Chapter 11 Chapter 6 Microtaxonomy, the science of species-1

Taxonomists group entities called species into genera and higher taxa.Species are the basic types of organisms that make up the diversity in nature, and they represent the lowest level of true discontinuity above the level of the individual.Song thrushes and fox sparrows and red oaks and pin oaks are all different species.The entity called a species may seem at first glance to be easy to define, but it is not.Perhaps no other concept in biology has been so continually contested as the concept of species. One might imagine that the intense debates of the post-Darwinian period would lead to clarification and agreement, at least that the new systems of the 1930s and 1940s would bring final clarification, but this is not the case.Even now some papers are published every year on species, and the differences of opinion about species are almost as much as they were a hundred years ago, if any progress has been made, it is only that the nature of the differences is more clearly defined than before .It is of particular note to the student of ideology that the history of the problem of species and the history of the problem of classification are largely independent of each other.The branch of systematics dealing with species issues can be called microtaxonomy.This chapter discusses the history of microtaxonomy.

When talking about species, people have plant and animal species in mind.In fact, the word species is often applied to all kinds of objects, referring to the meaning of "type". Chemists can talk about molecular types, and mineralogists can talk about ore types (Niggli, 1949; Hooykaas, 1952).However, the species concept of chemistry and mineralogy is fundamentally different from that of modern biological systematics.Generally speaking, the names of species in mineralogy are names of categories, which are named according to a series of basic properties of members of a category.The species of inanimate objects thus corresponds more or less to Linnaean or pre-Linnaeus species and has nothing in common with the modern biological concept of species.

Even when limited to biological species, however, there is considerable disagreement, in part because the species level has very different functions in different areas of biology.For the practical taxonomist, the taxonomic unit of species is the basic "category" (species) to be identified and classified; An organism with species-specific characteristics; for evolutionists, a unit of evolution (Monod, 1974b); for paleontologists, a segment of a family tree.Different experts are at best emphasizing different aspects and at worst drawing wildly different conclusions.The result has been constant debate.

One of the most basic human desires is to understand the different kinds of objects that make up their environment.Even though primitive humans had names for various birds, fish, flowers, and trees, the species they identified were generally identical to those identified by modern taxonomists (Gould, 1979).The naming of species is possible because diversity in nature is not continuous but consists of individual entities separated by discontinuities from one another.What people encounter in nature is not only individuals but also species, that is, groups of individuals that share certain characteristics with each other.

The concept of "species" is needed because the word "kind of" is imprecise.The division of species taxa relative to taxa of higher and lower meta-levels is a matter of demarcation.Distinguishing true biological (scientific) species within a genus is therefore a problem of delimitation relative to broader taxa.But each biological species contains many isotypes (phena, that is, variants in past literature; different phenotypes in a single population, including sex, age and seasonal variation, etc.), and these isotypes are very different from each other , were previously described as distinct species.If the word "(species)" is equated with "different kinds", then there is no distinguishing standard to distinguish different "kinds"

Undoubtedly attributed to the same type of species, species and genus in these three levels.The concept of species is the scale or standard for the correct classification of "kinds". This immediately raises the question: what are the characteristics of the species?Or according to what characteristics can individuals be assigned to species?This question is easy to answer when the difference between two species is as clear as the difference between a lion and a tiger.In many cases, however, individuals within a species differ at first glance to the same extent as between species; The different stages of different species (such as caterpillars and butterflies), the alternation of generations, and other forms of individual differences have caused great difficulties for the division of species.If we want to solve these problems, we not only need to have enough biological knowledge, but also have a good understanding of "species"

The meaning of the word has a definite concept. Looking back at history, it is evident that a great deal of confusion has been created by the use of the same word "species" in two fundamentally different logical categories.The two concepts have only been able to be distinguished since the introduction of the neologism "taxon" (taxon).Taxa is a specific animal or plant object, such as foxes, blue (robin) birds, flies and other individual taxa are species taxa (see Chapter 4). When the taxonomist first encounters individuals or specimens in nature and attempts to assign them to a species, he is dealing exclusively with zoological or botanical problems.He wonders whether these individuals are members of a certain region of the same population?

What he is concerned about is not the issue of the hierarchy of species, but the issue of species division.If the animal entity he was studying was white geese and he was trying to determine whether white geese and orchid geese were descended from the same gene pool, he was working not only on a species problem but also on an ontological one.Is the animal a species member of a class? Ghiselin (1974b) emphasized that all descendants of a species' gene pool are part of the species (rather than members of a class) and that species are ontologically an individual in general.Perceptive zoologists have long referred to species taxa as having different status, not classes.A taxon is an individual in the sense that each species has temporal and spatial (temporal) unity and historical continuity (Hull, 1976; 1978).Each species has its reasonably distinct boundaries, and at the same time has its cohesion, and has range and continuity throughout.

The practical problems associated with species taxa are twofold: (1) assigning individual varieties (isotypes) to the appropriate species taxon, and (2) separating species taxa from each other, especially deciding which single random Seed groups that vary in time and space are included in a single species. A strict distinction must be made between species taxa and species order.A species class is a category whose members are species taxa.The particular definition of species class adopted by a scholar determines which taxa he must arrange as species.The question of species order is purely a matter of definition.How to define the word "species" is constantly changing, and the changing definitions represent the history of the concept of species.

