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Foreword

Do species exist?

For Ernst Haeckel, in the midst of nineteenth century, the point was quite easy: In the beginning, there was a plant and an animal. Evolution started extending and varying their principal organizations. Accordingly, species were regarded to be just notes given by man to the continuous flow of evolving life. Classifying man looked for these entities to allow him a qualification of the ongoing diversification of life. Taxonomy, thus, supporting such an evolutionary biology should form out something like a phylogenetic systematics that may outline the quality of that continuing process of evolution in its details. Haeckel did not succeed to from out such a new taxonomy even though he was one of the great taxonomists of the late nineteenth century. In an evolutionary perspective, addressing that continuous flow of evolution, species, thus, could be described only as categories formed by the human mind. With the introduction of population genetics into Darwinism, the situation changed. Genetics allowed to address a certain base of heredity. Accordingly, the continuous flow of evolving forms could be outlined in a much more substantiated way, allowing to address how far genes were established, varied, or deleted in the course of generations. In fact, more and more extending insights into the functional organization of life forms established new ideas about life organization, in general. Thus, today, the situation of taxonomy is far less clear than it was at Haeckel's time. Not only that the last unified common ancestor had to be a prokaryote or we had to address mushrooms and eventually such prokaryotes and some others of such organizations as principally differing basic types of life. We have to integrate morphology, population biology, and molecular genetics and their different accesses toward a species concept. Thus, we have to address the question of what a species really is, again. And we have to clarify several problems: How we can evaluate biological diversity if species do not exist? How we can understand evolution if speciation is not really the motor of ongoing evolutionary development? And how we can compare the accesses of molecular biology, the analysis of palaeontology, cladistics, and morphological analyses directed toward the species concept in one run? In his book, Werner Kunz describes and analyzes all such various interpretations and concepts dealing with species. Thereby, he shows us that the different ideas of Neo-Darwinism, taxonomy, and genetics do not fit into each other. Even worse, he is aware that species have been formed out in various solutions within different evolutionary settings. A plant species is not directly comparable with an animal or mushroom species. In an evolutionary perspective, all these species are the outcome of differing evolutionary strategies. That is not true for their differentiation in taxonomic regard, but it is true in regard of what species mean in life. Thus far, a species not only might be characterized by a set of attributes allowing a reliable classification but may also offer principal differing materials for an ongoing evolution. There, species of plants, various animals, prokaryote species, and mushrooms may each react differently in evolution. Already, sexuality is organized in all these life forms in a different way. In any case, there is just something transferring a set of genes, certain morphological and functional specifications from one to the next generations. The modes by which this is being practised are different, however. What should be done in such a situation?

Werner Kunz did not offer a philosophical solution. He is presenting facts, and he is doing that in a comparative perspective. One point Kunz makes is that the taxonomists rely on Linnaeus who had a completely different view on nature from that we have today. For Linnaeus any species is part of creation. If nature is such a creation done by God, any entity in nature is reflecting an absolute ordering scheme. Systematic will outline this scheme. Thus, live forms a thought to be organized like the terms in a baroque encyclopedia. There, any term outlines a basic idea. Its true meaning is intelligible when the order, in which it is used, is made obvious. The underlying structure by which such ideas could be combined is the idea of a universal topic reflecting the concept God has had in mind in setting out his creation. To combine such a scheme with the Darwinian idea of a continuously varying world is not possible. Nevertheless, idealistic morphology in the start of twentieth century tried to do this. The result was a logical scheme adopted in principle by Willi Hennig. He formed out an abstract pattern that allowed a proper classification, but was not interested to integrate that view with a historical reconstruction of what actually happened in the course of evolution. Cladism adopted Hennig's idea, which is now forming the conceptual framing for evolutionary interpretation of DNA analyses. When such an idea of a logically consistent scheme is combined with evolutionary population biology, problems occur. Accordingly, in an attempt to combine such approaches, one has to address anew the question what a species really is. If species are individuals, evolutions will deal with them, resulting in new species. If species consist of populations, and if microevolution works on the level of such population, situations might become more difficult. What a species meant, cannot just be a taxonomic entity without any functional relevance in evolutionary biology. Would that be the case, we could not describe evolution as a process resulting in speciation. If species are actually something evolution worked on, then, however, species themselves (as structural units) might be entities that have been evolved as such ones within various evolutionary constraints. Accordingly, a species might address something different in plants, mushrooms, bacteria, viruses, and animals. On the other hand, what is meant by such different concepts regarding the functionality of the species? The resulting idea, describing evolutionary relevance of species within a certain evolutionary process, might differ from what a species meant for other life forms. A rodent will eat certain plants and will avoid poisonous ones. Thus, the species' concept somehow is actually valid for it irrespective of the different evolutionary dynamics of such a poisonous plant and its own species.

