taxa at the time (and indeed remains so today, apart from molecular evidence).
In defense of paleontology, it must be pointed out here that although catastrophists and those who support the process of natural generation make up the overwhelming majority of paleontologists, there are indeed a small number of gradualists and those who support natural selection.As early as 1894 W. B． Scott strongly defended the gradual nature of evolution against Bateson. Scott once said that although in all species there are more or less obvious variations around the "normal", new changes in the germline do not arise from extreme variations but from gradual changes in normal individuals (Scott, 1894 :359). Osborn and other proponents of "straight generation theory" also support gradual evolution and oppose "catastrophe theory".
Natural selection also has supporters.While most paleontologists agree that natural selection is insufficient to explain macroevolutionary phenomena, there are some (eg, Dollo, Kovalevsky, Abel, Goodrich, Matthew) who actively support natural selection.It is not clear from their writings, however, whether they considered natural selection alone to be sufficient to explain all evolutionary phenomena.Determining this would require adequate analysis of their and other contemporary macroevolutionists' writings.
In his introduction to Tempo and Mode in Evolution (1944), Simpson said his book was an attempt to synthesize paleontology and genetics.Efforts to bridge the two fields have been doubly difficult because geneticists have focused almost exclusively on changes in gene frequency (based on the assumption that non-additive gene effects are of minimal importance).This idea works well when explaining some macroevolutionary questions, such as evolutionary tendencies, but not others, such as the origin of diversity.
The synthesis between genetics and paleontology can be said to be carried out in two steps according to the following questions: (1) Are there any macroevolutionary phenomena that obviously contradict the genetic explanation of Darwin's theory? (2) Can all the principles of macroevolution be formulated only through the study of gene frequency in the population?The answer to these two questions turns out to be no.
The first task of Darwinian macroevolutionists is to refute arguments against Darwinists' assertion that there are some macroevolutionary phenomena that contradict the formula of "genetic variation and natural selection."This task was accomplished brilliantly by Renji and Simpson.Both of them (Lian Gang Huxley) pointed out that evolutionary trends do not need to resort to some mysterious natural occurrence factors to explain, but the increase of the whole body of animals, changes in the proportion of individual structures (such as teeth), certain structures Some declines (e.g., the toe of a horse, the eye of a cave animal) and some other long-term evolutionary regularities can be explained by natural selection.They have since revealed that both genetic and functional constraints also enhance the ability of natural selection to control evolutionary trends (Reif, 1975).
Since Geoffroy Saint-Hilaire some scholars have put forward many "laws" of evolution.In all cases these laws can be explained by natural selection.For example, Dollo's so-called "Irreversible Law", which means that the structures lost during evolution can never be completely regained in the original way.This view is clearly a consequence of the fact that genotypes are constantly changing in evolution, and that if a need arises for a structure that has previously disappeared, this structure will be inherited by another genotype quite different from the genotype that produced the original structure. As a result, the new structure is not exactly the same as the one that disappeared (Gregory, 1936).
Most evolutionary phenomena are concerned with complex structures, organ systems, whole individuals, and populations.In order to fully explain these phenomena, nothing is more helpful than the reductionist research method, which expresses everything in terms of gene frequency.Yet such reductionism is completely unnecessary for neo-Darwinism.Once the purely reductionist line of research is abandoned, the vast majority of objections against the Darwinists become heaven.
Simpson places particular emphasis on the speed of evolution.He has pointed out that some evolutionary lines change quickly, others change slowly, and most evolutionary lines have intermediate speeds.In addition, he also pointed out that the speed of a certain line (Phyletic line) may increase or decrease during the evolution process.Simpson called the fastest evolutionary evolution "quantum evolution" (quantum evolution, or translated as leap evolution), which is defined as "a biological population in an unbalanced state transitions to an equilibrium state relatively quickly, and this equilibrium state Significantly different from the state of its ancestors"
(Simpson, 1944:206).Simpson argues that it explains the well-known phenomenon that "significant evolutions can indeed occur in short periods of time and under special circumstances at a considerable rate" (p. 207). From his 1944 and subsequent works (1949: 235; 1953 : 350; 1964b: 211), Simpson considered mainly the problem of greatly accelerated evolution in a germ line.Simpson's thinking was clearly influenced by Wrisht's (1931) model of periods of maladaptive genetic drift followed by natural selection.Extreme changes in the rate of evolution are of course well documented in fossil history.Bats apparently evolved from insectivores in just a few million years, but no major structural changes occurred in the ensuing 50 million years.It also took only a few million years to go from dentate reptiles to Archeopteryx, but birds as a whole had the first modern birds about 70 million years ago, and haven't really changed much since then.The dramatic change in the rate of evolution does not in any way suggest that there is any contradiction between Darwin's theory and the origin of morphotypes in bats or birds.