Determination of species status is a two-step process.The first step is to divide the proposed taxa of species from each other, and the second step is to arrange the designated taxa into the appropriate hierarchy, such as "population", "subspecies", or "species". It is only the past few decades that the fundamental distinction between taxonomic units and species defenses has been clearly recognized, and has finally removed a major source of confusion, at least in principle.Much of the controversy over the concept of species actually concerns the identification of species taxa and the assignment of individual varieties (or other isotypes) to species taxa.For example, polytypic species (Polytypic species) is not a separate class of species, but only a special type of taxonomic unit.Most taxonomists, myself included, were confused about this question until a few years ago.

Ancient people did not recognize the biological integrity of each species.For example, Aristotle believed in the crossbreeding between species, such as the crossbreeding between fox and dog, tiger and dog.Aristotle and Theophrastus also believed in the folklore that the seeds of one plant can germinate into another (species mutation).Most herbalists and early botanists also believed these claims or at least did not refute them. Albertus Magnus once mentioned five ways of transforming one plant into another. Given the lack of clarity about the nature of the species, it is not surprising that there is a lack of consistent terminology.According to existing dictionaries, the Greek word for species is eidos and genus is genos, but Plato uses these two words completely interchangeably. He never uses the word eidos in the sense of a "species" belonging to the class "genus."Aristotle did make a distinction between the two words, but mainly in his logical writings.In his biological works, although the word genos is used 413 times, 354 times it is used to refer to the species of animals, and it is only used to refer to this level in the remaining cases.Of the 96 uses of the word eidos, only 24 are used to denote species of animals.Therefore, only 6% of the 378 cases use the word eidos to indicate the animal species, and the word genos is used in the rest of the cases.Therefore, Balme (1962) once pointed out: "It has been traditionally believed that Aristotle did divide animals into genera and species. ... But this view lacks evidence." In Greek philosophy the terms "genus" and "species" are used mainly in logical discussions.In logical classification genera are divided into species regardless of genus rank.Thus, the dog would be a species in the genus Carnivore, and the poodle would be a species in the genus Dogs. The terms "genus" and "species" are used to regulate the inclusion of members into larger classes. This use of emphasizing relative rank lasted from antiquity to Linnaeus. Adoption of Christianity and Creationism did not change the situation much in the first place.St. Augustine declared that on the third day of God's creation of the world plants brought a cause (causalter), that is, the earth accepted the ability to produce plants.This gave rise to spontaneous generation, heterogony, and all other forms of change in the subsequent history of the earth.His definition of species (i.e. similar and having a consistent origin) prefigured Ray's definition. Attitudes to species changed considerably after the Reformation.The fixedness and immutability of species become firm beliefs.A literal interpretation of Genesis in the Bible must believe that every plant and animal was created individually before Adam was created.The species is thus the unit of creation.The rapid development of natural history at that time was also conducive to the development of the above-mentioned situation.The vast majority of herbalists in their studies of wild plants have likewise come to the conclusion that species are definite units of nature, that they are invariant and distinctly different from each other. 6.2 The essentialist concept of species The Christian fundamentalist creationist interpretation of species is quite consistent with the essentialist conception of species, which holds that each species is characterized by its unchanging essence (eidos) and is related to other species by distinct discontinuities. the difference. Essentialism assumes that the diversity of both the non-living world and the living world is a very limited reflection of unchanging general principles (Hull, 1975).This notion eventually returned to Plato's concept of eidos, and is what later scholars had in mind when speaking of the nature of an object or living being.Objects that share the same essence belong to the same species. The existence of the same essence is inferred from similarity.Therefore, the definition of a species is simply a cluster of similar individuals that are not identical to individuals belonging to other (species) species.Species understood in this way represent different "types" of living things.According to this concept, there is no special relationship between individuals and individuals, they are only manifestations of the same essence (eidos).Variations are the result of imperfect manifestations of the essence. The criterion of similarity is quite appropriate when sorting out types of ores and other non-living objects.Similarity, however, is a very unreliable criterion in the classification of highly variable organisms.How can one know that two individuals have the same essence?It can be assumed that two individuals who are very similar, having common features, have the same essence. But what should be done in the following situations?Sexually dimorphic animals, in which individual females and males differ widely; juveniles and adults in invertebrates; and distinct varieties often found within a species.Methods of inferring from similarity are completely useless in cases of significant sex and age variation and polymorphism of any kind.Are there other criteria that can be used to determine "common nature"? Ray first offered a biological answer to this question: In order to proceed to compile a detailed list of plants and correctly establish their classification, some criteria must be established for what is called a "species".After long and careful research, I have found that there is no more certain criterion for determining species than the characteristics of self-propagation from seeds.Therefore, no matter what changes occur in individuals or species, as long as they come from the seeds of the same plant (plant), these changes are accidental and cannot be used to distinguish species... Similarly, animals of different species always maintain their own different species; one species must never be derived from the seed of another, and vice versa. ("History of Plants" 1686) Ray's above opinion is an excellent compromise between the practical experience of naturalists (what they can observe in nature belong to species) and the definition of essentialists (which requires a common essence).It is evident that the full range of variation which a pair of conspecific parents can exhibit in their offspring is contained in the nature of the underlying species.Reproduction (reproduction) is important in the concept of species because it allows inferences about the amount of variation compatible with the existence of a unique essence. Ray's definition has been enthusiastically embraced by generations of naturalists.Another advantage of this definition is that it fits well with the dogma of creationism. This is what Cuvier had in mind when he defined species as "offspring of common parents."He explained this in a letter to his friend Pfaff as follows: "We imagine that the species is all descendants of the first couple created by God, just as all people are the descendants of Adam and Eve. Now what do we have? Way to rediscover the process (pathway) of this pedigree (family lineage)? It will certainly not be structural similarity. In fact all that remains is reproduction and I insist that this is the only sure, absolutely infallible characteristic of a species ".This is actually Ray's standard, and later Cuvier himself admitted that similarity is the main criterion for dividing (species) taxonomies in practice.Clearly, there is no evolutionary implication in Cuvier's definition of species. From the beginning of Rui until the end of the 19th century, many definitions of species asserted the fixedness, permanence and insurmountable discontinuity of species while at the same time applied biological criteria to reconcile obvious variation with the existence of unique essence apparent contradictions.Many scholars of that period often used the term "common ancestor" because it had a purely blood-related connotation and not because of a belief in evolution.When von Baer (1828), a vehement opponent of the theory of evolution, defined species as "the totality of individuals connected by a common ancestry," he was obviously not referring to evolution, when Kant spoke of "natural classification involving descent ( pedigree) as well as when classifying animals by blood relationship" (Lovejoy, 1959d).In the case of creationists this simply means the family tree of a couple originally created. Lin Nai also reaffirmed such a "family background". Carl Linnaeus, the great Swedish botanist, has long been considered a proponent of the essentialist species.He did, but such a statement does not properly indicate his conception of species, since Linnaeus was a scientist with the experience of a regional naturalist, a devout creationist, and a believer in logical classification.Although all three components of his thought emphasize the invariance and strict demarcation of species, in order to fully understand Linnaeus' thought, it is necessary to remember the sources of these