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    Evolutionary Theory

An important aspect of the record of evolution, implicit in much that has already been said, is the rate at which it has occurred. On examination, this subject turns out to be unexpectedly complicated. Half the problem in learning about anything is asking the right questions. To ask "How fast has evolution occurred?" seems like a simple and proper approach to the present subject, but this is not the right question. In the first place, it must already be evident that there is no such thing as the rate of evolution. The record has demonstrated that evolution is not some over-all cosmic influence that has been changing all living things in a regular way throughout the periods of the earth's history. Some groups have been changed rapidly while others were remaining practically unchanged. The same group is commonly seen to have changed rapidly at some times in its history and slowly or not at all in others. Within a given group some parts may change while others are static. So the question "How fast has evolution occurred?" is meaningless unless we add, "The evolution of what group of organisms, of which of their structures, and at what time in their history?" These variations in rate are in fact important in themselves and should teach us something about the meaning of evolution.

A constant stumbling block in the way of attempts to understand evolution has been that its processes must not only explain adaptation but also absence of adaptation, the existence and persistence of apparently random as well as of clearly oriented features in evolution. This was, and remains, an unanswered argument against theories demanding the reality of purpose or the existence of a goal in evolution. It equally renders untenable all the other theories that attempt to explain evolution by the dominant or exclusive action of one single principle or another, such as the Neo-Darwinian insistence on natural selection as essentially the whole story. Modern understanding of evolution is not as simple as were these various theories, but their simplicity was factitious. They were bound to be wrong in seeking a simple explanation for something that is, in its nature and its phenomena, so far from simple.

Nonadaptive and random changes have another possible role in evolution that is important and that has so far been suggested only in passing. They have a bearing on changes in broad types of organization, the appearance of new phyla, classes, or other major groups in the course of the history of life. The process by which such radical events occur in evolution was the subject of one of the most serious disputes among qualified professional students of evolution. The question is whether such major events take place instantaneously, by some process essentially unlike those involved in lesser or more gradual evolutionary change, or whether all of evolution, including these major changes, is explained by he same principles and processes throughout, their results being greater or less according to the time involved, the relative intensity of selection, and other material variables in any given situation.

It is thus likely, to say the least, that major as well as minor changes in evolution have occurred gradually and that the same forces are at work in each case. Nevertheless there is a difference and many of the major changes cannot be considered as simply caused by longer continuation of the more usual sorts of minor changes. For one thing, there is excellent evidence that evolution involving major changes often occurs with unusual rapidity, although we have seen, there is no good evidence that it ever occurs instantaneously. The rate of evolution of the insectivore forelimb into the batwing, to give just one striking example, must have been many times more rapid than any evolution of the bat wing after it had arisen. The whole record attest that the origin of a distinctly new adaptive type normally occurs at a much higher rate than subsequent progressive adaptation and diversification within that type. The rapidity of such shifts from one adaptive level or equilibrium to another has suggested the name 'quantum evolution,' under which I have elsewhere discussed this phenomenon at greater length.

Evolutionary change is so nearly the universal rule that a state of motion is, figuratively, normal in evolving populations. The state of rest, as in bradytely, is the exception and it seems that some restraint or force must be required to maintain it.


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