Everyone describing biodiversity has to encounter diversification on the species level. He has to think about what it means to be extinct. He may even understand that a species is formed by populations and he has to understand that a population is not a species. The problems that come out of all that are addressed by Werner Kunz. He is not offering a new philosophy. He is following the ideas in biology to their consequences. This allows an understanding of the various uses and the significance of species concepts. This enables to address a lot of relevant questions and to understand conceptual constraints in modern evolutionary biology. The result is a great book that should be read by anyone who wants to understand evolution, biodiversification, and the meaning of species in those.

Ernst-Haeckel-Haus, Jena, April 2012

Olaf Breidbach

Preface

What is water? You would not answer “water is wet” or “water is a liquid” because these are properties of the water, not definitions. The answer “water is wet” does not explain what water is. Water is a substance consisting of H₂O molecules, and the formula H₂O is exhaustively explained by physicists and chemists. It is well known what water is.

However, what is a tiger? If you would answer “a carnivorous animal which is big and has black stripes,” you would describe only the properties of the tiger. You would not answer the question, what a tiger is. If you try to find the answer to the question what a tiger is, you will notice that the answer is very difficult, if not impossible to find.

The same becomes apparent if you ask the question “what is a species?” It is relatively easy to answer the question “what is a molecule?” A molecule is a group of atoms that are linked by chemical bonds, and atoms and chemical bonds are well defined by physicists and chemists.

Like a chemical molecule, a biological species is a group of organisms. But are the organisms of a species linked by bonds? If yes, the question arises what kind of bonds these are that hold the organisms of a species together. If there are no bonds, the question arises why the individuals of a species can form a group at all. If there are no groups, there are no species.

It is absolutely necessary to define the term group in taxonomy because there are several kinds of groups that organisms can form in nature. For example, migrating birds can form stable groups. You also can classify apes and monkeys including humans in groups according to their blood group alleles. But such groups of organisms are not species. What is it that makes a group of organisms to be a species? If you try to find the answer to the question what a species is, you will notice that the answer is very difficult, if not impossible to find. What is it that makes a tiger to be a member of the species tiger? And what is the species tiger?

If you consider all organisms that resemble each other in their traits to belong to a species, this view is immediately upset by the phenomenon of distinct trait differences between the two sexes of the same species that often exceed the species differences. If you consider all organisms to belong to a species that are able to crossbreed successfully, this view cannot be held out because several individuals of a species cannot crossbreed successfully for genetic or other reasons. If you consider all organisms to belong to a species whose genomes are similar in DNA sequence, this view will soon be corrected by the awareness that there exist phylogenetically young species that are genetically homogeneous as well as phylogenetically old species that are genetically heterogeneous. There may be larger genetic differences among the members of an evolutionary old species than among the members of different species if the species are evolutionary young. Evolutionary distance cannot be a reliable species criterion. At the end, all attempts to discriminate species from each other are blurred by the fact that all criteria for these discriminations also may apply to the individuals within a species. Intraspecific polymorphisms are a main obstacle for taxonomic classification.

It appears that, in contrast to chemical objects, taxonomic objects cannot be defined. What is it that makes this fundamental difference between chemical and biological objects? It is very remarkable that taxonomists in most cases abandon the species problem and are nevertheless quite able to work with species. How is it possible to do scientific work and to obtain reproducible results with objects without knowing what these objects are? Should it not be irritating or alarming to do research with objects that are not defined?