three aspects of his concepts.His first time. (1736) formulated his conception of species explicitly in the following famous argument: "There are as many species as have been created from the first, as there are different forms which we think". In 1751, he expanded his argument in "Philosophy of Plants", "In the beginning, God created as many species as there are different forms, and according to the law of reproduction, there are as many species that are always similar. Therefore, how many different species were there before us? There are as many species as the (overall) structure of When Linnaeus talks about being "created," he takes it literally.He mentioned in an article that he believed that "in the beginning of the world, only one pair of sexual partners was created for each kind of creature... A pair of sexual partners here refers to the sex of each species in which the individual is different. One female and one male. But there are certain animals which are naturally hermaphrodites, in which case initially only one individual of each was formed".Linnaeus made this conclusion not only on the basis of his religious beliefs, but also because it showed "modern" scientific findings at the time.At that time, Spallanzani and Redi denied the theory of spontaneous generation. Rui and Linnaeus believed that the seeds of one species could not be transformed into the seeds of another species (species variation), but St. Augustine's hypothesis was not confirmed. Species were never as important in Linnaeus' thought as genus.Therefore, when dealing with special species in his taxonomic catalog, he is often very casual. In the catalog of plants ("Plant Species", Specees Plantarum), and in the catalog of animals ("Natural Systematics", SystemaNaturae), he has many mistakes in listing species , requiring frequent revisions. The observations of naturalists, the claims of the Christian faith, and the tenets of essentialism all led to the conclusion of a definite and utterly unchanging species, a concept that would have a huge influence on the ensuing hundred years.As long as it is believed that species can easily change into other (species variation) species, or can easily arise by spontaneous generation, the whole problem of evolution will not happen. Poulton (1903), Mayr (1957), Zirkle (1959) successively mentioned that Linnaeus insisted on the authenticity of species, strict delimitation and invariance may be more encouraged than if he agreed that species have great plasticity. Subsequent studies of evolution were hindered, because it was his conception of species that created a contradiction that had to be resolved between many evolutionary phenomena in nature and the invariance of species. Oddly enough, Linnaeus later turned his back on the typologicalspecies concept with which his name is associated.He dropped the words "nullaespecies novae" ("no new species") in the twelfth edition of the Natural Systematics (1766) and crossed out the words "Natura non facit saltus" ("Nature is not prone to sudden changes").This change in his thinking is associated with many botanical discoveries.First he discovered a peculiar mutation in flower structure in the plant linaria (Peloria, abnormally regular flowers), which he thought was a new species and genus, and later he encountered some putative interspecific hybrids.This gave him the curious idea that perhaps only genera were created at first, and that species were the product of crossing these genera with each other.Of course, this assumption is not only inconsistent with everything he thought and said before, but also contradicts the essentialism.Not surprisingly, Linnaeus was immediately and sharply attacked from all sides, because the generation of new essences by interbreeding was unthinkable to any staunch essentialist.Kerr Luther was the most powerful botanist who attacked Linnaeus on this point. In a series of experiments (1761-1766), he proved that the hybrids produced by interspecific hybridization were not constant new species but were easily changed and passed through successive cycles. Crosses can be reverted back to the original parent species (Olby, 1966).These ideas of Linnaeus' later years were almost completely forgotten in the ensuing years and had no influence on later evolutionary thinking. His contemporary, Adanson, though radical in some respects, was completely orthodox in the concept of species.After a detailed analysis of the problem of species he came to the following conclusion: "On the accepted principle of the invariance of species, no evolution of species takes place in plants, nor in animals, nor even in minerals. evidence.” This quote makes it clear that even discerning and otherwise enlightened biologists cling to formalistic and non-biological notions of species. Taxonomists of the post-Linnaeus period adopted the essentialist concept of species almost unanimously.This concept