Problems related to the speed and tendency of evolution can be explained according to the formula of geneticists: evolution is the change of gene frequency.But this formula is meaningless with respect to a large number of other problems of macroevolution, which is one of the reasons why genetics has contributed so little to solving macroevolutionary problems.This poor formulation is also related to the time lag between evolutionary synthesis and the full study of some of these issues.
One of the most frequently raised arguments against Darwinian gradualism is that gradualism cannot explain the origin of the miracles of evolution.The so-called miracles of evolution refer to completely new organs, new structures, new physiological functions and new ways of behaving.It is often asked, for instance, how the underdeveloped wing was enlarged by natural selection, before it enabled the bird to take off?How, in fact, has any primitive organ been perfected by natural selection, and made to perform its functions adequately?Darwin (1859; 1862) believed that the function change of structure was the key to answering this question.His answer has been generally ignored until Dohrn (1875), Severtsov (1931) and Mayr (1960) have further elaborated this point of view to attract people's attention.
In this process of functional transformation a structure always passes through a stage in which it can perform two functions at the same time, e.g. the antennae of a (plankton) flea are both sensory organs and its swimming organs (caudal radials).This type of dual functioning is possible because genotypes are highly complex systems that always produce aspects of the phenotype that are not directly selected for, but are simply the result of the selected genotype. "by-product".Such by-products later emerge as mechanical parts of new functions.In this way, the quadruped's forelimbs (together with the wing membrane) can function as wings, and the fish lungs as swimming bladders.There are many "neutral aspects" of the phenotype of any organism that are "allowed" by natural selection (not excluded by selection) but not specifically selected for.Such components of the phenotype can assume new functions.There are also functional shifts in polymers and behavioral patterns, such as the switch from grooming to courtship in some birds.
As Severtsov pointed out, functional enhancement often enables a structure to accept an apparently new function.For example, the forelimbs of walking mammals transform into the digging tools of moles, the wings of bats, or the flippers of whales.
As the starting point of eye development, only photoreceptor cells need to be present.Natural selection would then help to acquire the necessary auxiliary mechanisms.This is why photoreceptors, or eyes, have evolved independently more than 40 times in the animal kingdom (Plawen and Mayr, 1977).Under most circumstances, no major mutation is required to achieve a new marvel of evolution; sometimes however some sharp mutation in phenotype seems to be the first step, such as the polymorphism of mimicry, but once this second In one step, finer adjustments can be made by small modifying mutations (Turner, 1977).Yet the key factor in achieving most of the miracles of evolution is behavioral change.
According to Lamarck, behavior is an important mechanism of evolution.The combination of physiological processes and inheritance of acquired traits caused by behavioral activities (use and disuse or use and disuse) is the cause of evolutionary change.Since the evolutionary mechanism he proposed was rejected by genetics, mutationists have gone to the other extreme.In their opinion, major mutations produce new structures that "go in search of a suitable function."Modern evolutionists do not accept either explanation.They argue that changes in behavior are indeed important leaders in evolutionary change.But the causal chain is quite different from what Lamarck or catastrophists imagined.The modern explanation is that changes in behavior generate new selective (pressure) forces, which then change the structures involved.
Mayer (1974a) pointed out that various behaviors have different roles in evolution.Behaviors that communicate information, such as courtship, must be rigid to avoid misinterpretation.The genetic program controlling such behavior must be "closed," that is, must be reasonably resistant to any change during the lifespan of the individual.Other behaviors, such as those controlling food or habitat choice, require some flexibility to adapt new experiences, and such behaviors must be controlled by "open" genetic programs.New selective pressures caused by behavioral changes may lead to morphological changes in favor of occupation of new habitats or new adaptation areas.For example, Bock (1959) pointed out that the original woodpeckers had changed their behavior to crawling on tree trunks and branches, although they still basically had the foot structure of their ancestors, but this new habit produced new selection pressure on several different woodpeckers, making The height of its foot and tail structure is specially designed for more efficient climbing activities.
Many, if not most, of newly acquired structures during evolution can be attributed to the selective pressure exerted by newly acquired behavior (Mayr, 1960).Behavior thus plays an important leader role in evolutionary evolution.Most adaptive radiation is apparently caused by behavioral changes.