Despite several promises, the species problem is not solved, and it cannot be ignored. This book elucidates the inconsistencies and contradictions that stand behind the conventional species concepts. In this book, it is emphasized repeatedly that it is a doubtful, if not an unscientific, way to practice taxonomic classifications while ignoring the foundations of the species problem. Furthermore, beyond these theoretic considerations, uncertainties and ambiguities of the species problem have considerable impact on the strategies of legislation in biodiversity conservation politics.

Düsseldorf, April 2012

Werner Kunz

Color Plates

Color Plate 1. Multivoltine Butterflies (photomontage)

Several butterfly species can be univoltine or bivoltine or even trivoltine. These are different morphs of the same species that differ genetically. Univoltine are individuals which produce only one imaginal generation per year; they live in the north. Bi- or trivoltine individuals generate two or three imaginal generations per year; in spring, early and late summer. They live in more southern regions. In Europe, well-known examples are the Scarce Swallowtail Iphiclides podalirius (top), the Common Swallowtail Papilio machaon (left), the Common Blue Polyommatus icarus (bottom left) and the Peacock butterfly Inachis io (bottom right).

Color Plate 2. Batesian Mimicry (photomontage)

Similarity is not kinship. Heliconiid and Ithomiid butterflies are bad-tasting and therefore avoided as a prey by birds and other predators. Representatives of completely different families of butterflies imitate the shapes and color patterns of the unpalatable species to be protected, although they are not unpalatable. Each of the four groups presents a Central American Heliconiid or Ithomiid together with their imitators. 1 Heliconius ismenius together with 2 Eresia eutropia (Nymphalidae); 3 the Heliconiid Eueides isabella together with 4 Dismorphia orise (Pieridae); 5 the Ithomiid Ithomia heraldica together with 6 Actinote anteas (Acraeidae); 7 Eueides isabella together with 8 Dismorphia amphiona (Pieridae).

Color Plate 3. Müllerian Mimicry (photomontage)

Similarity is not kinship. Different Heliconiid and Ithomiid species in Central America resemble each other in shape and color pattern, although they belong to two different butterfly families. Each of the four groups presents a Heliconiid together with Ithomiids. 1 Heliconius hecale together with 2 Mechanitis lysimnia; 3 Eueides isabella together with 4 Hypothyris lycaste; 5 Heliconius ismenius together with 6 Napeogenes tolosa and 7 Godyris zavaleta; 8 Heliconius hecale together with 9 and 10 Melinaea scylax and 11 Mechanitis polymnia.

Color Plate 4. Cryptic Species and Mimicry Morphs (photomontage)

Six different species of European Burnet moths (family Zygaenidae) are very similar in phenotype. They all are black with brilliant red spots. From left to right: Zygaena filipendulae, Z. ephialtes (photo: Jochen Rodenkirchen), Z. transalpina, Z. viciae (on top of blade of grass), Z. lonicerae (on yellow Birdsfoot Trefoil flower Lotus corniculatus) and Z. angelicae (on top of purple Oregano flower Origanum vulgare). Z. ephialtes, however, is also found in a completely different morph that has white and yellow spots (two examples bottom center and bottom right). This dark morph resembles the nine-spotted moth (Amata phegea), a moth belonging to the family Arctiidae (bottom left).

Color Plate 5. Sexual Dimorphism (photomontage)

Similarity is not kinship. Differences in traits between the morphs within a species can be much greater than species differences. Here sexual differences between males and females of Central American Tanagers are shown. 1 and 2 Cherrie's Tanager (Ramphocelus costaricensis) female and male; 3 and 4 Summer Tanager (Piranga rubra) male and female; 5 and 6 Passerini's Tanager (Ramphocelus passerinii) female and male.

Color Plate 6. Confusing Similarity between Doxocopa and Adelpha (photomontage)

Similarity is not kinship. In Latin American rain forest, the females of Doxocopa species (1 and 2) resemble the butterflies of another Nymphalid genus: Adelpha (3 – 5). They don't resemble the males of their own species, which are shining brilliant blue (6 – 8). 1 Doxocopa excelsa and 2 Doxocopa laurentia; 3 Adelpha zea, 4 Adelpha basiloides and 5 Adelpha cytherea; 6 and 8 Doxocopa laurentia, together with 7 Doxocopa clothilda.

