Creation by Evolution/The Evolution of Ants
THE EVOLUTION OF ANTS
By William Morton Wheeler
Professor of Entomology and Dean of the Bussey Institution for Research in Applied Biology, Harvard University
The term “evolution” is used by biologists to cover one of the aspects of “development,” which in turn merges into the universal phenomenon of “change.” Most people regard change as a matter of course, merely noting its occurrence and its various forms and adapting themselves to it, whether it occurs in their own lives, in the lives of other human beings, or in the lives of animals and plants, but to reflective observers, during the past three thousand years, change has always seemed so extraordinary as to constitute the basis of philosophy or the occasion for philosophical speculation. Since the wonderful complexity and diversification of the world is due to change, and since to us the outstanding features in this diversification are human beings and other organisms, it is easy to see why the origin and meaning of change should have been sought and discussed so ardently, and for so many centuries.
There are three groups of facts with which even the most casual observer of the constantly changing organic world is familiar. First, he knows of the development of animals from eggs and of plants from seeds. Second, he sees the possibility and the usefulness of making a rough classification of animals and plants, and he notes that among the various kinds of animals and plants there are in nature certain forms (species), some of which are very similar, though distinct—such as the various kinds of oaks, pines, deer, and ducks—and even the superficial observer knows that these species, though they may be very constant in many of their characteristics, are nevertheless more or less variable in others. And third, everybody knows that many of our breeds of domesticated animals and plants have given rise and are still giving rise under human control to other breeds, some of which show great differences from their ancestors, such as those, for example, seen among our dogs, pigeons, roses, and grapes. The facts of the first and the third group we can observe directly; those of the second group, showing classification, require explanation.
The resemblances and the differences between the kinds of animals and plants might be accounted for in two ways: either these several kinds were created independently, simultaneously or successively, or they were derived by natural descent from common ancestors, in the same manner as the various breeds of domestic animals and plants were derived from their ancestral forms. The first explanation is supernatural and nongenetic; the second is natural and genetic. There is no question as to which of these explanations the scientist and the philosopher must prefer, for, as Joseph McCabe says, “no plea for the supernatural origin of anything is valid so long as there is a possibility of a natural explanation of its origin.”
The changes noted in the three groups of facts discriminated above all come under the head of “development” in its general sense, but those of the first group comprise the development of individual organisms, whereas those of the second and third comprise the development of races. The term “development”, or “ontogeny,” is now commonly used of individual development; the terms “evolution,” or "phylogeny," of racial development. All reputable living biologists accept evolution either as proved or as so thoroughly substantiated as to be practically proved, but they differ as to the precise natural factors or conditions that have brought it about in any particular group of organisms. This point cannot be too strongly emphasized, because the discussions of biologists over the precise nature of the process of evolution are continually being misrepresented by ignorant or dishonest anti-evolutionists as confessions of disbelief in the occurrence of the process itself. There is surely nothing unusual about the discussions of evolutionary causes by biologists. Everybody now believes in what we call gravitation, but physicists, past and present, are by no means unanimous in their views of the precise nature or causes of gravitation. Those who are emotionally upset by the conception of a collateral generic relationship between men and the anthropoid apes may be reminded that although the difference in psychological and social behavior between the animals of these two groups is undoubtedly considerable, such behavior has exhibited great change even during historical time, and that the structural and functional differences between men and the anthropoids are trivial as compared with those which separate a frog's egg from an adult frog; yet this enormous gap is bridged by a continuous process that occurs under our very eyes. Endless confusion in the popular mind might be avoided if we could dissuade all journalists, politicians, teachers, and clerics from talking or writing on evolution till they had made an intensive first-hand study of the embryology of some animal or plant or a thorough investigation of some group of wild or domesticated animals or plants.