Orthodox phylogenetic studies focus almost exclusively on the evolutionary past.Its question is: what is the structure of the common ancestor?How can it be reproduced by studying the homologous characteristics of its descendants?The main purpose of this discipline is to demonstrate the validity of Darwin's theory of common ancestors.It is primarily concerned with determining the type of isolation (pattern) and where the lineage line should occupy on the phylogenetic tree.The focus of comparative anatomy research is the common ancestor, from T. H．This is true from Huxley and Gegenbaur to Remane and Romer.
Dissatisfied with the dwindling output of this line of research, a group of young evolutionary morphologists began to ask why.They came up with a new research method that can be said to invert the evolutionary tree, that is to say, using the common ancestor as the starting point of their exploration.They asked questions such as: Why did germline lines originating from a common ancestor branch?What are the factors that allow certain metamorphoses to move into new habitats and adaptation areas?Is behavioral change a key factor in adaptive change?The focus of this new line of research is clearly on the nature of selection (pressure).
Severtsov, Boker, Dwight Davis, Book, von Wahlert, Gans are the pioneers and representatives of this new evolutionary morphology.Their line of research builds a bridge between morphology and ecology, leading to the establishment of a new fringe discipline, evolutionary morphology, which is still young and on the threshold of further development.
Some noteworthy results of these studies may be mentioned here.One of these was the refutation of the notion of "harmonious development of types", which is the main tenet of idealist morphology.例如当发现南方古猿（Australopithecus）时解剖学家Weidenreich曾和我谈起它不可能是人类的祖先。它不可能是类人猿与人类之间的桥梁，因为它是“不和谐的类型”（进步的骨盆和四肢，原始的胞和面孔）。