Color Plate 7. Cryptic Species and an abnormal Mutant (photomontage)

Several European Fritillaries resemble each other largely, and it is very hard to determine their species membership. However, a rare mutant of Melitaea athalia is aberrant in its color pattern and deviates from the other members of its own species much more than the members of different species differ from each other. Above center: two Nickerl's Fritillaries Melitaea aurelia; above right: Assmann's Fritillary M. britomartis; center below the Nickerl's Fritillaries: two normal Heath Fritillaries M. athalia, and bottom left a mutant of M. athalia.

Color Plate 8. Water Frogs are “Kleptospecies” (photomontage)

The Water Frog Pelophylax esculenta (top and center) is a hybrid species between the Marsh Frog P. ridibunda (bottom right) and the Pool Frog P. lessonae (bottom left; photo: Benny Trapp). Whereas the somatic cells of the Water Frog contain the genomes of both parental species, one of the genomes is completely eliminated in the germinal cell line in the Water Frog prior to meiosis. As a consequence, the genes of one of the two parental species cannot be passed on to further generations. Therefore, the Water Frog “steals” one of its genomes from a foreign species and is termed a “kleptospecies” for this reason.

Introduction

In one of his most famous works, “Die Harzreise” (“The Harz Journey”) (Heine, 1826), the German poet Heinrich Heine found a beautiful flower on the Brocken, the highest peak in the German Harz mountains. Tourists were standing nearby in considerable numbers, and they all wanted to know the name of the flower. Heine expressed particular aversion to this demand and wrote:

“It always annoys me to see that God's dear flowers have been divided into castes, just like ourselves, and according to similar external features like differing stamens. If there has to be classification, people should follow the suggestion made by Theophrastus, who wanted to classify flowers in a more spiritual manner, that is, by scent. As for me, I have my own system of natural history, according to which I classify everything as eatable or uneatable” (The Harz Journey; English translation Heine) (Heine, 2006).

This almost two-century-old assessment of taxonomy is not as absurd as it might at first appear. Even today, taxonomic classifications are based on several subjective standards of valuation that do not withstand a thorough theoretical test. There are several reasons for this problem (Mallet, 1995). One reason is that, even today, no theory in taxonomy can determine and prescribe which traits may be used in taxonomic classification and which may not.

No binding rules exist for the classification of individuals into species, nor is it clear whether species exist at all. Notwithstanding the title of his famous work “On the Origin of Species,” Charles Darwin did not believe in the existence of species (Chapter 2). He took species for constructed units, defined to achieve a comfortable ordering of living beings that would not even exist in the absence of human principles of classification. Many recent authors treat the biological species in precisely the same way.

Several taxonomists agree that a definition of the term “species” will never be possible. Indeed, they state that this issue is merely an “academic” question and that it is not meaningful for a scientist to devote time to such a problem. To these taxonomists, the expectation that the nature of species can be understood represents an illusory goal that cannot be achieved.

As an alternative, they restrict taxonomy to the simple goal of identification. To work with species and to understand the function of a species and its ecological role, it would be sufficient to identify the units of biodiversity without knowing what these units are. Questions such as “what is a tiger?” or “what makes a tiger a tiger?” are considered to be senseless questions that hamper science.

This type of reductionism, “taxonomy is diagnosis or identification,” generated the recent technology of barcoding, a method that is evaluated as “the future of taxonomy” for several reasons. Supported by substantial funding, assisted by very successful public relations and preceded by the belief that this approach represents genuine “high-tech taxonomy,” the barcoding-technology approach to taxonomy has initiated a triumphal procession (Chapter 4).