Now it is easy to prove genetic continuity within existing species and between the breeds or races of domesticated forms, but it is very difficult to prove genetic relationships between similar species of wild organisms, for the process of racial development or evolution is so exceedingly slow that even some slight structural changes may have required millions of years, or at any rate periods far too long to fall under the observation of a being so ephemeral as man. The proofs of this very long historical evolutionary process are therefore indirect; they derive their value from the convergent and mutually corroborative inferences drawn from studies made in widely different fields of the great science of biology. At least five of these fields furnish significant historical inferences—the study of fossil animals and plants (palaeontology), the comparative study of the development and structure of existing forms (morphology, or anatomy and embryology), the study of the present geographical distribution of plants and animals (chorology), the study of the classication of plants and animals (taxonomy), and the comparative study of the behavior of animals (ethology). Obviously, the study of extinct or fossil species is of the greatest value, but the record of some species is deplorably fragmentary and most of the specimens found are imperfectly preserved. Although, therefore, all positive palaeontological data are precious, the fact that we have not yet found connecting or intermediate forms at particular geological horizons may be of slight significance. Comparative morphology and its shorthand expression, classification, are of enormous value in determining the possible genetic relationships between species, both living and fossil, and the distribution of living species as compared with that of their fossil allies is of great historical significance. Finally, the study of the behavior of existing animals and of the dependence of behavior on the structure and function of particular organs enables us to draw inferences in regard to the actual modes of life of their allied extinct species. After these very general statements we may turn to a study of the ants, which form one among a great many sources of inferences in support of evolution.
As a group, the ants are not so favorable for a study of evolution as their cousins the bees and wasps, because they constitute an unusually compact and homogeneous natural family and one which seems to have completed or nearly completed its evolution at an earlier date in geological time. This difference is indicated by the fact that all the six thousand or more known species, subspecies, and varieties of ants are eminently social, or live in organized colonies, whereas most of the wasps and bees are still solitary insects. There are also other reasons, which will be given later, for believing that the ants arose from a very ancient wasp-like stock and attained their present relatively high specialization a long time ago. We may now review some of the inferences derived from the study of their palaeontology, morphology, distribution, taxonomy, and ethology, which all agree in indicating not only that the ants have been subject to evolution but that this evolution has been of a particular character or pattern.
Many ants have been preserved in a fossil state in formations of Tertiary age, but none have yet been found in earlier formations. A small number of species have been found in Eocene deposits, which were laid down at the beginning of Tertiary time, but a much greater number have been collected from amber (a kind of resin) of Lower Oligocene age, found near the Baltic Sea, and from Miocene shales in Europe and in the United States, at Florissant, near Pike’s Peak, Colo. Several species of ants have been found in Sicilian amber which is also of Miocene age. I have studied no less than 10,000 specimens from the Baltic amber and at least 8,000 from the Florissant shales. Many of those in amber are exquisitely preserved (Fig. 1), having been enclosed in it much as insects are mounted in our laboratories in Canada balsam, so that they may easily be compared with existing ants, though the amber was formed millions of years ago. All this material, as well as that found in other formations and studied by others, shows that though the fossil ants, with a few doubtful exceptions, belong to extinct species, most of them belong to existing genera, and that none of the species is more primitive in structure and habits than many now existing. Indeed, many of them are quite as highly specialized as the most specialized existing forms. We are therefore unable to detect any significant evolution of the ants as a whole during the millions of years of Tertiary time, though many species have undoubtedly become extinct and others have arisen through relatively slight variations during that time and have given rise to the ants now living. We find, preserved in amber, even the larvae and pupae of certain ants, some of the plant lice which they tended, and a few characteristic ant guests (Paussidae) and parasites (mites). All this might seem to indicate that there has been no notable evolution of the group, but only a gradual extinction of species among a very considerable number that were suddenly created and distributed over the globe, but such a conclusion is unwarranted. We are bound to assume, on the contrary, that the significant vespoid or wasp-like forms among which the ants had their origin must have lived before Tertiary time—that is, during the Cretaceous period, or even during earlier Mesozoic time, which, unfortunately, is represented by few fossil insects, even of other groups. The only important conclusion we are at present justified in drawing is that the ants are a very old group of insects, which long ago attained essentially its present stage of evolution and has since been marking time or changing very slowly and imperceptibly. Probably the same was true of the ants of periods antedating the Tertiary, though there may have been in those periods occasional spells of acceleration and efflorescence of new forms.
