Evolution in Relation to Taxonomy

CHAPTER I EVOLUTION IN RELATION TO TAXONOMY Several bases of classification have come into use, each designed to serve a special purpose. Some of these grew out of the eco- nomic uses of plants; others were founded on gross structural resemblances such as habit of growth trees, shrubs, vines, and herbs. All these systems were fragmentary and incomplete, and those plants that did not fit the classification or had a different interest were ignored. For example, plants not regarded as medicinal received little consideration by the early herbalists. At the present time many wild plants that are not known to be either beneficial or harmful to agriculture are ignored even by those whose scientific training in agriculture has included considerable botany. Only one system of classification has made any pretense to completeness, and that is the one now in use which is based on natural relationships. Before the conception that existing species originated by evolu- tion had been proposed, " natural " relationships among plants and among animals were described. Certain families such as the Umbelliferae, Compositae, Gramineae, and Leguminosae were recognized through morphological similarities in flowers, inflo- rescences, leaves, etc., but the expression "natural " relationship did not have the significance that it now has. It meant merely similarity of parts. That the early conceptions of relationship among living things were based largely on comparative morphol- ogy is fortunate, for this has been found to be one of the best criteria of phylogenetic relationship. IDEALS IN CLASSIFICATION There are three conceptions of classification that must be understood in studying systematic botany. 1. Natural classification refers to the relationships that exist among plants as a result of evolutionary development, regardless of man's knowledge of the subject. These natural relationships 8 EVOLUTION IN RELATION TO TAXONOMY 9 actually exist and would exist if there were no human beings on earth to study them. 2. Taxonomic classification is the result of man's efforts to express or describe natural classification and put it into form for discussion and use. Inevitably it ib imperfect, incomplete, and subject to improvement, for it is built on incomplete evidence and even personal opinion concerning natural classification. 3. Artificial classification is a grouping, generally for convenience, that does not pretend to express natural relationships. Often it is the result of using a single character as a basis for classi- fication rather than a combination of characters, which is now recognized as the surer method of bringing out actual relationships. Making a single class that would include all thallophytes, or all spermatophytes that lack chlorophyll, but no others would result in an artificial group. Methods of Classification. The ideal classification must embody two qualities. It must show actual phylogenetic relationships, and it must be reasonably convenient for practical use. If it fails to show true relationship, it is artificial and does not satisfy the discriminating thinker, although it may be convenient for use. Too often, however, as in the bacteria and some groups of fungi and algae, an artificial grouping has had to suffice until a taxonomic classification could be perfected. If, on the other hand, a natural system is too involved and too difficult of comprehension, and especially if, in addition to these faults, the phylogenetic evidences are incomplete and debatable, such a system fails to gain general acceptance and may have to give way, at least temporarily, to one that is more artificial. Many bota- nists believe that different classes of fungi originated from as many groups of algae not closely related to each other and that a natural phylogenetic classification should show a closer relationship between certain fungi and certain algae than between the different groups of fungi. Nevertheless, they continue in actual practice to treat the fungi as if they were a homogeneous unit. One of the advantages of a phylogenetic classification over an artificial one is that it is safer to generalize in related groups than in unrelated ones. For example, if one is well informed concerning a few species of Pinaceae, Gramineae, Leguminosae, or Cucurbitaceae, he can assume the likelihood that some of his knowledge will apply to other members of the same family, 10 A TEXTBOOK OF SYSTEMATIC BOTANY although some verification will be necessary and differences in detail must be expected. On the other hand, if all plants with compound leaves were considered to be a family, this would give us an artificial group and no generalization would be safe except one regarding the leaves. Fortunately, in most of the spermatophytes phylogeny and convenience can be fairly well combined. Evolution as a Basis for Classification. The relationships of species or other groups are determined by the genetic lines running back from them to a common ancestor. Other things being equal, the shorter the genetic lines (in time) and the nearer the two species being considered are to the point where the two merge into a common ancestral line, the closer the relationship. However, if the two species are much alike, indicating that the two genetic lines have run nearly parallel, we call this condition parallel development, or parallel evolution, and these species are commonly thought of as being more closely related than others that show greater differences and consequent divergence of phylogenetic lines that may have been much shorter. In considering evolution as a basis for classification, a student in any branch of biology has to make a decision among three alternatives: (1) to reject the hypothesis of evolution and with it the idea of true natural relationships, (2) to accept this hypothesis blindly on faith in the judgment of his instructor, or (&) to look into the evidence bearing on the subject. The last named is the scientific method. EVIDENCES OF EVOLUTION At the outset the meaning of organic evolution should be made clear. Evolution is believed to have produced all existing forms of plant and