However, it seems to be forgotten that identification cannot be the ultimate goal of taxonomy. What result is ultimately obtained if groups of individuals are identified? The use of diagnostic tools can allow the identification of a number of cat-like animals as tigers or as lions. However, a number of individuals can also be identified as males or as females. Furthermore, a number of individuals can be identified and distinguished by differences in their blood groups. The results obtained by these three approaches to identification are certainly very different, and the identification procedures per se do not distinguish between intraspecific polymorphisms and species differences.

The attempts to identify animal or plant groups do not achieve the final goal of demonstrating that these groups are species. It does not suffice that the groups can merely be distinguished from each other. Why are the different sexes not different species? Neither the simplest conservative identification techniques nor the most modern molecular techniques can determine whether two clearly distinguishable groups are species. This consideration shows that the idea of species has additional significance. It is the goal of this book to elucidate the true nature of species. Species are not simply groups of individuals that can be distinguished. Species are something else entirely.

Educated by a number of identification guides or field guides that are available for most groups of animals or plants, we are misled to believe that individuals that clearly differ in traits must belong to different species. If a goose in Eurasia has a uniformly pink-to-orange beak and pink feet, it must be a Greylag Goose (Anser anser). However, if it has an orange beak with black margins together with orange feet, then it must be a Bean Goose (Anser fabalis). Nevertheless, these differences are not exhibited by each individual Greylag Goose or Bean Goose because mutants occur. Hence, why are those mutants still members of the species? Why are they not different species? Indeed, of what help are identifying traits for the understanding of the species?

Linnaeus stated that certain traits are essential to the species. A particular member of the species must possess these traits, or else it would not belong to the species. However, Darwin stated that the particular traits found in the individuals of a species change over time. This principle means that no single trait can be the essence of a species. It is not possible that both authors can simultaneously be correct.

As a consequence of Darwin's theory of evolution, it was necessary to conduct a thorough revision of Linnaeus's view of the species. This revision was achieved by Poulton in 1903, and extended by Dobzhansky in 1937 and Mayr in 1942. These authors replaced the Linnaean typological view of a species by the concept of a reproductive community. This concept is based on mutual lateral gene exchange by sexual contact. Organism A belongs to the same species as organism B if any of its offspring does receive genes from organism B.

The concept of the species as a gene-flow community is a species concept based on mutual relational connections among the organisms and their offspring (Chapter 6). It is not a typological species concept based on trait similarities. The concept of the gene-flow community is not easy to understand. It contradicts a type of cognitive presetting in the human mind (Chapter 2), and most importantly, it is very difficult to use. Ultimately, it is inapplicable for use in an operational and pragmatic everyday taxonomy. However, the concept of the gene-flow community appears to be the only species concept that reflects an entity that exists as reality as a delimited group in nature. According to this species concept, the borders between the species (although penetrable) exist in nature; they do not result purely from human constructs used for the purpose of grouping individuals.

The fundamental disagreement between a species concept that is logically consistent and a species concept that is applicable in practice is the primary reason for the existence of a “species problem” that could not previously be resolved. This deep conflict has its roots in the incompatibility between the claim to classify biodiversity according to taxa, in the sense of Linnaeus, and the scientific fact, introduced to the world by Darwin, that the traits of organisms change over time (Chapter 2).

This book has a long history. The book's inception occurred almost twenty years ago, when I became aware that the biological phenomenon of multiple allelic polymorphism implies a serious problem for taxonomic classification. How can organisms be classified into different species if single organisms already differ in hundreds or even thousands of their traits? Doesn't this mean that there must exist two different types of traits? One type of trait serves to discriminate among individuals within the same species, whereas other types of traits must possess certain unique qualities to be suited for species discrimination. However, two such types of traits do not exist (Chapter 4). Accordingly, what difference separates individual differences and species differences?

