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[A]lthough species may be discrete, they have no immutable essence. Variation is the raw material of evolutionary change. It represents the fundamental reality of nature, not an accident about a created norm. Variation is primary; essences are illusory. Species must be defined as ranges of irreducible variation.
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"Of Wasps and WASPs", p. 160

 

Stephen Jay Gould

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But all evolutionary biologists know that variation itself is nature's only irreducible essence. Variation is the hard reality, not a set of imperfect measures for a central tendency. Means and medians are the abstractions.

 

Stephen Jay Gould

 

Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.

 

Charles Darwin

 

Now the compound or composite may be supposed to be naturally capable of being dissolved in like manner as being compounded; but that which is uncompounded, and that only, must be, if anything is, indissoluble. ...And the uncompounded may be assumed to be the same and unchanging, where the compound is always changing and never the same? ...Is that idea or essence, which in the dialectical process we define as essence of true existence--whether essence of equality, beauty, or anything else: are these essences, I say, liable at times to some degree of change? or are they each of them always what they are, having the same simple, self-existent and unchanging forms, and not admitting of variation at all, or in any way, or at any time?

 

Socrates

 

If E1 is the cause of E2, then a small variation (a mark) in E1 is associated with a small variation in E2, whereas small variations in E2 are not associated with variations in E1. If we wish to express even more clearly that this concept does not contain the concept of temporal order, we can express it in the following form, where events that show a slight variation are designated E*:  E1E2,  E1*E2*,  E1E2* and never the combination E1*E2.

 

Hans Reichenbach

 

If, however, one factor is too successful, it will continue to be the winning factor regardless of the variation in the other factors over the range of variation in the conditions, and therefore will stifle the development of other advantageous factors until the conditions change sufficiently that it no longer is the winning factor. At this point, the whole population is ill prepared for the change, and may well perish entirely if the winning factor accidentally becomes the matching factor for a disease or a predator.

 

Erik Naggum