animal life from more ancient forms that were fewer in number and simpler in structure than those of the present day. It is not held that one group as it exists today came from another group as it exists today, but that similar groups had a common ancestor more or less like both. A great deal has been written in recent years on the evidences of evolution and also on the methods of its operation, concerning which some half-dozen plausible theories have been proposed. Space limits us here, however, to a brief outline of the best accepted evidence on which the belief in evolution is based. EVOLUTION IN RELATION TO TAXONOMY 11 Geological Evidence. It is well known that the surface of the earth is not smooth and unchanging. In the past slow but profound changes have taken place. Land has been lifted out of the sea, and mountains have been lowered by erosion the materials thus removed being finally deposited in the ocean as layers. These layers have later been lifted to form new mountains, and the process has been and is being repeated again and again. Some of the strata of earth and rock that have thus been formed contain many fossils, the remains or prints of prehistoric plants and animals. The forms of life represented by these fossils give an important historical record of the kinds of living things that have existed on the earth during the past ages. If all kinds of plants and animals, higher and lower, had been created and established at the same time and place, we should expect the fossilbearing layers, both older and newer, to contain representatives of all of these, but such is not the case. The older strata, those formed before the Cambrian era (see frontispiece and table on pages 12-13), contain limestone, graphite, and fossils of primitive forms of life only, especially marine algae and invertebrate animals. Strata not quite so old, those of the Paleozoic era, bear fossils of complex invertebrates, lower vertebrates, seaweeds, pteridophytes, and gymnosperms. Not before the Mesozoic era were fossils of mammals and angiosperms produced, while the apes and man left no fossils below the younger layers of the Cenozoic era. It is generally accepted as a fact that the time from the Archeozoic era to the Cenozoic era covered hundreds of millions of years and that new species of plant and animal life kept appearing on the earth throughout that extensive span. This leaves us with two alternatives: (1) that direct creation was repeated many times throughout all that vast period of time, up to the present, thus producing the hundreds of thousands of species now in existence and others that have become extinct, or (2) that the species that first came into existence were relatively few and simple and that they gave rise to newer and more complex ones by evolutionary processes. The latter seems the more plausible and is widely accepted. Morphological and Anatomical Evidence. The members of different groups of animals and plants show striking similarities in fundamental structure but vary in lesser details. The spinal 12 A TEXTBOOK OF SYSTEMATIC BOTANY EVOLUTION IN RELATION TO TAXONOMY 13 OO to 14 A TEXTBOOK OF SYSTEMATIC BOTANY column in all vertebrates and the same number of limbs in mammals, birds, and reptiles suggest relationship through a com- mon ancestor. In all the vertebrates, except the most primitive, there are four limbs, sometimes in a rudimentary, sometimes in a highly specialized, condition, and the skeleton of the arm and forefoot, for example, compares almost bone for bone with that in such superficially different structures as the walking organs of the bear, the swimming flippers of the whale, the wings of the bat, and the grasping organs of man. In the spermatophytes every part of the sporophyte is some form of root, stem, or leaf; and every plant has all three of these structures, though they may differ greatly in form and function. The chromosome number in these parts is always diploid. Given the general characteristics of the fibrovascular system, one can infer correctly in almost every case the number of cotyledons and the venation of the leaves. The tissues of normal leaves show a general similarity of plan with palisade cells above, in which respect they differ from cladophylls, which are leaf-like stems. The position of buds in the axils of leaves at nodes is likewise uniform. This similarity of ground plan points to a development from ancestral forms by a process of evolution, which would be likely to modify existing structures rather than to create new ones. A Embryological Evidence. special kind of anatomical evi- dence is found in the embryological development of higher ani- mals. The ancients believed that, in general, living things started with an appearance and structure much like miniature adults, growth and development being merely processes of enlargement of each individual organ. Scientific observation has revealed that such is not usually the case. The fern, beginning with the germinating spore, first resembles a green alga. From this develops a small thallus like those of the liverworts. This prothallium, in turn, is replaced by the leafy, mature form with which all are familiar. The frog in its metamorphosis goes from a one-celled state through the tadpole stage, more like fish than amphibian, and changes to adult form by loss of tail and gills and development of legs and lungs. Insect larvae have no wings, not even rudimentary ones, and resemble worms, which they are often called. But these larvae, EVOLUTION IN RELATION TO TAXONOMY 15 by losing certain leg-like structures, taking on permanent legs and wings, and developing a definite segmentation, become in adult life strikingly different creatures. Ontogeny is a term used to express a series of embryonic stages of individuals such as those just described. Phylogeny, on the other hand, expresses the evolutionary history of a race of beings from remote ancestors. HaeckeFs "law of recapitulation/' trans- lated into English, states that " Ontogeny is a brief repetition of phylogeny." This law has been highly useful in tracing relation- ships in animals and plants. The early stages of the advanced form and