When we carefully study the anatomy and development of the various species of ants we find that they are essentially wasps, and that they are closely allied to species of certain existing families of wasps, the Tiphiidae, Mutillidae, and Thynnidae. We must, indeed, suppose that the ancestors of these families produced also the ants, the Formicidae. But the members of these families, like most other wasps, are solitary, and, like most animals, possess only a single type of female; whereas among the ants each species presents two female phases, or castes, one of which, the “queen,” is fertile and nearly always winged, and the other, the “worker,” is always wingless and nearly always sterile. In only a few species of ants, and those highly parasitic species, do we find no worker caste. There is every reason to assume that in these species the worker has been lost or suppressed within comparatively recent time. We must therefore conclude that sexual trimorphism—that is, the presence in each species of three castes, male, fertile female, and sterile female, or worker, which were perfectly developed also in the known fossil ants of Tertiary time—was first established among the Mesozoic ancestors of the family Formicidae. A similar trimorphism has arisen independently among the social bees and social wasps, but it has evidently been of much more recent development, for among these insects the worker is much more like the fertile female and always has wings. Then, too, the differentiation of fertile and sterile females among certain tropical wasps is so feeble that the evolution of the two castes may be said to be still uncompleted.
When we arrange all the species of living and fossil ants
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Fig. 1.—Male ant embedded in amber. Although this insect lived ages ago, the details of its structure are wonderfully preserved. |
Fig. 3.—Dinoponera grandis of Brazil. Worker, about natural size. (Photographed by C. T. Brues.) |
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| Fig. 2.—One of the famous bull-dog ants of Australia (Myrmecia tarsata). Of a beautiful deep-blue color. Mandibles yellow and tip of abdomen orange-red. (Photographed by C. T. Brues.) | |
according to their structure we find that they fall into some seven subfamilies and that these may be most naturally regarded as seven large branches that arose from a single main trunk representing the most primitive and most wasp-like forms (Figs. 2 and 3). The existing species correspond to the green twigs and leaves at the tips of the branches of this "Stammbaum," or phylogenetic tree, and the new species that are discovered from time to time may be placed very naturally among their nearest allies according to this arborescent scheme. Now such an arrangement of the six thousand known Formicidae is the only one that will adequately represent the similarities or the relations of the forms, and the attempt to represent the morphological affinities of the species of any other group of organisms invariably produces the same kind of arrangement. This arrangement, moreover, would seem to admit only of a genetic or evolutionary interpretation.
It is, of course, impossible to give here any adequate account of the distribution of ants. With the exception of a few species that have been accidentally transported within recent times by man from one to another country, all ants are confined to rather narrow areas of the earth's surface, and their distribution agrees in general with that of other organisms, suggesting that the genera and species arose at different periods during geological time and then, with more or less modification, radiated to other regions, except as natural obstacles, such as large bodies of water or high mountain chains, may have prevented. The facts that most ants nest in the soil, that they avoid soil that is too constantly wet, that they are fond of warmth, and that they are abundant in certain arid regions suggest that they had their origin as a group on rather high continental areas during Mesozoic time. Many species, however, have since become adapted to life in dry deserts on the one hand and in moist, tropical jungles and rain forests on the other. Many of the ants in these jungles and forests, owing to the seasonal drenching of the soil, build their nests in trees or inhabit the pith cavities of twigs and branches. We also observe that regions of the globe like Australia, which are inhabited by the most primitive mammals and birds (duck-bills, echidnas, marsupials, emus, etc.), are also inhabited by the most primitive ants (bull-dog ants of the genera Promyrmecia and Myrmecia), whereas countries like Europe and North America, which have highly specialized mammalian and bird faunas, are similarly inhabited by highly specialized and dominant ant faunas, with which, however, are intermingled a small number of primitive forms, which were once widely distributed but are now rare and are in process of extinction. Such a distribution can be explained only on the theory of evolution and is in complete agreement with all we know about the geological history and morphology of other organisms.
