Charles Darwin's landmark book The Origin of Species, which presented to scientists and the lay public alike overwhelming evidence for the theory of natural selection, ironically never explains where new species come from.

      Species are names given to extremely similar organisms, whether animals, plants, fungi, or microorganisms. Because we need to identify poisons, predators, shelter materials, fuel, food, and other necessities, we have long bestowed names on living and once-living objects. Species names of organisms you probably know are listed in Table 1.1. Until the Renaissance, however, names of live beings varied from place to place and were seldom precisely denned. The confusion of local names and inconsistent descriptions led the Swedish naturalist Carolus von Linne (1709-1789) to bring rigor and international comprehensibility to the descriptions. Since Linnaeus (his Latin name) imposed order on some 10,000 species of live beings, scientists use a first name (the genus—the larger, more inclusive group) and a second name (the species—the smaller, less inclusive group) to refer to either live or fossil organisms.

      Most Linnaean names are Latin or Greek. By todays rules the species and genus names are introduced into the scientific literature with a "diagnosis," which is a brief description of salient properties of the organism: its size, shape, and other aspects of its body (its morphology); its habitat and way of life; and what it has in common with other members of its genus. The diagnosis appears in a published scientific paper that describes the organism to science for the first time. The paper also includes details beyond the diagnosis, called the "description." To be a valid name not only must the names, diagnosis, and description be published but a sample of the body of the organism itself must be deposited in a natural history museum, culture collection, herbarium, or other acknowledged repository of biological specimens.

      Fossils are dead remains, evidence of former life. The word comes from fosse in French, something dug up from the ground. Fossil species, like the enigmatic trilobite Paradoxides paradoxissimus, also are given names and grouped on the basis of morphological similarities and differences.

      The word "species" comes from the Latin word speculare, to see—like spectacles or special. Everyone, knowingly or not, uses the morphological concept of species—dogs look like dogs, they are dogs, they are all classified as Canus familiaris. The problems come when we try to name coyotes (Canus latrans), wolves (Canus lupus, gray wolf, or Canus rufus, red wolf), and other closely related animals.

      Zoologists, those who professionally study animals, have imposed a distinct concept of species, which they call the "biological species concept." Coyotes and dogs in nature do not mate to produce fully fertile offspring. They are "reproductively isolated." The zoological definition of species refers to organisms that can hybridize—that can mate and produce fertile offspring. Thus organisms that interbreed (like people, or like bulls and cows) belong to the. same species. Botanists, who study plants, also find this definition useful.

      A third concept of species is in vogue today: the "phylogenetic," "evolutionary," or "cladistic" species idea. Groups of organisms, again like people or corn plants or chickens, considered to be all descended from the same ancestors ("clade") are classified as members of the same species. Such organisms are called "monophyletic" because they are descended from "a single common ancestor."

      We all have a strong sense of species—our ancestors needed it to recognize food, potential mates, thatch grass, poisonous snakes, and many other organisms in order to survive. Two-year-old children delight in recognizing domestic animals, birds, and even fish; witness the popularity of stuffed teddy bears and dinosaurs. An instinctive evolutionary cognition of life forms has been crucial to our and other species' survival.

      Evolution, the study of changes of life through time, is largely the tracking of the origins of species. We argue here that of these three concepts of species, the traditional, morphological one is still the best. The morphological species, we will show, is the external manifestation of the symbiogenetic species.

      The assignment of similar animals of compatible genders to the same species if, in their natural habitat, they can mate and produce viable offspring is adequate for mammals and many other animals but it is not general. Here we widen the concept of speciation to include all organisms. Our symbiogenetic definition of species is as follows. We suggest that if organism A belongs to the same species as organism B, then both are composed of the same set of integrated genomes, both qualitatively and quantitatively. All organisms that can be assigned to a unique species are products ofsymbiogenesis. That is, because A and B share the same number of the same different kinds of integrated genomes they are assigned to the same species. Since no bacterium (whether eubacterium or archaebacterium) evolved from symbiotic integration of formerly independent cells, bacteria lack species; the process of speciation began with the earliest eukaryotes (the first protists, or organisms with nuclei). The concept that all bacteria are interfertile (they can transfer their genes from one to another no matter how different are the recombining partners) has been well argued for over thirty years and is newly explained in Prokaryotology, by Sonea and Matthieu, 2000. Ironically the popular evolutionist's view that organisms evolve by the accumulation of random mutation best describes the evolutionary process in bacteria. All of the larger, more familiar organisms originated by symbiont integration that led to permanent associations. The once-separate symbiotic components become genetically integrated to make new whole individuals, always in populations. As we see from the work on karyotypic fissioning in Chapter 12, the ancient microbial symbionts became so stripped-down in their capabilities and morphology that their true nature can be revealed only by fervent sleuthing. Whatever the origin of the evolutionary variation under study, it is natural selection that relentlessly eliminates the beings whose form, physiology, behavior, and chemistry are not suited for that given environment at that given time and place (whatever the details). Bacterial cells have single genomes that acquired their sets of genes, usually a thousand or more, from compatible prokaryotes one or two at a time. Eukaryotes acquire and integrate entire complete genomes to form "individuals." For example, all plant cells have at least four integrated genomes: 1) the motile eubacterium and 2) the protein-synthesizing archaebacterium that formed the nucleated cell (the first protist) followed by 3) the oxygen-respiring proteobacterium that became the mitochondrion and finally 4) the cyanobacterium that became the chloroplast.

      The biologists' "interbreeding" requirement (the "biological species definition") is extremely useful but mostly for land-dwelling mammals, other closely related forms like birds and snakes, and many plants. The requirement for potential mating (hybridization) is probably related to the way the animals have evolved by karyotypic fissioning, the subject of Chapter 12. But it does not apply to at least four-fifths of all life. The biological concept of species should be renamed the "zoological-botanical concept of species." In zoology the concept is, indeed, indispensable, and in botany it is useful.

      However, the "phylogenetic" or "evolutionary" or "cladistic" concept of species is entirely wrongheaded, and its adoption interferes with understanding how species arise. The long-term symbiosis that led to species origin by symbiogenesis requires integration of at least two differently named organisms. No visible organism or group of organisms is descended "from a single common ancestor."

      The purpose of this book is to explain, with abundant evidence collected by scientists around the world, this new concept of how new species really come into being.

Reference
Margulis, L., and D. Sagan. 2002. Acquiring Genomes: A Theory of the Origins of Species. 240 pp. Basic Books, NY.