that of the simple form, presumably ancestral, are much alike, but the higher form continues its development farther and thus produces a complex individual. Vestigial Structures. In a close examination of the higher plants and animals one finds many structures wholly useless and sometimes actually detrimental to the individual. Of what use to plants are scale-like leaves on rhizomes and tubers, stamens without anthers, and the antheridial cells of germinating pollen Why grains? should the legs of the horse include so many little bones, some of which by pressure against others become inflamed and cause splints and spavins? Why the easily infected vermi- form appendix, the coccyx bones, and the scattered body hairs of the human being? Why so many useless structures in higher forms of life and so few in lower forms? As a result of direct creation such structures would seem absurd. Evolutionary development, however, would almost inevitably be accompanied by such vestiges of once useful organs. Great and varied environmental changes have taken place during the eras since life appeared on the earth. The changed conditions profoundly affected the existing plants and animals. Some species unable to adjust themselves were exterminated, and but for their fossil remains we should never know that they had existed. Other species became adaptedsto the new conditions by structural modification. Some organs, such as bones, for example, were lengthened, shortened, o\otherwise modified, and in some cases were entirely lost. Useless structures became encumbrances, and the species best fitted to survive were those in which these superfluous parts were rapidly reduced and made unobtrusive or entirely lost. It sometimes happened, however, that another change of environment reestablished a need for the 16 A TEXTBOOK OF SYSTEMATIC BOTANY organ in its original or equivalent form. If it was too far gone or too greatly modified, the victim perished, as doubtless happened in numerous instances, for by the law of irreversible evolution (see page 28) animals and plants do not ordinarily go back to earlier states. Had the bodies of living things been so unstable that organs would quickly and entirely disappear with a brief change in the climate or the food supply, organic adaptation would have been oversensitive and caused destruction rather than protection. That myriad forms of life did survive profound environmental B C D E FIG. 3. Vestigial structures in plants. A, potato tuber with a scale-like leaf at an "eye." B, rhizome of peppermint with scale-like leaves. C, Indianpipe, which is colorless and saprophytic and has scale-like loaves. D, staminode vestige of stamen in beard-tongue, in contrast with normal stamen. E and F, lodicule vestige of petal of wheat. (F, after Robbins.) changes is evidence t>f the tenacity with which unused organs persisted. Modification rather than destruction of organs appears to have been the fortunate rule. Furthermore, by this hypothesis vestigial structures would be more frequent in the versatile, highly complex spermatophytes and vertebrates and more rare in the simple, conservative, lower forms. Thus it can be seen that by a theory of evolution these vestigial or rudimentary structures can be explained naturally, while by a theory of direct creation they are inexplicable and inharmonious. Recent Productions. It must not be supposed that evolution has come to a standstill, that it began when the world was young, EVOLUTION IN RELATION TO TAXONOMY 17 continued for a few million years, and then ceased to operate, leaving us with a great array of unchanging species. There can be no doubt that it has been continuous from the start, is still going on, and will continue into the indefinite future. That new species of animals and plants are forming all the time is a wellaccepted fact supported by observation and experiment. Variation from the parental characters appearing in offspring, followed by the survival of the fittest (a Darwinian principle), still goes on in nature and can be hastened experimentally by the geneticist. Of the offspring obtained by crossing related subspecies and species, many fail to reproduce or are poorly constituted to meet the conditions of their environment and soon disappear; some are sterile, and others, especially among plants, are fertile, surviving and multiplying; and their new combinations of inherited char- acteristics, if firmly established, may be sufficient to constitute them new species. The mutation theory of de Vries assumes that most evolutionary changes take place in this way. These processes go on among wild plants, and some of the resulting products are collected and given specific or varietal names. By the same processes other products are made by plant breeders. Varieties of corn, fruits, and vegetables, wonderful flowers, breeds of dogs, pigeons, and goldfish all these and many more existing forms of life we know have actually come from ancestry that was very different. That we now call these differ- ent cultivated and domestic forms varieties rather than species is of little significance, for there is no doubt that many of them, if found wild, would unhesitatingly be given different specific names. The fact must not be overlooked that these recent productions have been made in a relatively brief time. Compared with the duration of life on the earth, the existence of civilized man has been as a day is to a century. One could not expect, therefore, that a considerable number of new species would become estab- lished in so brief a span. ACCEPTANCE OF THE DOCTRINE OF EVOLUTION The popular conception that Charles Darwin was the first to suggest the origin of species through a modification of previously existing species is an error. During the first half of the 18 A TEXTBOOK OF SYSTEMATIC BOTANY nineteenth century at least a score of 1 writers, including both scientific men and theologians, gave some expression to this idea, mostly in the form of brief comments incidental to a discussion of some other subject. The tendency of these early writers was to express their belief, sometimes supported by evidence, that cer- tain species had originated