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Quotations
Evolutionary Theory
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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.
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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.
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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.
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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.
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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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