实际上类型和谐发展的概念以往曾被驳斥过很多次。在研究。始祖鸟（爬虫类与鸟类之间的桥梁）时de Beer（1954）指出它在某些特征上（如羽毛及翼）已经很像后来的鸟类，但在另一些特征上（牙和尾）又仍然是爬虫。他将这种进化速度不相同的情况称为“镶嵌进化”（mosaic evolution）。然而这也并不是新发现。Abel（1924：21）曾经相当详细地讨论过这一原理，而他又是从Dollo（1888）了解到的，Dollo本人则又深受拉马克的影响。拉马克（1809：58）曾说过：“事实上对生命并不重要或并不是必需的某些器官并非总是处在完善或退化的同一阶段；因而如果我们追索某个纲的所有物种我们就会发现在任何一个物种中某一器官已经达到它的最完善的程度，而在同一物种中另外的器官却发育很不完全或很不完善，但在另外的某些物种中这个（县外的）器官却是高度完善的。”我们现在的论据和拉马克的虽然非常不同，但是他所观察到的不同的结构和器官系统具有极不相同的进化速度则是完全正确的。
关于类型以不同的速度进化这一论断中最值得注意的是它往往涉及某一特殊的特征，也就是与新的变化有关的关键性状。由爬虫类进化到鸟类，羽毛的发展至关重要，它几乎肯定是在飞翔之前就已完成。由水生两栖类进化到陆生爬虫类，这一关键性状就是体内受精。在研究高级分类单位的进化中探索关键性状是一项主要任务。就人类进化而言，从树栖的类人猿阶段过渡到现代人阶段就涉及一系列关键性状。直立姿势，灵活的手，制造工具，捕捉大型有蹄类动物，以语言为基础的信息交流系统等等，就是这样一类相连续的关键性状。
唯心主义形态学派的解剖学家总是强调类型的保守性。就构成脊椎动物类（型），哺乳动物类，或鸟类特征的总体而言的确是极其保守。现在已弄清楚大多数进化实际上只限于关键性状以及少数与之有关的性状。蝙蝠除了对飞翔的适应（包括有关的感觉器官）而外，就其全部结构来说，仍然很像食肉动物。甚至鲸也是除了适应在海洋中生活以外仍然是哺乳类。反过来，也几乎没有任何一种哺乳类性状不能直接追溯到爬虫类。
“模式（和谐）一致”显然有其遗传基础，这基础就是基因之间的相互作用以及调节基因的保守（如果不是近于惰性的话）作用。
宏观进化的一个最典型的特点是转移到新适应区的速度相当快，例如从食肉动物到蝙蝠或从爬虫类到鸟类。当某一种系线进入新适应区，例如当鸟类进入飞翔区，它一开始就要经历迅速改组形态的阶段直到达到新的适应水平。一旦到达这新的级后，它就能辐射到各式各样的小生境，用不着在基本结构上作重大改变。例如所有的鸟类在解剖结构上彼此都很相似，只是在某个特殊性状上发生变异。级这一现象的重要性早已知道（Bather，1927），后来又被赫胥黎加以强调（J．S．Huxley，1958）。
清醒地认识进化遗度极不相同（这已由辛普森在1953年特别加以强调）而且进化又和异常稳定的阶段（由级这个词表示）相轮替对分类学说（见第五章）以及解释进化与生态学之间的关系都很重要。
动物的进化形态学还处在发展的初期阶段。它的最重要成就可能是澄清了某些概念。
这包括明确分清了结构的功能和结构对于环境的生物学作用；预适应这一概念业已重新下了定义用来表示某一特征适应新功能和新生物学作用的可能性；Bock（1959）发展了多重途径概念；迈尔（1960）澄清了多重功能概念。这一新思维的主要着重点是生物的结构、生理、行为特征的生物学意义以及选择力是通过什么途径得以逐渐改变这些特征。
达尔文如果九泉有知也会为所有这些研究的最后结论深感欣慰；这结论是：哪怕是最激烈的结构改造也是逐渐进行的，特别是当种群（包括创始者种群）进入新的栖息地并由它们本身开拓新生境时更是如此。
虽然植物学家尽了最大的努力，但是植物系统发育的再现（重建）仍然滞后于动物的。这主要有两个原因：（1）大多数植物类群的化石记录远远少于动物，尤其是在鉴定上很重要的植物繁殖系统的残留物远远少于植物营养系统。（2）被子植物内部解剖（维管系统）上的差别远远小干24门动物内部解剖方面的差异。然而对化石花粉以及植物的某些化学成分与高分子的研究，正在开始为植物系统发育开拓全新的领域。由于植物形态学者所面临的困难，只是近一二十年才有可能进行（进化动物形态学者早已开展的）关于进化动因的研究．在这新的动因形态学中开拓性的工作是史太宾斯（1974a）关于显花植物的进化研究。为了探寻每一种结构的适应意义，他总是问“什么样的生态条件和环境变化最有可能引起所观察到的形态差异？”像这样强调性状的适应意义和传统的分类学家的路线根本不同，后者只关心研究共同祖先的线索。当然，同样的适应性特征可以通过趋同现象在无关的种系线中反复出现，这对干分类学者来说十分讨厌，但对研究进化原因的学者而言却是宝贵的信息来源。进化植物形态学研究中的另一重要贡献是Carlquist（1965）对海岛植物的趋同适应（例如木质）以及木质部进化的生态策略（1975）。