by successive changes in ancestral lines without claiming that such a method of origin was universal or even general. A few even went so far as to include man among the products of evolution, and it seems surprising that they remained unchallenged. FIG. 4. The Darwin-Wallace medal given by the Linnaean Society in recognition of the work of these two great pioneers in the study of organic evolution. (Courtesy of Popular Science Monthly.) In 1858, Alfred Russel Wallace, a young English naturalist, sent to his older and better known countryman, Charles Darwin, a hastily prepared manuscript setting forth his views on the ori- gin of species through variation and natural selection, closely similar to the unpublished conclusions of Darwin but reached independently. Upon the insistence of his friends Darwin pre- pared a summary of his own work, which was 2 published jointly with the paper submitted by Wallace. This was followed a year later by Darwin's book in two volumes on the " Origin of Species/' which was the product of some 20 years of study. It is a matter of history that the theory of evolution, when first set forth by \ Most of these are mentioned in the Historical Sketch of Darwin's "Origin of Species." 2 DARWIN, C. f and A. WALLACE, On the tendency of species to form varie- ties; and on the perpetuation of varieties and species by natural means of selection, Jour. Linn. Soc. London, 3 : 53, 1858. EVOLUTION IN RELATION TO TAXONOMY 19 Darwin and his associates, was seriously challenged because it seemed so revolutionary. At the present time, however, it is given the same recognition as that accorded to other principles in all fields of science. Its adoption has revolutionized the classi- fication of plants and animals, and what is most needed now is more information as to the methods of its operation and the course it has followed, in order that we may complete our knowl- edge of phylogenetic taxonomy. THE MECHANISM OF EVOLUTION The fact that organic evolution takes place is generally accepted, but the explanation as to what causes it to go on has been difficult to ascertain. The endeavor to do so has involved numerous researches in genetics, cytology, and ecology, and yet our knowledge on the subject is far from complete. While it is generally believed that the rate at which evolution operates has varied appreciably during different periods of geologic time, it is also thought that in terms of a human lifetime it has been very slow. In the evolutionary process many patterns and materials have been used, most of which have been discarded. The present life on the earth is the product of this long process of trying, sorting, selecting, and discarding. The experimental biologist interested in this subject is able to study only the relatively elemental steps in the process, such as those that take place within the species. On rarest occasions he has been able to reproduce artificially an existent natural species, but the genetic relationships of genera, families, and higher groups at present are closed to experimental investigation. Their evolution has gone so far that it is doubtless now impossible to repeat the steps with the living forms that remain on the earth. Variation and Natural Selection. Darwin recognized two great principles of evolution, vaiiation of the offspring from the parental characters and survival of the fittest by natuial selection, but he did not fully understand the explanation of either. Vari- ation is the active agency that develops new forms upon which evolution may build; natural selection is ever operative upon this variation, eliminating many forms and perpetuating, in general, those best fitted to survive in that environment in which they find themselves. 20 A TEXTBOOK OF SYSTEMATIC BOTANY The Kinds of Natural Variation. Variation in plants is of two sorts, heritable and nonheritable. Failure to differentiate clearly between these has been a source of some confusion in taxonomy. The heritable variations, which are transmitted from parent to offspring, are the only kind that have significance in evolution. The nonheritable variations are acquired during the life of the individual as direct effects of the environment. These are temporary and reversible and consequently of no significance to either evolution 01 taxonomy, but it is not always possible to distinguish them from heritable variations without performing an experiment. Undoubtedly, new specific names have been given to some of these, and they can be found in herbaria so named. Variation is common to all plants whether they reproduce asex- ually or sexually. It is to be expected, of course, that there is relatively much less variation among those forms that depend upon vegetative reproduction, whether it be by simple fission, as in the lowest plants, or by aerial bulblets, layering, or development of seed without fertilization (apomixis), as in the higher plants. Self-pollinated forms are likewise less variable than those that must be cross-pollinated. The genetic variations among plants of a given kind in a single population are often trivial in appearance and in themselves of no evolutionary or taxonomic significance. When plants of all the populations of a common hereditary pattern in an area are considered together, however, and compared with those of a somewhat different pattern found growing in another climatic region, then the evolutionally significant climatic races or subspecies of a single species are disclosed. The hereditary patterns of differ- ent species are ordinarily less similar than those of the climatic races of one species, but the degree of morphological difference alone is not the final mark of a species. The genetic relationship is a sounder criterion of origin than is morphology but more difficult to determine. The species of a genus have a number of characteristics in com- mon that mark them as a group distinct from other groups. The explanation for this lies in their common evolutionary origin. Experiments show that in most genera many species are still closely enough related to be able to hybridize to some extent. In this way they intermix their hereditary materials