proposes that species have four characteristics: (1) species are composed of individuals with a common essence; (2) each species is fractal with other species according to obvious discontinuities; (3) each species should be fixed from beginning to end (4) Any species has strict limits on possible variations.Such is the case with Lyell's species concept. Although Buffon's species thought is earlier in time, it is closer to modern views than Linnaeus and Cuvier.It is difficult to give a brief summary of Buffon's ideas of species, not only because they are scattered throughout the voluminous Natural History, but also because of the development of his ideas over time (1749-1766). Variety.Therefore, Buffon's students have different explanations for it. Buffon's original opinion on species was strongly nominalistic, and it seems that he emphasized the individual rather than the species, and the continuity between them: "Nature proceeds in unknown gradual steps, and thus passes from a species to a Another species (and often from one genus to another) does not accept the absolute dividing line we have laid down. It is inevitable that there will be a large number of undetermined species and transitional specimens who do not know the arrangement. where" ("Oeuvr. Phil": 10). In fact this passage in the first volume of the Natural History is part of a critique of Linnaeus' system of classification.In the other two volumes of the Natural History (volumes 1, 2, and 3 were published simultaneously in 1749), Buffon supported the notion of invariant and clearly demarcated species.Although some experts have denied it, but Buffon's species is produced from the essentialist thinking. Each species is characterized by a species-specific internal model (moule interieur), which, though of different origin, shares many properties with Plato's eidos.And each species is clearly separated from the others: Every species in nature has a universal prototype (Prototype), and all individuals are shaped according to this prototype.However, the individual has changed or improved according to the environment in the course of his reality.There are some characters which occur irregularly in the lineage of individuals, but which are at the same time markedly constant in the species as a whole. The first animal, such as the first horse, was at the same time an external model and an internal model from which past, present, and future horses arose. ("Natural History", IV: 215-216) Buffon regards the lineage of individuals as the most important characteristic of species, because each lineage of individuals is strictly distinguished from all other species: It is in the characteristic differences of species that the intervals of subtle differences in nature are most evident and best marked.It may even be said that these intervals between species are the most uniform and least variable, since a dividing line can always be drawn between two species, that is to say, between two lines of individuals which are infertile to each other.In natural history the distinction is most profound...Each species--each lineage of individuals capable of interbreeding successfully with each other--will be considered as a unit and treated individually. A constant lineage of identical individuals bred together. (Natural History, IV: 384-385) Buffon discovered a criterion for determining whether two very similar "species" were different at all. This supplemented Ray's criterion, which showed that extremely dissimilar Common ancestors can belong to the same species.For example, are donkeys and horses the same species?Buffon's answer to this question is that individuals that fail to produce fertile offspring belong to different species. "We should regard two animals as belonging to the same species, if by mating they can perpetuate themselves and preserve the similarity of the species; if they cannot produce offspring by the same method, we should regard them as of different species" (Natural History, II: IO).The obvious novelty of Buffon's concept of species is that the standard of conspecificity is no longer the range of morphological variation of offspring of a pair of parents like Ray's concept of species, but the ability to produce fertile offspring.By introducing this new standard, Buffon took a big step towards the concept of biological species.However, in terms of seeing species as constant, Buffon still clings to the essentialist concept of species. Buffon differs from Linnaeus and other orthodox taxonomists in another respect in that he downplays the importance of morphological characters (characteristics) and emphasizes habits, temperament and instincts, which he considers to be far more important than purely morphological characters. More important species traits.He once said that it is not enough to identify a species based on only a few key characters; if you want to understand an animal, you must