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p.349 There are fashionable problems and there are neglected problems in any field of research. The problem of the emergence of evolutionary novelties has undoubtedly been greatly neglected during the past two or three decades, in spite of its importance in the theory of evolution.
p.349 Is not evolution characterized by the continuous production of complete novelties, such as the lungs of vertebrates, the limbs of tetrapods, the wings of insects and birds, the inner ear of mammals, and literally thousands of structures in all the phyla of animals and plants?
p.350 it was asked, how can an entirely new structure originate without complete reconstruction of the entire type? And how can a new structure be gradually acquired when the incipient structure has no selective advantage until it has reached a considerable size and complexity?
p.351 Tentatively, one might restrict the designation "evolutionary novelty" to any newly acquired structure or property which permits the assumption of a new function. This working definition must remain tentative until it is determined how often it is impossible to decide whether or not a given function is truly "new."
p.352 In the days of classical comparative anatomy, the term "evolutionary novelty" referred unequivocally to a new structure. With the broadening of biology, attention has been directed to evolutionary novelties that are not morphological or at least not primarily morphological. New habits and behavior patterns are very often as important in evolution as are new structures.
p.355 most mutations appear to have only a slight, if not an invisible [JLJ - see Wagner's neutral spaces for a continuation of that line of thought], effect upon the phenotype. More penetrant mutations are usually disruptive and produce disharmonious phenotypes... and will therefore be selected against. The real function of mutation is to replenish the gene pool and to provide material for recombination as a source of individual variability in populations.
p.358-359 Most evolutionary changes take place without the origin of new structures... Most differences are merely shifts in proportions, fusions, losses, secondary duplications, and similar changes which do not materially affect what the morphologist calls the "plan" of the particular type. An intensification of the running function has led to a conversion of the five-toed mammalian foot (or hand) to the two-toed foot of the artiodactyls or the one-toed foot of the perissodactyls. Many glands are the result of intensified function and local concentration of previously scattered secretory cells. The intensification of function in these cases does not lead to the emergence of anything that is basically new, and yet it may result in a reorganization of the phenotype so drastic that the first impression is that of the emergence of an entirely new organ. Of importance to the evolutionist is the fact that no essentially new selection pressure is involved but merely the intensification of a previously existing selection pressure.
p.359 The improvement of a single key component of a structure may result in an "evolutionary avalanche."
p.360 It is often difficult to say to what extent a structure is new or merely an improvement on an old one.
p.360-361 By far the most important principle in the interpretation of the origin of new structures is that of the "change of function." ...Darwin recognized quite clearly that the possibility for a change of function usually depended on two prerequisites. The first of these is that a structure of an organ can simultaneously perform two functions... The other one is the principle of duplication... two distinct organs... may simultaneously perform in the same individual the same function... A change of function is easily explained on the basis of these two premises, either a simultaneous multiple functioning of a single structure or the performance of the same function in different or duplicated organs.
p.364 Both cases have the essential feature in common: that an existing structure is preadapted to assume a new function without interference with the original function. This is preadaptation, as now understood (Bock, 1959). The term "preadaptation" has been applied to diverse concepts.
p.364-365 an organism, as a whole, may be preadapted to undertake a major habitat shift. The aquatic branch of the vertebrates that gave rise to the first partially terrestrial amphibians must not only have had a crawling locomotion, but must also have been partially air-breathing and have had other characteristics of skeleton, epidermis, and sense organs which preadapted them for the habitat shift.
p.368 The active shift of an organism into a novel niche or entirely new adaptive zone will set up a powerful array of new selection pressures. An organism must have a special set of characteristics to cope with the demands of the new environment. It must be "preadapted" for the new world in which it will henceforth live. The change from water-living to land-living is a particularly instructive illustration of this.
p.368 Among all the marine animals, only benthonic [JLJ - The collection of organisms living on or in the bottom of a body of water] ones, because they already lived a somewhat "terrestrial" life underwater, were able to emerge onto land. The peculiar pedunculated fins of the crossopterygian fishes, presumably used in part for moving along rocks and over the bottom, were ideally preadapted for locomotion along land. A similar situation is probable for the arthropods (Manton, 1953).
p.371 To sum up the evolutionary aspects of a shift into a new niche or adaptive zone: such a shift can occur only if the organism is preadapted for it. However, as soon as the shift has been achieved, a whole new set of selection pressures will tend to modify all those structures that are particularly concerned with life in the new environment. The more drastic the change in environment, the more rapid will be the evolutionary change and the more far-reaching, in general, the structural reorganization.
p.371 A shift into a new niche or adaptive zone requires, almost without exception, a change in behavior.
p.372 Many functions are performed, not by simple structures, but by a combination of structures. For an articulation, for instance, a minimum of two bones is needed, as well as the muscles that move these bones and the ligaments which help to bind them. To achieve efficient vision, a highly complex organ is needed, consisting of a receptor and its nervous connections, a lens and other focusing devices, pigments, etc. It is probable that some evolutionary novelties have emerged as the result of a more or less incidental coming-together of such components.
p.372-373 Let us now consider a specific case of the quasi-accidental coming-together of two structures, resulting in a new character complex with a unified function of high selective advantage.
p.374 The three cited cases have in common the essential feature of preexisting building blocks, which, when pieced together, give rise to an "improbable" new character complex of high selective value. The particular organisms are preadapted to acquire the new character complex because they already possess the potentiality for it, that is, the individual building stones. The role of natural selection in these cases is apparently not the bringing-together of the individual units; this is done by forces independent of the prospective new structure. Natural selection enters the scene as soon as the pieces have been combined into a new complex which can function as a unit and can respond to natural selection as a unit.
p.375 There is perhaps no better way to learn how evolutionary novelties emerge than by carefully comparing similar structures that have evolved independently in response to similar selection pressures. The fact that so many independent answers may be found to satisfy a single need proves three points: (a) the ever present pressure of selection, (b) the opportunism of evolution, and (c) the potential variability of any structure. Whichever structure is the first to vary in a desirable direction will be the one on which natural selection can work. That component of the variation of accessory structures will be favored by natural selection which best fits with the modification of the primary structure.
p.375-376 Natural selection comes up with the right answer so often that one is sometimes tempted to forget its failures. Yet the history of the earth is a history of extinction, and every extinction is in part a defeat for natural selection, or at least it has been so interpreted. Natural selection does not always produce the needed improvements.
p.376 Natural selection can operate only when it has a choice between alternate phenotypes.
p.377-378 The tentative answer to our question "What controls the emergence of evolutionary novelties" can be stated as follows: Changes of evolutionary significance are rarely, except on the cellular level, the direct results of mutation pressure. Exceptions are purely ecotypic adaptations, such as cryptic coloration. The emergence of new structures is normally due to the acquisition of a new function by an existing structure. In both cases the resulting "new" structure is merely a modification of a preceding structure. The selection pressure in favor of the structural modification is greatly increased by a shift into a new ecological niche, by the acquisition of a new habit, or by both. A shift in function exposes the fully formed "preadapted" structure to the new selection pressure. This, in most cases, explains how an incipient structure could be favored by natural selection before reaching a size and elaboration where it would be advantageous in a new role. Mutation pressure, as such, plays a negligible role in the emergence of evolutionary novelties, except possibly on the cellular level. Yet the structure of the gene complex is important: too great a genetic and developmental homeostasis will result in too stabilized a phenotype and will tend to prevent a response to new selection pressures. Any population phenomenon that would tend to counteract excessive stability of the phenotype may favor evolutionary changes.
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