研究微生物的进化是更加新近的领域，其中分为两条战线。一条战线是由Barghoorn，Cloud，Schopf发起的化石微生物的研究；另一路是对真菌，原生生物以及原核生物的高分子和代谢途径的比较研究。遗憾的是，由干篇幅有限，甚至只是提到这些研究所吐露的一系列激动人心的问题也不可能。
宏观进化有一个方面，自从达尔文以后一百多年长期被忽略，这就是高级分类单位的起源问题，换句话说就是宏观进化多样性起源的问题。即使在进化综合时和以后，这个问题也一直被古生物学家忽视，他们会高谈阔论适应辐射但从不研究辐射到不同生境和适应区的分类单位是怎样起源的问题。这种忽视有很多原因（还从来没有被分析过），我想挑出其中的两个原因加以介绍。
第一个原因当然是普遍存在于形态学家中的本质论思想，尤其明显的是在唯心主义形态学派中。这些解剖学者对构成形态学类型（模式）或祖型（不论是哺乳类，脊椎动物还是节肢动物类型）的全部特征的保守性印象极深。一旦这样的类型逐渐形成，正如Schindewolf（1969）和其他古生物学家所正确强调的，它就几乎不再进行重大改组。
另外，不同类型之间的中间阶段，无论是现存的还是存在于化石记录中的，都极其罕见或根本不存在。种群遗传学的基因频率研究方法对这一起源问题也不能提供．任何答案。
关于新类型起源研究停滞不前的第二个原因在于古生物学家专注于直线种系进化，也就是进化的“纵向”成分。综合前的所有着名古生物学家如Cope，Marsh，Dollo，Abel，Osborn以及Matthew都主要研究进化定律，进化趋向以及适应的进化。所有这些会导致更好的适应但并不能引起更多的多样性。新的多样性是怎样起源的，是按本质论者的骤变来解释或者还是根本不提。后一种情况即使对辛普森来说也是真实的，（Simpson，1944；1953），他的进化（即纵向进化）物种定义使得他难于分析种系线的分支问题。
奇怪的是，这个问题的答案其实自从进化综合后就已经有了（Mayr，1942；1954），但被古生物学家忽略不顾，直到Eldredge与Gould（1972）在他们的所谓“间断平衡”
（Punctuatedequilibria）模型中加以运用。他们指出在检视地质记录时就会发现绝大多数化石属于广泛分布的稠密物种，这些物种在时间量纲（time dimension）上很少变化直到它们灭绝。一部分系谱要经历纵向种系进化过程（Gingerich，1976），在这个过程中某一时间层次的物种演变成后裔的亚种或下一个时间层次的物种。更常见的是尚存的物种由化石记录中突然出现的新物种补充，灭绝的物种则由这新物种代替。在正统文献中这突然形成的新种一般归之干瞬时骤变。但是Eldredge与Gould接受了迈尔的解释，即这样的新物种是在某个隔离区（外围或不是外围）的某个地方起源的，如果它们是成功的就能够扩散得很远很广。对“引进新种”（莱伊尔在150年前这样称呼它）的这种解释和化石记录很相符（Boucot，1978；Stanley，1979）。这样的新类型起源并不是纯粹推测，在现有的动物区系中外围隔离的新的较小类型的起源已被证实。
Gould和Eldredge在一个方面和迈尔根本不同。他们坚持间断平衡是由相当于Goldschmidt的有希望的畸形生物那样的不连续性造成的：“宏观进化是经由有希望的畸形生物罕见的成功而进行的，并不是在种群内经过不断细小变化进行的”（Gould，1977：30）。Goldschmidt主张（似乎也得到Gould支持）新物种或高级分类单位是通过单个个体一步产生的。相反，迈尔认为创始者种群中的进化是种群性过程，是按人类的时间尺度逐渐进化的（Bock，1979）。它只是在用地质时间尺度衡量时才似乎是骤变性的。毫无疑问调节基因参与了这些变化或者大部分与之有关，但这并不需要骤变。
最关紧要的是当创始者种群发生遗传剧变并在上位与调节系统解体之前就为新物种腾出了位置。这就大大有利并加速获得新的适应。上述情况当然不是一步取得，而对它们加以改进的自然选择过程也一直继续进行。它甚至可以因后裔创始者种群的建成而加速。现在还不知道这样的进化转变究竟需要几代，几十代，几百或几千代才能完成（也许时间是可变的），但是肯定无疑要比古生物学文献中所描述的传统的种系进化（需要几百万年）快几个数量级。即使这样，通过创始者种群的演变而引起的进化也不是骤变过程而是渐进进化。新思维的重要不同处是将它作为种群现象来对待。
在现代情况下幸而还有一些地理的和生态的机会使我们能够证实这样渐进的、一步一步的宏观进化的起源。夏威夷列岛从西（考艾岛）到东（夏威夷岛）的岛屿群上都有动植物移殖，提供了这样的几乎是渐进进化步骤的画面。这已由Book（1970）就管舌鸟（DrePanididae）的种和属的研究证实，Carson和Kaneshiro（1976）对果蝇的研究也证实了这一点。