know all its characters.Only the naturalist working in the field took this advice seriously; the great flourishing of the natural history of living animals (especially birds) in the ensuing ages owed much to Buffon's concept.A species can often be identified based on its life history characteristics.Species are thus natural and real, in contrast to Linnaeus's genus, which is entirely subjective and arbitrary. In his later years (after 1765) Buffon somewhat revised his concept of species, giving the word "species" a narrower and more restrictive definition (Roger, 1963:576).When he found, especially in the study of birds, that there were groups closely related to species, some of which apparently produced fertile hybrids, he assigned to them certain attributes which he had previously assigned only to species. "family" (families).But Tongcai still retains a clear concept of species at a lower level.This search for a new concept of species though heralds the idea that the classification of species may be unified due to a common ancestry.But it obviously didn't impress the readers at that time, and it didn't have any further effect on the historical development of the concept of species in the future. On the other hand, Buffon's very strong biological view of species had an important impact. Zimmermann himself claimed (1778) that he was a follower of Buffon, Blumenbach, and Spallanzani in adopting the backcrossing method as the standard of the breed, and attributed all dogs to the breed "first because they were mated with each other, and more importantly It is capable of producing fertile offspring. Second, because all types of dogs have a common nature, are loyal to people and are easy to domesticate.” This concept of biological species was widely popular between 1750 and 1860, and it is reflected in Among the writings of Pallas, Gloger, Faber, Altum, and the great naturalists of the nineteenth century.At the same time, however, a strict essentialist notion persists, especially among collectors, who regard each variety as a new species; for example Brehm (the parish priest) found no fewer than 14 species in the small village in which he lived. "Species" house sparrow, a French freshwater clam expert regards the variants in one species as more than 250 species.These people regard types as species, and consider any consideration of species as populations to be inadmissible.Such species concept is often referred to as typological species concept (pattern concept) in systematic literature.There are hardly any higher taxa of animals and plants without one or two such "Species mongers" active, which is why there are hundreds of homonyms (Mayr, 1969: 144-162) . This is more the case in botany than in zoology, where variation becomes an excuse for describing large numbers of new species, especially in so-called "difficult" genera such as Rubus or Crataegus.This situation is aggravated by the almost universal inability of botanists to distinguish individual variations from geographical varieties.This situation did not begin to improve until the International Botanical Conference held in 1867 adopted de Candolle's proposal to recognize subspecies, varieties, and other subspecific classifications.Subsequent works by Kerner (1866; 1869) and Wettstein (1898) were also helpful in clarifying the situation. But even after the rise of the new systematics, many botanists continued to use the word "variety" indiscriminately to refer to geographical groups and within-population varieties in general. 6.3 The nominalist concept of species Objections to the essentialist concept of species come from both naturalists and philosophers.The two philosophers who had the greatest influence on the early and mid-18th century, Leibniz and Locke, were skeptical of the notion of a distinct and strictly separate species.Locke did not deny the existence of species, he just said: "I think it is still true that the boundaries of species that people use to classify are artificially set." He claimed that he could not see why the two breeds of dogs Unlike pugs and elephants, which are different species, how unreliable the boundaries of animal species are to us. When the concepts of perfection (Plenitude) and continuity came to dominate thought in the Western world in the post-Leibniz period (Loveioy, 1936), the concept of discontinuous systematic categories, including the concept of species, became a disadvantage, philosophy Scientists fall back on the nominalist concept of species.For nominalists, only individuals exist, while species and any other "categories" are artificial constructs. Nominalism is a medieval school of philosophy that denies the essentialist notion that the same thing has the same substance (essence), and claims that all categories of the same thing share only one name. Some scholars in the 18th century applied this explanation to species as well.For example, Robinet