Research methods in ecology

by Frederic E. Clements

The general application of field experiment to evolution will render the current methods of recognizing species quite useless. It will become imperative to establish an experimental test for forms and species, and to apply this test critically to every “new species.” Descriptive botany, as practiced at present, will fall into disuse, as scientific standards come to prevail, and in its place will appear a real science of taxonomy. In the latter the criteria upon which species are based will be obtained solely by experiment.

184. Fundamental lines of inquiry. There are two primary and sharply defined fields of research in experimental evolution, namely, adaptation in consequence of variation (and mutation), and hybridization. The latter constitutes a particular field of inquiry, which is not intimately connected with the problems of evolution in nature. In the study of specific adaptation, two questions of profound importance appear. One deals with the effects of ancestral fixity or plasticity in determining the amount of modification produced by the habitat. These are fundamental problems, and a solution of them can not be hoped for until exact and trustworthy data have been provided by numerous experimental researches. It thus becomes clear that the principal, if not the sole task of experimental evolution for years to come is the diligent prosecution of accurate and prolonged experiment in the modification of plant forms. It seems inevitable that this will be carried on along the lines that have already been indicated. Plants will be grown in habitats of measured value, or in different intensities of the same factor. The relation between stimulus and adjustment will form the basis of careful quantitative study, and the final expression of this relation in structural modifications will find an exact record in drawings, photographs, exsiccati, and biometrical measures. The making of an accurate and complete record of the whole course of each experiment of this sort is an obligation that rests upon every investigator. Studies in experimental evolution will prove time-consuming beyond all other lines of botanical research, and the work of one generation should appear in a record so perfect that it can be used without doubt or hesitation as a basis for the studies of the succeeding generation.

185. Ancestral form and structure. The significance of the fact that some species have been found to remain unaltered structurally under changes of habitats that produced striking modifications in others has already been commented upon. It is hardly necessary to indicate the important bearing which this has upon evolution. The very ability of a plant to undergo modification, and hence to give rise to new forms, depends upon the degree of fixity of the characters which it has inherited. Stable plants are less susceptible of evolution than plastic ones. The latter adapt themselves to new habitats with ease, and in each produce a new form, which may serve as the starting point of a phylum. There is at present no clue whatever as to what calls forth this essential difference in behavior. This is not surprising in view of the fact that there have been no comparative experimental studies of stable and plastic species. Until these have been made, it is impossible to do more than to formulate a working hypothesis as to the effect of stability, and an explanation of the forces which cause or control it is altogether out of the question.

186. Variation and mutation. New forms of plants are known to arise by three methods, viz., variation, mutation, adaptation. The evidence in support of these is almost wholly observational, and consequently more or less inexact, but for each there exist a few accurate experiments which are conclusive. Origin by variation and subsequent selection is the essence of the Darwinian theory of the origin of species. According to this the appearance of a new form is due to the accumulation, and selection, through a long period, of minute differences which prove advantageous to the plant in its competition with others in nature, or are desirable under cultivation. Slight variations appear indiscriminately in every species. Their cause is not known, but since they are found even in the most uniform habitats, it is impossible to find any direct connection between them and the physical factors. In the case of origin by mutation, the new form appears suddenly, with definite characteristics fully developed. Selection, in the usual sense of the term, does not enter into mutation at all, though the persistence of the new form is still to be determined by competition. Mutations are known at present for only a few species, and their actual appearance has been studied in a very few cases. Like variations, they are indiscriminate in character. The chief difference between them is apparently one of degree. Indeed, mutation lends itself readily to the hypothesis that it is simply the sudden appearance of latent variations which have accumulated within the plant. De Vries regards constancy as an essential feature of mutation, but the evidence from the mutants of Onagra is not convincing. Indeed, while there can be no question of the occurrence of mutation in plants, a fact known for many years, the facts so far brought forward in support of the “mutation theory” fall far short of proving “the lack of significance of individual variability, and the high value of mutability for the origin of species.”[^[19]] Mutations do not show any direct connection with the habitat, but their sudden appearance suggests that they may be latent or delayed responses to the ordinary stimuli. Origin by adaptation is the immediate consequence of the stimuli exerted by the physical factors of a habitat. This fact distinguishes it from origin by variation, or by mutation. The new form may appear suddenly, often in a single generation, or gradually, but in either case it is the result of adaptation that is necessarily advantageous, because it is the result of adjustment to controlling physical factors. Origin by adaptation is perhaps only a special kind of origin by variation, but this might be said with equal truth of mutation. New forms resulting from adaptation are like those produced from mutation, in that they appear suddenly as a rule and without the agency of selection. They are essentially different, inasmuch as their cause may be found at once in the habitat, and since a reversal of stimuli produces, in many cases at least, a reversion in form and structure to the ancestral type.