This book addresses biologists and philosophers, although it is much more a biological than a philosophical book. During the long time of the progress of this book, I benefited greatly from Markus Werning (now University of Bochum), who taught me several basic elements of philosophy. I also thank Gerhard Schurz (Düsseldorf), who opened the door for me to enter the philosophic scientific community. A decisive role in the continuation of my efforts to bind taxonomy to philosophy was played by Hartmut Greven (Düsseldorf), who encouraged me not to give up. He eased the difficulty for me, as a geneticist and molecular biologist, to gain entry into the taxonomic scientific community by inviting me to give lectures and to publish preliminary papers on the species problem. I also thank Sebastian Löbner (Düsseldorf), who is a linguist, not a biologist, but his invitation to be a member of his research group on functional concepts had a great impact on the understanding of taxonomic class formation presented in this book. Finally, I thank Gregor Cicchetti and Andreas Sendtko for their decision to support the processing of this book by the Wiley-Blackwell publishing company.

A number of the color plates in this book would not have been created if I had not enjoyed several holidays on the Finca Hamadryas in Costa Rica, where Paul Gloor helped me to identify the butterfly species that I photographed there. The book was first completed in German language, and afterwards translated into English. The first versions of this translation were done by Christian Feige. The art work of the black and white graphs in the book was done by Karin Kiefer. The color plates are photo montages of my own outdoor photographs that are inserted into the correct habitat. The technical art work was done by Monika Dörkes. I also thank Albert Kaltenberg who was the "soul" of my computer whenever the machine did not follow my advices.

This book presents no novel scientific data, nor does it present new philosophical conclusions. The material on which the book is based has previously appeared in biological books and papers or in philosophical books and reviews. The novel feature of this book is that it combines the fields of biology and philosophy. In this book, philosophical reasoning is explained for biologists and applied to unsolved or controversially disputed taxonomic problems. This book will raise biologists' awareness of one of the most difficult problems of taxonomy, namely, how to arrange the existing diversity of living organisms into cohesive and delimited groups.

Remarkably, many taxonomists are not genuinely interested in the species problem, although they are affected. In contrast, philosophers are more engaged with the foundations of taxonomy, although they are not as directly affected in their daily research. It was Albert Einstein who once said: “Science without philosophy is blind, and philosophy without science is empty.”

Chapter 1

Are Species Constructs of the Human Mind?

In 1926, Reagan defined the species as a purely pragmatic principle of classification: “A species is what a good taxonomist says it is” (cited from Huxley, 1942). In 1996, Hawksworth did not see the biological species any differently: “Species are groups of individuals separated by heritable character discontinuities and which it is useful to give a name to” (cited from Heywood, 1998). Even today, more than twenty different species concepts are still practiced concurrently (Mayden, 1997). This observation shows either that the biological species does not exist or that the particular species concepts define something different from the one truly existing species.

Since Darwin and Wallace, it has not been possible to unite Linnaeus's taxonomic principle of classification into rigid classes with the theory of evolution. Simply consider the implications of the title of a famous publication by Alfred Russel Wallace “On the tendency of varieties to depart indefinitely from the original type” (Wallace, 1858). Does this title in itself not mean “There are no species?”

With these considerations in mind, it would now be consistent and simple to accept the reality that species are fictitious human constructs made to sort genuine biodiversity into manageable but artificial units. However, a large majority of field biologists, insect collectors and “tickers” and “twitchers” among the hundreds of thousands of bird watchers believe in the real existence of species. All of the modern field guides to the birds of Europe and the adjoining regions contain approximately 800 bird species. None of these books identify the species concept that was used to obtain this number. They do not explain whether the term “species” means morphotypes, ecotypes, reproductive communities or descent communities. Instead, the impression is conveyed that these 800 species exist in reality and that each species simultaneously satisfies the classification principles furnished by each species concept.

Of course, the field guides do contain disputed borderline cases, for example, the recently undertaken separation of the Balearic Shearwater (Puffinus mauretanicus) from the Yelkouan Shearwater (P. yelkouan) or the separation of the eastern Mediterranean Black-eared Wheatear (Oenanthe melanoleuca) from the western Mediterranean Black-eared Wheatear (O. hispanica) to give two distinct species. However, these are isolated incidents. In the main, the books convey the general consensus that species exist without posing the question of the nature of species. Otherwise, no consistent field guide could appear on the market. Nevertheless, these apparently unambiguous species are not defined anywhere in the field guides. Except for observations that certain species diverge from each other genetically or that there are diagnostic-typological differences, the reader does not learn why particular varieties are delimited from each other as species.