Conclusions from a comparative study of the habits of ants, or ant behavior, which is necessarily restricted to living forms, agree closely with the conclusions reached in the fields mentioned. Although all ants are social, they exhibit different degrees of social organization. This diversity is shown in different degrees of division of labor in the colonies as coordinated with their size and in differences shown by their component individuals. Thus among the most primitive ants many of the colonies are very small and the fertile females and workers are much alike in size and structure, but in the most highly socialized species (Dorylinae, Formicinae, and Myrmicinae) the colonies may be very large and the workers may be unlike the females and may even exhibit a differentiation of the worker into secondary castes, major and minor workers (Fig. 4), or soldiers and workers proper. Along with this advance in diversity of form, there is a notable change in feeding habits, the primitive forms being purely carnivorous, like their ancient wasp-like ancestors, and the more advanced types having become increasingly vegetarian. The vegetable feeders are best developed in regions where competition for insect food is keenest—that is, in deserts, where insect food is scarce or limited to a short season, and in the tropical rain forests, where the ants must enter into close competition with many other predatory insects and with insectivorous reptiles, birds, and mammals. In the deserts of the world (in southwestern United States, Mexico, Sahara, South Africa, Central Australia) we find that two kinds of ants have become adapted to a vegetarian diet, the harvesting ants, which feed largely or exclusively on the seeds of plants, and the honey-ants, which store in the crops of a special caste of worker a sweet liquid ("honeydew") collected from plant-lice, scale-insects, and oak-galls. In the tropical and subtropical forests of the New World a peculiar tribe of ants (Attini) have acquired the habit of making mushroom gardens in which they grow fungi as food. The garden beds are made of pieces of leaves, which they cut from the trees, or from the collected excrement of caterpillars or other insects that feed on plant tissues (Figs. 5 and 6). Fully a hundred species of these attine ants are known, and some of the larger species are at times very injurious to the agriculturist and the horticulturist, because they use the leaves of cultivated plants (sugar-cane, orange trees, etc.) as material on which to grow their food-fungus. Many of the most highly specialized termites, or "white-ants," in the Old World tropics have independently developed a similar habit of growing fungi. Among these insects, however, the substratum of the fungus gardens consists of triturated wood, which has been passed through the intestines of the workers.
One of the most striking of the evolutionary habits of ants is social parasitism, which leads colonies of different species to live very near or actually with one another. One of these_p220_Mycetosoritis_hartmanni.png)
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Fig. 4.—Part of a colony of a common highly specialized ant (Camponotus americanus) of the eastern United States.
Somewhat enlarged. The winged forms are virgin queens; the wingless forms with large heads are major workers; the wingless forms with small heads are minor workers. (Photograph by J. G. Hubbard and O. S. Strong.)
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Fig. 6.—One of the fungus chambers of the nest of Mycetosoritis hartmani, showing the garden, which is suspended from small rootlets left by the ants when they are excavating the chamber. Enlarged about one-fourth. (Photograph by C. T. Hartman.)
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That the activities of ants in response to particular environments have led to the development of highly specialized habits is shown also by many interesting examples of “convergent” or “parallel” evolution in species that are not closely related. One striking example is furnished by the tropical ants that inhabit silken nests on trees. These nests are really constructed by the young larvae, which their worker nurses use as weaving-shuttles. Throughout the East Indies, Northern AustraHa, India, and equatorial Africa the “tree ants” of the genus Oecophylla (Fig. 7) have attained great proficiency in the art of thus using their larvae for spinning adjacent leaves together (Fig. 8). A similar habit has also been acquired by certain species belonging to two other genera, Polyrhachis, in the tropics of the Old World, and Camponotus, in central and northern South America. The nests made by C. senex and C. formiciformis in the forests of British Guiana, Panama, and Guatemala are extraordinarily like those made by Oecophylla longinoda in the forests of the Congo and by Oecophylla smaragdina in the jungles of India. The structure as well as the behaviour of remotely related species of ants has been similarly modified by convergent or parallel evolution in response to identical environment. A fine example is furnished by certain ants in which the head is cylindrical, constructed like the cork of a bottle, with a hard, roughened, truncated anterior surface, and used for closing the circular orifice of the nest, which leads to galleries excavated in sound wood or in hard soil. Species of at least four different genera in different parts of the world (Camponotus, Pheidole, Crematogaster, and Epopostruma) exhibit this identical form of head. A similar modification of the head is seen in a number of worms, bees, beetles, toads, and tree-frogs; and in certain spiders, beetles, caterpillars, snakes, and armadillos the posterior end of the body is similarly modified for use as a barricade for closing the burrows in which they live and thus preventing the entrance of enemies.