A valid distinction between forms or species upon the basis of constancy is impracticable at the present time. It is doubtful that such a distinction can ever be made in anything like an absolute sense, since all degrees of fluctuation may be observed between constancy and inconstancy. In all events, it is gratuitous to make constancy the essential criterion in the present state of our knowledge. So little is certainly known of it that it is equally unscientific to affirm or to deny its value, and even a tentative statement can not be ventured until a vast amount of evidence has been obtained from experiment. Accordingly, there is absolutely no warrant, other than tradition, for limiting the term species to a constant group. In the evolutionary sense, a species is the aggregate ancestral group and the new forms which have sprung from it by variation, mutation, or adaptation. It should not be regarded as an isolated unit for purposes of descriptive botany; indeed, its use in this connection is purely secondary. It is properly the unit to be used in indicating the primary relationships which are the result of evolution.

On the basis of their actual behavior in the production of new forms, species may be distinguished as variable, mutable, or adaptable. The new form which results from variation is a variant; the product of mutation is a mutant, and that of adaptation, an ecad. The following examples serve to illustrate these distinctions. Machaeranthera canescens, judging from the numerous minute intergrades between its many forms, is a variable species, i. e., one in which forms are arising by the gradual selection of small variations. It apparently comprises a large number of variants, M. canescens aspera, superba, ramosa, viscosa, etc. Onagra lamarckiana is a mutable species: it comprises many mutants, e. g., Onagra lamarckiana gigas, O. l. nanella, O. l. lata, etc. Galium boreale is an adaptable species: it possesses one distinct ecad, Galium boreale hylocolum, which is the shade form of the species.

187. Methods. The best of all experiments in evolution are those that are constantly being made in nature. Such experiments are readily discovered and studied in the case of origin by adaptation; variants present much greater difficulties, while mutants are very rare under natural conditions. The method which makes use of these experiments may be termed the method of natural experiment. The number of ecads which appear naturally in vegetation is limited, however, and it is consequently very desirable to produce them artificially, by the method of habitat culture. This method, while involving more labor than the preceding, yields results that are equally conclusive, and permits the study of practically every species. The method of control culture, which is carried on in the planthouse, naturally does not possess the fundamental value of the field methods. It is an invaluable aid to the latter, however, since it permits the physical factors to be readily modified and controlled. All these methods are based on the indispensable use of instruments for the measurement of physical factors.

METHOD OF NATURAL EXPERIMENT

188. Selection of species. Species that are producing variants or ecads are found everywhere in nature; those which give rise to mutants seem, however, to be extremely rare. Consequently, mutants can not be counted upon for experimental work, and their study scarcely needs to be considered. When a mutant is discovered by some fortunate chance, the mutable species from which it has sprung, and related species as well, should be subjected to the most critical surveillance, in the hope that new mutants will occur or the original one reappear. On account of the suddenness with which they appear, mutants do not lend themselves readily to natural experiment, and after they have once been discovered, inquiry into the causes and course of mutation is practicable only by means of habitat and control cultures. Among variable species, those are most promising that show a wide range of variation and are found in abundance over extensive areas. A species which occurs in widely separated, or more or less isolated areas, furnishes especially favorable material for investigation, since distance or physical barriers partly eliminate the leveling due to constant cross-fertilization. The individuals or groups which show appreciable departure from the type are marked and observed critically from year to year. The direction of the variation and the rapidity with which small changes are accumulated can best be determined by biometrical methods. Representative individuals of the species and each of its variants should likewise be selected from year to year. After being photographed, these are preserved as exsiccati, and with the photographs constitute a complete graphic record of the course of variation. When the latter is made evident in structural feature also, histological slides are an invaluable part of the record.

Polydemic species are by far the best and most frequent of all natural experiments. In addition to plants that are strictly polydemic, i. e., grow in two or more distinct habitats, there are a large number which occur in physically different parts of the same habitat. The recognition of polydemics is the simplest of tasks. As a rule, it requires merely a careful examination of contiguous formations in order to ascertain the species common to two or more of them. The latter are naturally most abundant along the ecotones between the habitats, and, as a result, transition areas and mixed formations are almost inexhaustible sources of ecads. Many adaptable species are found throughout several formations, however, and such are experiments of the greatest possible value. Not infrequently species of the manuals are seen to be ecads, in spite of their systematic treatment, and to constitute natural experiments that can be readily followed. Finally, it must be kept in mind that some polydemics are stable, and do not give rise to ecads by structural adaptation. They not only constitute extremely interesting experiments in themselves, but they should also be very carefully followed year by year, since it seems probable that the responses are merely latent, and that they will appear suddenly in the form of mutants. In natural experiments it is sometimes difficult to distinguish which form is the ecad and which the original form of the species. As a rule, however, this point can be determined by the relative abundance and the distribution, but in cases of serious doubt, it is necessary to appeal to experimental cultures.