Adherence to any species concept is never fully consistent. If the reproductive community, the classification according to apomorphies or the classification of equal-ranking kinship were actually taken seriously, then many animal and plant groups would be split much more deeply into separate units than current practice supports (Chapter 2). An unspoken agreement appears to sanction “generously” combining mosaic-like fragmented reproductive communities or nested cladistic bifurcations to construct inclusive species boundaries because this approach yields readily manageable units. In critical cases, pragmatism proves to be a highly dominant principle in taxonomy. Pragmatism determines taxonomy's direction, and consistent reasoning has only a marginal importance in taxonomy (Chapter 2).

The introduction to a remarkable review article by Martin L. Christoffersen titled “Cladistic taxonomy, phylogenetic systematics, and evolutionary ranking” in the journal Systematic Biology contains the following statement that could equally be an opening theme for the present book (Christoffersen, 1995):

“The ancient discipline of biological taxonomy has been very slow to incorporate major shifts in world views . . . Impervious to the derision of scientists in the more glamorous fields of research, many taxonomists today simply take for granted secular traditions of describing and naming the diversity of nature. They may persist stoically for a lifetime in such a self-appointed descriptive role, avoiding theory, philosophy and explanation. Some of these taxonomists may venture intuitive classifications for their named groups but will often delegate to others the task of deriving evolutionary meanings from their proposals.”

Of course, one can use the traits employed for identification to recognize particular species and to distinguish them from other species. However, this procedure already implies that these particular species do exist, and that one needs only to learn how to identify them. If there were no species, it would be meaningless to identify them. Moreover, if two groups of organisms were not different species, but instead were one and the same species, it would be meaningless to identify and distinguish them. This observation demonstrates that the process of defining a species must precede the process of identifying that species (Chapter 2). Taxonomy cannot defend its reputation as a serious science if it relies exclusively on species identification. More scientific than the diagnosis of a species is the “why” of a species (Mayr, 2000). It is not sufficient to identify two organisms belonging to two different species by their diagnostic traits. It is more scientific to be able to explain the reasons that the organisms belong to two different species.

There is an important difference between that which something is and that by which something can be identified. Two human beings are not brothers because they have similar traits, but because they have the same parents. Half a century ago, George Gaylord Simpson stated this difference as follows: “The well-known example of monozygotic twins is explanatory . . . Two individuals are not twins because they are similar but, quite the contrary, are similar because they are twins” (Simpson, 1961). Stated precisely, individual organisms do not belong to the same taxon because they are similar, but they are similar because they belong to the same taxon.

The anthropologist and psychologist Scott Atran stated resignedly: “Perhaps the species concept should be allowed to survive in science more as a regulative principle that enables the mind to establish a regular communication with the ambient environment than as an epistemic principle that guides the search for nomological truth” (Atran, 1999).

It appears that species are simply pragmatic principles of classification. Furthermore, the principles of classification are not the same in higher animals, for example, antelopes in Africa, and in more primitive animals, for example, rotifers. However, under these conditions, the species of different animal and plant taxa are not mutually comparable. It would be meaningless to contrast the species richness of certain beetle families (Coleoptera) with the species poverty of certain families of frogs (Anura). Nevertheless, such comparisons are made.

Taxonomy pursues the intention of classifying organisms according to personal standards. In contrast, scientific correlations, as they nomologically exist in nature, are a different matter. To research such correlations serves a different objective and disagrees with taxonomy's goal of forming a stable classification (see Section “The constant change in evolution and the quest of taxonomy for fixed classes: can these be compatible?” in Chapter 2). There is a distinct difference between a definition that serves pragmatic intentions and the reality of organismic diversity, which fits only imperfectly into all recent definitions.

George Gaylord Simpson had already expressed this dilemma half a century ago: “Taxonomy is a science, but its application to classification involves a great deal of human contrivance and ingenuity, in short, of art. In this art there is leeway for personal taste, even foibles, but there are also canons that help to make some classifications better, more meaningful, more useful than others. . . .” (Simpson, 1961).