A different modification is seen in certain ants that live in the narrow pith-cavities of the twigs and smaller branches of tropical shrubs and trees. In these insects the whole body becomes very long and slender, or even thread-like, or
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Fig. 8.—Two small nests of weaver-ants (Oecophylla longinoda) of the Congo, made by employing the larvæ to spin the terminal leaflets of a pinnate leaf together with silk. Some ants are seen on the surfaces of the nests. (Photograph by H. Lang.)
filiform. This singular modification, too, is seen in several different genera in the tropics of both hemispheres. Finally, attention may be called to the development of a peculiar beard, consisting of long, forward-sweeping hairs on the lower surface of the head in several unrelated genera of desert ants. The hairs are rather stiff and form a kind of crate or basket, in which the ants carry up the dust or sand that they loosen while they are excavating their burrows. To account for all these exquisitely adapted forms or features there are only two hypotheses: either they have been developed gradually, in response to the environment in which the insects have long been living, or they were created at the same time as their possessors by a being having a prevision of their ultimate function. If the latter hypothesis is accepted we can only marvel at the Creator’s meticulous solicitude for the welfare of ants and His failure to provide adequate prophylactic measures against the many common diseases and calamities that have for thousands of years decimated the paragons of His creation.
Of course, the conclusions we have reached in regard to evolution among ants, though based on many more observations than those briefly cited here, relate nevertheless to a very small part of the animal kingdom. But during the last sixty years essentially the same conclusions have been reached by hundreds of other students, each of whom has investigated some particular group of organisms; and the combined labors of all these workers may be said to cover the whole extent of the plant and animal kingdoms, man included. Are we to suppose that this conclusion, unanimously reached by so many men who have devoted their lives to minute and conscientious observation and experiment, is the result of some marvelous unanimous hallucination, and that the truth lies with those who have given little or no study to the organic world, but have accepted blindly the doctrine of special creation fostered by equally unobservant ancient Hebrews, Babylonians, and mediaeval priests? In this matter, however, mere appeal to authority, either scientific or theologic, is unnecessary. The innumerable unequivocal facts that have convinced all competent biologists of the reality of the evolutionary process are recorded in thousands of volumes, which are open to the perusal of all who cannot find opportunity to make independent observations of their own.
REFERENCES
- Donisthorpe, H. S. J. British Ants, their Life History and Classification. Plymouth, Brendon & Son, 1915. 2nd ed., London, Roudedge & Sons, 1927.
- Escherish, K. Die Ameise, Schilderung ihrer Lebensweise. 2nd ed. Braunschweig, Viehweg, 1927.
- Forel, A. Les Fourmis de la Suisse. 2nd ed. La Chaux-de-Fonds, 1920. Le Monde Social de Fourmis du Globe. 5 vols. Genève, Kundig, 1921–1923 (English translation by C. K. Ogden in press).
- Wheeler, W. M. Ants: their Structure, Development, and Behavior. Columbia University Press, 1910. 2nd impr., 1926. Social Life Among the Insects, New York, Harcourt Brace & Co., 1923. Les Sociétés d’Insectes, Leur Origine, Leur Evolution. Paris, G. Doin, 1926 (English edition in press). Foibles of Insects and Men (in press).
“The Anthropoid apes no doubt approach nearer to man in bodily structure than do any other animals; but when we consider the habits of ants, their social organization, their large communities and elaborate habitations, their roadways, their possession of domestic animals, and even, in some cases, of slaves, it must be admitted that they have a fair claim to rank next to man in the scale of intelligence.”—Sir John Lubbock.