declared: "There are only individuals, and there are no kingdoms, classes, genus, and species" ("Nature", IV; 1-2).The same remarks can be found in the writings of certain French naturalists, starting with Buffon, but also Lamarck (Burkhardt, 1977) and Lacepede (1800). Buffon soon abandoned this notion (if he ever believed it at all), and other naturalists such as Lamarque and Lacepede took a completely orthodox attitude when they discussed species in their taxonomic works.In his later years (after 1817), Lamarck realized the importance of species more and more.He emphasized that the species of inanimate objects is quite different from the species of living things.Biological species are complex systems of molecules of different kinds, which accounts for their ability to mutate and change.Finally, Lamarck also proposed the evolution of species and "whether species diversify without reproduction" The problem.This kind of prediction about the species problem is very different from Lamarck's early nominalist view that only individuals exist. This nominalistic notion of species was still common among botanists in the 19th century. Schleiden and Nageli are among the main supporters."Disordered" genera such as Rubus and Licorice were the most commonly cited evidence in defense of their point of view.Nominal species concepts are also popular among paleontologists, especially in the study of so-called "exotics" such as conodonts, where demarcation of species taxa is often indeed difficult (artifacts) scholars.The famous botanists and paleontologists Cronquist and A·B·Shaw in the past ten years have successively published articles to support the view that species is regarded as a completely subjective custom.The botanist Bessey (1908) put this view more bluntly: "Nature produces only individuals and nothing else... There are no species actually existing in nature. They are only concepts in the mind and nothing else is just It was conceived so that we can collectively designate large numbers of individuals." Some recent opponents of the concept of biological species (such as Sobal and Crovello, 1970) also still cling to ideas that are essentially nominalistic, although they are absolutely few.The evidence for inherent discontinuity between natural populations with overlapping ranges is so persuasive that most researchers of regional flora and fauna have adopted the biological species concept. The reason why biologists in the eighteenth and nineteenth centuries who were dissatisfied with the concept of an essentialist species adopted the nominalist concept was not because the latter was superior but simply because there was no alternative.With the advent of the biological species concept, it lost its edge and is now out of fashion, at least among biologists. 6.4 Darwin's Species Concept No other scholar has more vividly reflected the efforts to establish the concept of species than Darwin.他在青年时期作为一个博物学家和收藏家在舒兹伯利，爱丁堡、剑桥等城市所见到的物种是当地动物区系的模式的、“无因次”的（nondimensional）物种。这也是他的研究甲虫的朋友以及亨斯娄和莱伊尔所见到的物种。当他于1835年9月16日在加拉帕戈斯群岛登陆时仍抱有这种物种概念。他搭乘的贝格尔号访问了四个岛屿（Chatham，Charles，Albemarle，James），彼此相距都不超过一百里。由于达尔文以前并不了解地理变异，所以他以为这些相邻海岛的动物区系都相同，因而很自然地将他采集到的标本都简单地标记为来自“加拉帕戈斯群岛”。当地的西班牙人能够分辨每个岛上的巨龟的种类这一事实起初显然并没有引起达尔文注意，当时他主要关心的是地质学问题。随后当达尔文对他所收集到的鸟类标本进行整理分类时，就面临了怎样区分不同海岛上的种群这个问题。例如加拉帕戈斯群岛上的每个海岛都有小嘲鸫（Mimus），但是某个岛上的这种鸟和其它岛上的多少总有一点不同。因而达尔文就提出了这样的问题：不同海岛上小嘲鸫是同一个种还是变种？毫无疑问它们是不同的分类单位，因为它们的差异可以观察到并加以描述。这是一个安排等级的问题，即把它们安排到适当的阶元中去的问题。这一点在分析达尔文的关于物种的论述时必须记住。更为重要的是应当认识到达尔文的物种概念在19世纪40年代和50年代发生了相当大的变化（Kottler，1978；Sunoway，1979）。 19世纪30年代达尔文的物种和物种形成的概念几乎完全受动物学证据支配。的确，他认为物种是由生殖隔离来保持的。这是达尔文当时对物种的想法，这一点直到他的笔记被发现后才被研究达尔文的学者们认识到，在以前是一直不清楚的。例如达尔文在笔记中写道：“我的物种定义丝毫也不涉及杂种状态（hybridity），而仅仅是一种保持分隔开的本能冲动，这无疑是应当克服的（否则就不会产生杂种），但是在此以前这些动物就是不同的种”（NBT，C：161）。由之可以看出达尔文对由行为隔离机制保持着的生殖隔离作了清晰的阐述。在达尔文的笔记中还反复指出物种对异种交配的互相“敌对” 现象。“两个物种彼此嫌恶现象显然是一种本能，这样就避免了繁殖”（NBT，B： 197）。“种的定义：充分地保有固定不变的性状，连同具有非常近似结构的生物”（B： 213）。达尔文在这些笔记中反复强调种的地位和差异程度无关。“因此真正的种在外观性状上可能并没有任何差异”（B：213）。他在这里指的是鸣禽欧柳莺的两个姐妹种（棕柳莺和林柳莺），它们是1768年GilbertWhite在英国发现的，但由于彼此非常相似以致分类学家到了1817年才正式予以确认。因此可以毫不夸张地说达尔文的物种概念在19世纪30年代就已经非常接近现代的生物学物种概念。 当谈到并查阅一下其中关干种的问题达尔文是怎样写的时就使人不禁感到是在和完全不同的另外一位学者在打交道。（Mayr，1959b）。因为直到重新发现达尔文的笔记以前从1859年出版以后达尔文正是这样被生物学界认识的，所以从历史上来说引述一下达尔文在这本书中是怎样写的就很重要： 迄今为止还没有一个物种定义能满足所有博物学家的需要；然而每个博物学家又都模糊地知道当他谈到一个物种时他指的是什么（原书44页）在确定某一生物究竟应当是安排为种还是变种时，只能遵循判断力很强而又具有广泛经验的博物学家的意见。（原书47页）从上述意见可以看出我把种这个词看作是为了方便起见任意指定给彼此极其相似的一组个体，它和变种这个词并没有什么基本区别，变种指的是差异比较不明确而且波动较大的生物。（原书52页，469页）因此差异程度在确定两类生物究竟是应当安排成种还是变种上是一个非常重要的标准。（原书56－57页）变种具有和种相同的一般性状，因为它们无法和种相区别。（58页、175页）由此可以证明能育性和不育性都不能在种和变种之间提供任何明确的区别界限。 （原书248页）总之，我们将和博物学家处理属的问题那样来处理种的问题；博物学家认为属只是为了方便所作出的人为组合。（原书485页）达尔文在给胡克的一封信（1856年12月24H）中写道：“最近我将种的定义比较了一下，我发现当博物学家们在谈到'种'时各有各的想法，真是可笑；有些博物学家认为相似性就是一切，是最根本的，亲缘关系毫不重要；有些人认为相似性毫不重要，而神创论则是左右一切的；有些人认为亲缘是关键；另一些人认为不育性是最可靠的检验标准，而其它的人则又认为这一文不值。我认为这都是由于强求去为无法下定义的东西下定义的结果”（《达尔文通讯集》，II：88）。 什么事情使得达尔文的物种概念发生了180度的大转弯？他的读物和他的通讯表明在1840年以后，尤其是1850年以后，达尔文日益受到植物学文献以及他和植物学界朋友的通讯的影响。