Introduction

Many of the contributions to evolutionary thought in the past century can be put in one of two opposed groups. One group emphasizes natural selection as the primary or exclusive creative force. The other minimizes the role of selection in relation to other proposed factors. R. A. Fisher (1930, 1954) showed that many of the proposed alternatives could be discounted with the acceptance of Mendelian genetics and a logical investigation of its relation to selection. Even without Mendelian genetics, Weismann (1904) effectively championed natural selection against some of its rivals of the nineteenth century. His only serious errors are traceable to his ignorance of the Mendelian gene.

The contest was decisively won by natural selection, in my opinion, when by 1932 the classic works of Fisher, Haldane, and Wright had been published. Yet even though this theory may now reign supreme, its realm still supports some opposition, perhaps more than is generally realized. Many recent discussions seem on the surface to conform to the modern Darwinian tradition, but on careful analysis they are found to imply something rather different. I believe that modern opposition, both overt and cryptic, to natural selection, still derives from the same sources that led to the now discredited theories of the nineteenth century. The opposition arises, as Darwin himself observed, not from what reason dictates but from the limits of what the imagination can accept. It is difficult for many people to imagine that an individual's role in evolution is entirely contained in its contribution to vital statistics. It is difficult to imagine that an acceptable moral order could arise from vital statistics, and difficult to dispense with belief in a moral order in living nature. It is difficult to imagine that the blind play of the genes could produce man. Major difficulties also arise from the current absence of rigorous criteria for deciding whether a given character is adaptive, and, if so, to precisely what is it an adaptation. As I will argue at some length, adaptation is often recognized in purely fortuitous effects, and natural selection is invoked to resolve problems that do not exist. If natural selection is shown to be inadequate for the production of a given adaptation, it is a matter of basic importance to decide whether the adaptation is real.

I hope that this book will help to purge biology of what I regard as unnecessary distractions that impede the progress of evolutionary theory and the development of a disciplined science for analyzing adaptation. It opposes certain of the recently advocated qualifications and additions to the theory of natural selection, such as genetic assimilation, group selection, and cumulative progress in adaptive evolution. It advocates a ground rule that should reduce future distractions and at the same time facilitate the recognition of really justified modifications of the theory. The ground rule — or perhaps doctrine would be a better term — is that adaptation is a special and onerous concept that should be used only where it is really necessary. When it must be recognized, it should be attributed to no higher a level of organization than is demanded by the evidence. In explaining adaptation, one should assume the adequacy of the simplest form of natural selection, that of alternative alleles in Mendelian populations, unless the evidence clearly shows that this theory does not suffice.

Evolutionary adaptation is a phenomenon of pervasive importance in biology. Its central position is emphasized in the current theory of the origin of life, which proposes that the chemical evolution of the hydrosphere produced at one stage an "organic soup" of great chemical complexity, but lifeless in its earliest stages. Among the complexities was the formation of molecules or molecular concentrations that were autocatalytic in some manner. This is a common chemical property. Even a water molecule can catalyze its own synthesis. Only rarely would a molecule be formed that would produce chance variations among its "offspring" and have such variations passed on to the next "generation," but once such a system arose, natural selection could operate, adaptations would appear, and the Earth would have a biota.

The acceptance of this account of the origin of life implies an acceptance of the key position of the concept of adaptation and at least an abstract criterion whereby life may be defined and recognized. We are dealing with life when we are forced to invoke natural selection to achieve a complete explanation of an observed system. In this sense the principles of chemistry and physics are not enough. At least the one additional postulate of natural selection and its consequence, adaptation, are needed.

This is a very special principle, uniquely biological, and must not be invoked unnecessarily. If asked to explain the trajectory of a falling apple, given an adequate description of its mechanical properties and its initial position and velocity, we would find the principles of mechanics sufficient for a satisfying explanation. They would be as adequate for the apple as for a rock; the living state of the apple would not make this problem biological. If, however, we were asked how the apple acquired its various properties, and why it has these properties instead of others, we would need the theory of natural selection, at least by implication. Only thus could we explain why the apple has a waterproof wax on the outside, and not elsewhere, or why it contains dormant embryos and not something else. We would find that an impressive list of structural details and processes of the apple can be understood as elements of a design for an efficient role in the propagation of the tree from which it came. We attribute the origin and perfection of this design to a long period of selection for effectiveness in this particular role.

The same story could be told for every normal part or activity of every stage in the life history of every species in the biota of the Earth, past or present. For the same reason that it was once effective in the theological "argument from design," the structure of the vertebrate eye can be used as a dramatic illustration of biological adaptation and the necessity for believing that natural selection for effective vision must have operated throughout the history of the group. In principle, any other organ could be used for illustration although the adaptive design of some parts may not be as immediately convincing as that of the optics of the eye.

This book is based on the assumption that the laws of physical science plus natural selection can furnish a complete explanation for any biological phenomenon, and that these principles can explain adaptation in general and in the abstract and any...