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Natural selection is a theory of evolutionary change in organisms that was defined by Charles Darwin in 1859 [Darwin, 1859]. This theory rests on two initial premises:
The corollary of these premises is that progeny with inherited variations that are favoured by their environments will survive and pass these variations on to their progeny in turn. Thus, over time, populations of organisms will fit better into their changing environments.
Stephen Jay Gould [Gould, 1991] argues that two additional constraints are needed to ensure that natural selection is a creative force and not just a removal mechanism. The first is that "variation must be random ... if variation comes prepackaged in the right direction, then selection ... merely eliminates the unlucky individuals who do not vary in the appropriate way" [Gould, 1991, p. 12]. The second is that variation "must be small relative to the extent of evolutionary change in the foundation of new species ... if new species arise all at once, then selection only has to remove former occupants to make way for an improvement that it did not manufacture" [Gould, 1991, p. 12].
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