经由地理性物种形成过程不断形成新种（Stanley，1979），之所以可能是因为同时还有使物种稳定消失的灭绝现象。因此灭绝是物种形成的对立面（莱伊尔早就认识到这一点），是同等重要的问题，对生态学家来说更是如此。
当人们看到拟态物种是多么一丝不爽地模拟其原型（正模）的哪怕是十分偶然的特征时，便会相信自然选择是无所不能的。但是这和自然界中经常出现灭绝现象又相矛盾。
当三叶虫，菊石或恐龙这样一些曾一度高度繁荣的动物门和目灭绝时，为什么自然选择不能在这些大的分类单位中重新组建哪怕是一个物种使之生存下去？事实上菊石在以前已经经历过四次大规模的灭绝，在这几次灭绝中每一次都有一个种系（lineage）存活下来形成新的适应辐射。但是在最后一伙灭绝中没有任何一个物种具有合适的基因群体能够成功地应付它所遇到的环境挑战，姑且不论这挑战具体是什么。
现在看来越来越清楚，灭绝是一个极其复杂的问题。恐龙的灭绝只是在几十种或几百个物种最后灭绝之后才发生。因而就引出了这样的问题，为什么这整个高级分类单位被淘汰了？从动植物的门和目的历史来看就可以发现它们被灭绝的难易程度极不相同。
Van Valen（1973）曾指出，可以对灭绝的方式或格局定出明确的规律性。我本人深信灭绝和遗传型的内聚性有某种关系。在生物的不同物种中突变速度无疑应当大致相同。
然而有些物种的遗传型集成（整合）得如此完善，因而变得如此不灵活，以致再也不能产生与传统的正常标准有差异的、能够在资源利用或对付竞争对手或病原体方面发生重大转向的个体。这些当然还只是议论或设想，要作出完满的解释还有待于对真核生物遗传型结构和它的调节系统有更深入的了解。
动物或植物的多样性取决于物种形成和灭绝之间的平衡。近年来由于对化石生物区系的知识大量增加才有可能通过地质年代探索物种的多样性。分析结果表明在有些地质年代中多样性呈指数增长，例如寒武纪早期和奥陶纪；在有些地质年代中呈现稳定状态，即在几百万年（如果不是上亿年）中多样性基本保持稳定不变以及大规模灭绝的年代（Sepkoski，1979）。最值得注意的可能是某些生态群落复合体（ecologicalassociations）的极端稳定性。物种的多样性不是这些动物区系的逐渐增多，而是在整个地质年代中基本保持不变，它的更新大都是由于灭绝接1：l的比例由新移殖的物种取代。奥陶纪的“物种爆炸”可能是由于泛食者被专食者取代的结果；更近期的变比（特别是海洋中）可能是由于板块运动，气候性事态（包括冰期）以及浅大陆架海域范围等因素。近年来的开拓性研究显然还仅仅是一个开端。
另外有一些大规模灭绝的地质年代，例如二送纪末期和白垩纪末期。实际上古生代末期和中生代末期都是以大规模灭绝来划分的。关于灭绝的地球外原因曾经有过很多设想，例如地球穿过宇宙尘。另外也有人认为灭绝是由于气候急剧变化，而这种变化则是源于板块结构。白垩纪与第三纪之间的交替时期铱的沉积大大增加这一发现，促使Alvarez及其同事（1980）提出了一种假说，假定地球被一外星体击中，尘暴将太阳光遮断达数年之久。这个假说乍看起来似乎很有吸引力，然而它却引起了很多尚未解答的问题，例如怎样解释哺乳类，鸟类，被子植物以及非恐龙类爬虫等仍然存活了下来。很明显对灭绝的研究仍然是一个有待开发的广阔领域。
13．6人类的进化
再也没有别的思想比人类可能来自猩猩更不合维多利亚时代的胃口。纵使其他一切生物的进化都能被证实，人类以其独有的特征也必定是被特地创造的。即使华莱士也反对将人类进化归因于自然选择，这的确使达尔文感到很惊讶。实际上解剖学家都很清楚在形态上人类和类人猿十分类似。这就是为什么林奈毫不迟疑地将他本人归入灵长类的原因。出版后的短短几年内，海克尔（1866，1868）在德国，T． H．赫胥黎（1863）在英国先后出版了主张人类来自猩猩的着作。甚至莱伊尔（1863）最终也至少承认了人类的历史很悠久。 1871年达尔文出版了重要着作《人类由来》，对人类的进化问题作了相当详尽的讨论。
就在同时（实际上是在出版以前）发现了第一批化石猿人，特别是尼安德特人（1856）。海克尔以其惯有的浪漫想像力甚至在人类与猩猩之间重现了“缺少的环节”，并将这一过渡动物定名为猿人（Pithecanthropus）。在寻找这缺少的环节的热潮中竟然出人意料的很快就取得了极大成功。先是荷兰军医、业余人类学家E.Dubois于1891年在爪哇发现了直立猿人（现在列入人科中）的头骨。自此以后人类化石的新发现层出不穷，其中最重要的是1924年Dart在南非汤恩（地名）发现的非洲南方古猿（Austrabnlthecus africanus）化石，又称为“汤恩幼儿”（Taung child）。随后由Broom、Leakey兄妹以及其他人的许多新发现使得有可能重现这种特殊的过渡动物。