正如他自己所说的：“我的关于物种是怎样变化扩一切观念都来自长期不断地研究农艺学家及园艺学家的着作和与之对话的结果”（《通讯集》，II：79）。 对达尔文的思想影响最大的植物学家莫过于William Herbert，他曾讲过：“在种和固定不变或可以查知的变种之间并没有真实的或自然的区别界线…也不存在任何可靠的特征以区别两种植物究竟是种还是变种。”（1837：341）。从那个时候起直到现在的植物学文献中都可以发现几乎完全与此相同的议论。当时试图去分清分布区重叠与分布区不重叠情况的极为罕见。Herbert并不把杂交可育性放在形态类似程度之上予以重视，因为他认为：“杂种的可育性更多地取决于体质上而不是亲本在植物学上的更为相似”。 （1837：342）。在这里作为种的地位的标准不是生殖隔离而是差异程度。对Herbert来说属是唯一的“自然（的）”阶元。 如果将“变种”这个词改写为“地理隔离种”则达尔文的许多上述言论都是完全合理的。现在和达尔文时期一样，对地理隔离种（尤其是其中易于辨别的）的安排是主观随意的。鸟类中实际上有成百甚至上千的地理隔离种，1970年前后有些鸟类学家将之安排为种，另一些鸟类学家将之看作是亚种。 如果达尔文所要讲的是说安排隔离种群很困难而且往往是不可能的，那么谁也不可能加以挑剔。地理隔离种确实是端始种。遗憾的是，达尔文所用的全是模式语言，用了“类型”和“变种”，而不是“个体”或“种群”，因而引起了混乱。而且他也不是一贯地使用“变种”这词表示地理种，他往往（尤其在后期着作中）还用它来指变异体或异常的个体。由于扩展了“变种”这词的涵义，达尔文就混淆了两种十分不同的物种形成方式：地理（隔离）物种形成，同域型物种形成（sympatric speciation）。 达尔文在中关于种所作的论述令人读后产生这样的印象，即他认为种完全是主观随意的，只是为了方便分类学家而创出的。他的某些议论使人回忆起拉马克所说的种并不存在，存在的是个体。然而在分类学研究中（拉马克研究软体动物，达尔文研究的是藤壶类），他们两人又完全按正统方式对待种，就好像这些种都是独立产生的。我认为这是十分自然的，因为在这些分类学专着中他们列出了并描述了物种分类单位，而且除了在分类上模棱两可之间的情况下以外也用不着考虑物种阶元。 不知道是怎么一回事，达尔文为已经“解决了”种的问题而感到特别开心：由于种在不断地进化，因而无法下定义，它们单纯是随意起的名称。分类学家再也用不着去为种是什么而苦恼。“当这本书中所包含的观点被普遍接受后，系统学者们…将再也不会被虚幻的疑虑不断地困扰，这疑虑就是从本质上来说究竟是这一种类型还是哪一种类型是一个物种。我根据经验可以肯定地说这对分类学家将会如释重负”（，484）。这就是为什么达尔文不再试图为种下定义的原因。他完全按模式概念将“差异程度”作为种的特征。Ghiselin（1969：101）说得很对：“并没有可靠的证据表明达尔文把种看作是生殖隔离的种群。”就达尔文撰写毫物种起源。的当时来说这是千真万确的。 还应当进一步记住在中达尔文是在物种逐步起源的范围内讨论物种。 达尔文当时有一种强烈的愿望（虽然可能是不自觉的）想要论证物种并不像神创论者所声称的那样是固定不变和具有独特性的。因为物种如果像达尔文的反对者在此后一百多年不断宣扬的那样是严格分隔开的而且是由“不可跨越的鸿沟”分开的，那末它们又怎样可能是通过自然选择逐渐变化的结果？因此，否定物种的独特性是一种很精明的策略。 如果仅仅按差异程度而不是按生殖隔离来为种下定义，如果对地理变种和种群内变种不加区分，则支持反对达尔文观点的还可以搜罗不少。若按物种是通过自然选择逐步变化的产物这种观点来看，则新种的起源就不是一个无法解决的问题。然而由19世纪30年代达尔文的物种概念向50年代达尔文物种概念的转变却为历时一个世纪的论战理下了火种。 6．5生物学物种概念的萌芽 的出版使研究物种的学者处于一种尴尬的两难局面。按达尔文的观点物种显然是通过缓慢渐进的过程由共同祖先而来的。然而地区的博物学家则认为种在自然界中是由无法弥补的裂缝分隔开的，完全不像达尔文在中宣称的那样是由标本的任意集群构成的。因此关于物种问题仍然不停地被提了出来，就好像从来没有过进化学说似的。在博物馆的分类学家之中本质论的物种概念仍然占统治地位。由于按照这种物种概念是以形态差异程度作为标准来判定哪些个体属于同一个种或不属于同一个种，所以被称为形态学物种概念。直到1900年，一批有名的英国生物学家和分类学家，包括R·Lankester，W·F·R．Weldon，贝特森，华莱士都一致支持严格的形态学物种定义。由华莱土提出的定义——“种是在一定的变异限度内产生与本身相似的后代的一群个体，它们并不和最邻近的有关种经由不可察知的变异联系起来”——将会把每个地理隔离种提升到单独的种的地位。一旦遇到变异的情况就按照瑞的意见处理，即一对同种的亲本无论在其后代中能生出什么都应当将后者看作是同种。这一物种概念不仅被大多数分类学家采纳，而且在实验生物学家中也处于支配地位。德弗里的月见草（Oenothera）种就是依据这样的形态学物种概念而确认的，近至1957年Sonneborn拒绝把草履虫的“变种”看作种，尽管根据生物学性状和生殖行为完全有充分证据说明这些变种确实是种，这一点Sonneborn本人最后也承认了。 在野外进行实地调查研究的博物学家的着作中随时都可以发现一种远比形态学物种概念更好的物种概念。例如F． A． Pernau（1660－1731）和J． H． Zorn（1698－1748）在他们的居处周围详细地研究了鸟类生物学的各个方面而且毫不怀疑地认为所有这些鸟按生物学性状（鸣声，鸟巢，迁移方式等）以及生殖隔离分别属于不同的鸟种。 Zorn和瑞一样，还是遵循自然神学传统，而且在随后的15O年中关于自然界物种研究的最优秀着作都是出自自然神学家之手。在这一段时期中最着名的鸟类学者如G·White、C·L·Brehm，B·Altum等也都是牧师或教士。在研究自然界中昆虫的种方面自然神学家如William Kirby也走在前列。正是从事野外工作的博物学家的这种传统一旦变成自觉的和科学化的时候就逐步发展成为生物学物种概念。 依据形而上学的本质（essence）概念的旧物种概念和生殖隔离群体的生物学物种概念是如此根本地不同，因而由前者逐渐地转变为后者是不可能的。为此就需要自觉地放弃本质论者的概念。当研究物种的学者们在试图运用差异程度这个标准并清楚地认识到遇到许多困难的时候反而促成了这种转变（Mayr，1969）。第一个困难是无法找到本质存在的证据，而本质却是自然界不连续性的根源。换句话说，就是没有办法确定物种的本质，因而在模棱两可的情况下无法运用本质作为衡量标准。第二个困难是由明显的多形性引起的，也就是说自然界中出现明显不同的个体，虽然按这些个体的生活史或繁殖习性可以证明它们属于一个繁殖种群。第三个困难和第二个相反，即自然界中有一些类别虽然在生物学方面（行为，生态）明显不同而且在彼此之间存在着生殖隔离，但是从形态上却无从加以区别（姐妹种；见下文）。 在研究历史上的有关物种问题的讨论时就会发现某些早期学者的观点是多么接近于生物学物种概念。就现代生物学家看来Ray的修正后的本质论概念（“种是一切变体的集群，这些变体可能是同一亲本的后裔。”）和单单依据繁殖种群概念的物种定义只有一步之差。更为接近的是布丰的定义，“种是能够共同繁殖的相似个体的不变系列（世系）”而且它们的杂种是不育的。然而布丰仍然从本质论的观点认为种是固定不变的，Girtanner（Sloan，1978）和Illiger（Mayr，1968）的一些论述同样也很接近生物学物种，然而他们也不能摆脱思想中的本质论框架。19世纪很多学者也是如此。他们之中没有一个人再前进一小步去接生殖隔离的种群给种下定义。为什么拖延了这样之久？ 生物学种有三个方面要求采用新概念。首先是不能把种设想为模式（type），而应当看作是种群（或种群集群），也就是说从本质论转变到种群思想。其次是不能按差异程度为种下定义，而应当按独特性、即生殖隔离下定义。第三，不能按内在性质给种下定义，而是要按它们和共存的其它种的关系下定义；这种关系既表现在行为上（非相互配育），也表现在生态上（致死竞争）。在实现了这三项概念转变之后就可以明显看出种的概念只是在非因次情况（nondimensional situation）下具有意义：在划分物种分类单位时考虑多因次（多维）很重要，但在概念标准的拟订上则并非如此。这一概念被称为生物学概念并不是因为它涉及生物分类单位而是因为它是生物学定义，完全不能用于非生物（无生物）物体的种类。还应当提醒的是决不能把和物种分类单位有关的问题与和物种阶元有关的问题混淆在一起。 直到本世纪的40年代和50年代才明确地提出生物学物种概念并对其特征作了认真的分析。然而它的基本点却早已就被一些先驱掌握。最先明确提出生物学物种概念并为之下定义的是昆虫学家K·Jordan（1896，1905）和Poulton（1903）。Poulton将种定义为：“相互配育的种群、有性生殖的，”Jordan写道：“由血缘关系联系起来的个体在一个地区形成单独的动物区系单位…由某个地区动物区系组成的单位彼此之间由不能被任何东西弥补的间隔分隔开（1905：157）。 着名的鸟类学家如斯垂思曼及壬席在20年代和30年代就一贯使用生物学物种概念。 斯垂思曼于1919年就强调物种的特征不是差异程度而是那样的类型，当它们在地理隔离中上升到物种等级并在生理上彼此差异如此之大以致它们…不通过相互配育又能聚集在一起。”杜布赞斯基的物种定义实际上也与此相同：“生理上不能互相配育的那些类型”。迈尔的1942年定义：“物种是实际（或可能）能够相互配育的自然种群的类群，这些类群和其它类似的类群彼此被生殖隔离分隔开。”仍然有其不足之处。这“实际或可能”几个字是不必要的，因为“生殖隔离”就意味着具有隔离机制，而这和物种地位是无关的。更确切的定义是：物种是在自然界中占有特定生境的种群的生殖群体（reproductive community of populations），和其它种群的生殖群体被生殖隔离分隔开。 这个定义并不告诉怎样去划分物种分类单位。它所能起到的作用是确定分类单位的阶元等级。对比之下，形态差异程度并不是恰当的标准，这已由姐妹种或明显的型（morphs）所证实。生物学物种概念表示了种群之间的关系，只有在非因次情况下才有意义并能真正应用。它只能通过推论扩展到多因次情况。 生物学物种定义中“生殖隔离”这几个字是关键词。这立即提出了生殖隔离的原因问题，这个问题随着隔离机制概念的发展才得到解决。这概念的雏形可以回溯到布丰的不育性标准，直到20世纪这一标准在植物学家中还普遍采用。然而动物学家，尤其是鸟类学家和研究蝴蝶的学者，发现在野外不育性障碍在动物中极少检测到，同种（canspecificity）一般是由行为相容性（behavioral compatibility）来决定。随着时间的推移，越来越多的防止种间杂交的策略或措施被发现，例如繁殖季节或花期不同以及占有不同的生境（栖息地）。瑞典植物学家Du Rietz（1930）首先就种间杂交障碍列出了详细目录与分类。然而这类研究由于缺乏恰当的学术谁汇而严重受阻。杜布赞斯基提出了“隔离机制”这个术语表示“阻碍个体类群杂交的任何原因…隔离机制可以分为两大类，地理隔离和生理隔离”（1937：237）。虽然杜布赞斯基也认识到地理隔离“和任何生理隔离是处在不同的水平”，但是他并不了解只有后者才是物种的真正属性。 因此，迈尔将“隔离机制”这个术语限制为物种的生物学性质，明确地排除了地理障碍（1942：247）。这仍然还剩下一个难题：就其它方面来说完全是纯种的生物中个别的个体可能杂交。换句话说，隔离机制只能保证种群的完整性，而不是每一个单独的个体。