Popular Science Monthly/Volume 77/July 1910/Paleontologic Evidences of Adaptive Radiation




THE law of adaptive radiation[1] is an application of paleontology J-of the idea of divergent evolution as conceived and developed successively in the studies of Lamarck, Darwin, Huxley and Cope. It is more than divergence because it implies evolution in every direction from a central form. The idea of radii, or radiations from a central form greatly assists the imagination, because a distinctive feature of paleontology is that we are constantly dealing with fragments of history. The radiations which have been discovered must be supplemented by those which remain to be discovered, and it is very remarkable how in group after group of animals these missing "radii" have turned up.

Radiation actually begins in certain single organs, and the first principle to be observed, as shown in the accompanying diagram, is

Main Lines of Adaptive Radiation of (a) limbs and feet, (b) teeth among mammals.

that radiation of different parts of the body is not necessarily correlated; that is, that the adaptive divergence of the feet and limbs may take one direction, while that of the teeth and skull may take another direction. Thus great variety in combinations of characters may arise, bringing about the very antithesis of Cuvier's supposed "law of correlation"; for we find that while the end results of adaptation are such that all parts of an animal conspire to make the whole adaptive, there is no fixed correlation either in the form or rate of development of parts, and that it is, therefore, impossible for the paleontologist to predict the anatomy of an unknown animal from one of its parts only, unless the animal happen to belong to a type generally familiar. For example, among the land vertebrates the feet, which are associated with the structure of the limbs and trunk, may take one of many lines of adaptation to different media or habitats, either aquatic, terrestrial, arboreal or aerial; while the teeth, which are associated with the structure of the skull and jaws, also may take one of many lines of adaptation to different kinds of food or modes of feeding, whether herbivorous, insectivorous or carnivorous. Through this independent adaptation of different parts of animals to their specific ends there have arisen among vertebrates almost unlimited numbers of combinations of food and tooth structure.

Alternations of Habitat.—In the long vicissitudes of time and procession of continental changes animals have been subjected to alternations of habitat either through their own migrations or through the "migration of the environment itself," to employ Van den Broeck's epigrammatic description of the profound and sometimes sudden environmental changes which may take place in a single locality. The traces of alternations of anatomical adaptation corresponding with these alternations of habitat are recorded both in paleontology and anatomy. For example, Huxley in 1880 briefly suggested the arboreal origin of all the marsupials, a suggestion which has been confirmed abundantly by the detailed studies of Dollo and Bensley, according to which we may imagine that the marsupials have passed through a series of phases, as follows: (1) a very early "terrestrial or ambulatory" phase, (2) a "primary arboreal" phase as exemplified by the tree phalangers of the present day, (3) a "secondary terrestrial" phase as exemplified by the kangaroos and wallabies, (4) a "secondary arboreal" phase as exemplified by the tree kangaroos.

Each one of these phases has left its anatomical record in the structure of the feet and limbs, although this record is often obscured by adaptation.

Louis Dollo especially has contributed most brilliant discussions of this theory of "alternations of habitat" as applied not only to the interpretation of the anatomy of the marsupials but of many kinds of fishes, and to such reptiles as the herbivorous dinosaurs of the Upper Cretaceous.

This brief consideration of the external features of adaptation leads us to glance at groups of animals. We here observe the influence of geographic distribution; we observe the adaptive radiation of groups both continental and local.

Continental Adaptive Radiation.—Among the Tertiary mammals we can actually trace the giving off of radii in several, sometimes in all, directions for the purpose of taking advantage of every opportunity to secure food, to escape enemies, and to reproduce kind, the three phenomena of the struggle for existence. Among such well-known quadrupeds as the horses, rhinoceroses and titanotheres the modifications involved in these radiations can be clearly traced. Thus the history of the life of continents presents a picture of contemporaneous radiations in different parts of the world. We observe the contemporaneous and largely independent radiations of the hoofed animals in South America, in Africa and in the great continent comprising Europe, Asia and North America.

Through the laws of parallelism and convergence each of these radiations produced a greater or less number of analogous groups.

While originally independent, the animals thus evolved separately as autochthonous types in many cases finally mingled together as migrant or invading types.

We may thus work out gradually the separate contributions of the great land masses of North America, South America, etc., to the mammalian fauna of the world. As a rule the greater the continents the more important and fundamental the orders or larger groups of mammals which have radiated in them; the lesser land masses and continental islands, like Australia, have been less favorable to wide adaptive radiation. One of the most interesting features of adaptive radiation is that it may also occur locally.

Local Adaptive Radiation.—On a smaller scale are the local adaptive radiations which occur through segregation of habit and local isolation in the same general geographic region wherever physiographic and climatic differences are sufficiently great to produce local differences in food supply or other local factors of change. This principle is well known among living animals, and it is now being demonstrated among many of the Tertiary mammals, remains of four or five distinct genetic series having been discovered in the same geologic deposits.

The existence of multiple phyla of related animals, as of the rhinoceroses, horses and titanotheres in the same localities is due partly to the operation of the law of local adaptive radiation.

This is conspicuously the case among the titanotheres, for example, the chief evolution of which can be traced in the Rocky Mountain region. In the Eocene we discover four or five independent local phyla; again in the Oligocene we discover five or six independent local phyla. The evolution of these animals appears to have been chiefly American.

In other cases, however, the polyphyletic condition appears to have been through the mingling with local phyla of phyla evolved in other countries. This is illustrated in the case of the Middle Miocene rhinoceroses of America, which are invaded by rhinoceroses of Eurasiatic or European origin.

In studying the herbivorous quadrupeds, therefore, we must keep in the imagination constantly the production of local phyla through local radiation and the intermingling of foreign phyla through migration. There are a few very striking and profound differences between quadrupeds which recur so frequently that where we discover one form we may surely anticipate the discovery of the opposite or antithetic form: in other words, there are extremes of structure shown in the proportions of the skull, of the teeth, of the limbs, and groups of quadrupeds are constantly tending through adaptive radiation to reach these extremes. Some of the contrasting extremes are the following: brachyodonty vs. hypsodonty, dolichocephaly vs. brachycephaly, dolichopody vs. brachypody.

For example, a local adaptive radiation observed in the horses is that the forest-living types are brachyodont, or possess short-crowned teeth, while the desert-living horses are hypsodont, typically grazers, with long-crowned teeth.

Extremes of long-headedness and short-headedness, of long-footedness and of short-footedness, comprise a very large part of the mechanism of adaptive radiation; but we have to do also with long-necked and short-necked types, and with many other chances of proportion which are correlated with different feeding habits.

  1. Osborn, H. F., "The Law of Adaptive Radiation," Amer. Naturalist, Vol. XXXVI., No. 425, pp. 353-363.