就它的骨盆和后肢来看和现代人类几乎没有什么区别；它的牙列和面庞大致介于猩猩与人类之间；而它的脑（约为450毫升，现代人约为1500毫升）则基本上仍然处于猩猩的水平。
在东南亚、埃塞俄比亚、肯尼亚以及坦桑尼亚的一些发现使我们现在有可能从最古老的南方古猿经过能人（Homo habills）、直立猿人（Homo erectus）、到现代（智）人（Homo sapiens）组成一条几乎不中断的链索。按年代次序和形态学考虑，南方古猿可能是一个多型种和被隔离种群，由它引出粗壮南方古猿（Australopithecusrobustus）（旁支）和能人。似乎很少有可能会发掘出足够的化石来判定这些人种在其中发生演变的隔离区的位置，也无从断定是什么原因促使它们和南方古猿分离。和能人共存的粗壮南方古猿早在一百万年以前便已灭绝。虽然现在能够回溯南方古猿到四百万年以前，但是这一类人猿种系又在多少个几百万年以前从进化线上分岔成非洲猩猩、黑猩猩、大猩猩则还有争议。最后的决定在很大程度上取决于化石拉马古猿（Ramapithecus）所处的位置以及它是否只是类人猿的祖先或者它又是非洲猩猩的祖先，或者它是一个劳支。从类似猩猩的祖先（拉马古猿？）转到类人猿这一进程可能很快，也许近在五百万到七百万年以前。只有进一步发现更多的化石才能对此作出明确答复。
令人感到惊讶的是，人类和非洲猩猩在分子特征和染色体结构上非常相似。这是镶嵌进化的一个明显例证，其中遗传型的某些部分（基本的高分子）固定不变，而控制一般解剖结构、尤其是中枢神经系统的另外部分则以非常高的速度演化。但是目前认为类人猿种系经是从导向非洲猩猩的种系线分岔出来的这一重要事实则无可置疑。
较之年代次序上的不确定性更为重要的是我们对从类人猿进化到人类的中间步骤的了解日益深入。当我们的祖先从树上降到地面时采用直立姿势显然是第一步而且可能是快定性的步骤。它使前肢解脱出来执行操作功能，使前肢能携带物体并比任何一种猩猩能更广泛地使用工具、最终制造工具。猎捕大的猎物和真正语言的发展显然是人类进化的另外重大步骤。采用意识、意志、智力等衡量标准作为人类特征并不是特别有效，因为有很多证据表明人类和猩猩以及许多其他动物（甚至狗）在这些特征上的区别只是量的差异、程度不同而已。语言比其他任何东西更能使知识世代相传，从而促进非物质性的文化得以发展，因此语言是人类最典型的特征。往往有人说文化是人类最独特的特点，实际上这完全是一个定义问题。如果把文化定义为年长的个体将某种知识、技能（通过示范和学习）传给年轻的个体，那么文化在动物界就是很普遍的（Bonner，1980）。因此在文化的发展进化上动物和人类之间也并没有断然的差异。虽然文化对人类来说更重要（可能高几个数量级），但对文化的包容力并不是人类所独有，它也是渐进进化的产物。
人类学研究最出人意料的发现之一是人类进化的速度极高。即使把躯体同时相伴增大考虑进去，类人猿的脑从450毫升增加到1600毫升也是非常迅速的。也许值得同样注意的是一旦达到现代（智）人阶段（约在十几万年前），脑的容积就不再有明显的增加。
为什么原始人类一开始就要选择如此完善的头脑使十万年后的笛卡儿、达尔文、康德得以作出重大成就，莎士比亚、歌德得以完成其文艺杰作，发明计算机，访问月球等等，这的确令人难以理解。但是对人类未说，人类当然永远是一个谜，他永远也无法了解自己。
承认自然选择（并且只是自然选择本身）将人从猩猩这一层次提高到人类的这种认识促使高尔敦于达尔文去世不久之后就想到可以运用这选择原理使人类从生物学上得到改进。这一乌托邦式的议想（Gallon称之为优生学）一开始就有很多人支持。事实上很多遗传学家和其他生物学家在他们的着作中都同意通过促进物种中“最优秀”成员的繁殖并阻止患有遗传疾病的个体或其他方面低劣的个体繁殖的办法来改善人类是一种高尚的思想。实际上必须分清两种优生学。消极优生学（Negative eusenics）致力于在种群中减少有害基因的数量，办法是阻止显性基因携带者的繁殖和降低隐性基因的杂合体携带者的繁殖率（当这样的杂合体能被诊断时）。积极优生学（Postive euuenics）则着重促进“优秀”个体的繁殖能力（Haller，1963；Osborn，1968）。在阅读这些早期的优生学信奉者的着作时，他们的理想主义和人道主义精神予人以滚刻印象。他们在优生学中看到了除教育可以办到的以外的另一种改进方式并且提高生活水平。起初并没有政治偏见渗入优生学，因而得到了从极左派到极右派的一致支持。但是这种情况并没有延续多久。优生学很快就变成了种族主义者和反动分子的工具。它并不是严格按种群思想而是按模式思想来解释；不久之后在没有提出任何证据的情况下人类的各个民族就分别被划定为优等或劣等民族。最后终于导致了希特勒大屠杀的血腥恐怖。
自从1933年以来正是由于这种结果便几乎无从客观地讨论优生学问题。但是这并不否定正是通过自然选择，人才具有人的属性这一事实，而且我们也知道除了选择以外也并没有别的方法可以改进人类的遗传型。然而对人施用人工选择的办法至少在目前还是不可能的，这有很多原因。第一个原因是目前还很不清楚非体质性的人类性状在多大程度上具有遗传基础。第二，人类社会是在其成员的天资与能力的多样性的基础上繁荣兴盛的，纵使我们拥有管理选择的能力，但是我们对究竟需要什么样的人材特殊混合结构却毫不清楚。最后，大多数西方人对人在遗传上是各不相同的概念（即使将来在科学上比今天能更好地论证这一概念）是无法接受的。平等概念和优生学之间在意识形态上是完全相抵触的。我们必须记住美国宪法原则所依据的是启蒙运动领导人物的着作，他们的理想是高尚的，但是，说得客气一点，他们的生物学知识是不够的。正像贝特森在很多年前所说的，“甚至早期基督教领袖的着作中所包含的背离生理学知识的幻想也赶不上“百科全书派”的理性主义者所据以制订社会方案的那些幻想之背离生理学事实”
（Batesan，19147）。优生学目前是一门沉寂的学科，除非种群思想能更广泛地被采纳以及对人类性状的遗传基础有更多的了解，否则它将仍然处于这种状况。
如果要问目前进化研究最突出的方面是什么，我们会用互相作用来回答。在还原论者方面注意力放在作用和单个基因的适合度上；现在则越来越注意基因的互相作用，调节机制以及作为一个活跃系统的遗传型。孤立个体的适合度研究已扩大到亲属选择（kinship selection），总适合度，互相利它（reciprocal altruism），亲子关系等等。动物和植物进化的研究由于对它们共同进化（coevolution）的探索（Ehrlich andRaven，1965）而更为充实。食草动物的进化只有作为对其所食用植物进化的一种反应才能深入了解。过去经常引用的地质年代第三纪中马及其他温带哺乳动物在进化中由啃食（browsing）转变为啮食（grazing）的例子就表明共同进化早已为人所知。自白垩纪以后大多数昆虫的进化和被子植物的进化密切有关。社群（社会）系统和生态系统进化的研究主要着重于相互作用的效应。所有这些当然都是自然选择的结果。自然选择是由环境施行的，即由环境进行选择。个体的环境不仅包含非生物性的环境，还包括同一物种的其他个体，其他物种（动物和植物）的个体。因此进化过程中相互作用的大多数研究归根到底只不过是自然选择研究的扩大运用。这一点已由近年出版的进化生物学（Futuyma，1979），行为学（Alcock，1980）和生态学（Rickleffs，1978）等教科书予以充分说明。
进化生物学家常被人问起在他的研究领域中还有哪些问题没有解决。这些问题很少涉及基本原则，因为随着我们对生命现象的了解日益深入和增多，能够代替达尔文主义的别的学说也日益无法成立。至于谈到尚未解决的问题，似乎可以列出如下：在所观察到的生命变异性中有哪些部分是自然选择的产物，又有哪些其余部分是机遇（随机）过程的结果？更加专门的问题是生命的起源（核酸和多肽是怎样联结在一起的），病毒的起源，原核生物通过哪些步骤或过程转变成真核生物，真核生物染色体的功能，各种DNA的分类（结构性，调节性，重复DNA等等）及其在进化和物种形成中的各自功能，植物和无脊椎动物主要类型的相互关系和种系发育，种内和种间竞争在进比中的各自作用，各种不同行为的进化及其在进化中的“标兵”（Pacemakers）作用，灭绝十分频繁的原因（为什么自然选择在防止灭绝上无能为力？），此外还可加上各种专食性生物（specialists）。一个特别广阔的研究领域是进化中的多元化（pluralism）、即多样途径。对环境提出的几乎每一项难题，不同的进化路线都各有不同的答案。这不同的答案（例如节股动物的外骨骼与脊椎动物的内骨骼）对这些进化路线的未来进化施加了什么约束或影响？进化约束（力）（evolutionary constraints）的整个领域还几乎是一片未经开发的处女地。进化生物学与生态学、行为生物学、分子生物学的融合引出了几乎无尽无休的新问题。可是，在这里再重复一遍，任何新的发现很少有可能会迫使在进化综合中所建成的基本理论框架作重大的改动。
13．7现代思维的进化
进化主义者阵营内部经常而又往往很激烈的争论常常把非生物学家弄得莫名其妙。
他们因而对进化的全部概念或至少是达尔文的自然选择学说感到怀疑。因此便很自然地产生了这样的问题：进化和达尔文主义在现代思维中究竟扮演什么角色、起什么作用？
为了回答这个问题一开头就指出凡是有见识的生物学家都不再怀疑进化这种说法可能是公允持平的。事实上很多生物学家认为进化并不是一个学说而是活生生的事实，这事实是由基因