It is an incontrovertible fact that organisms have changed, or evolved, during the history of life on Earth.
Readers who first encounter such passages in the evolution literature may be surprised. Is this not a radical broadening of the very definition of evolution? How can mere change over time, which even the Genesis account calls for, be counted as evolution? This may seem to be a concession. Have evolutionists dropped the claim that strictly naturalistic explanations are sufficient to account for the origin of species?
No, evolutionists have made no such concession. It is typical to find in the evolution genre apparent logical disconnects such as this. In his article, Lenski explains evolution as the usual unguided biological variation coupled with natural selection (along with dozens of other occasional mechanisms, as needed).
So how should the reader understand and interpret the apparent disconnect. How can Lenski define evolution as mere change over time, but then swap back to the traditional understanding of evolution as strict naturalism? Is this an equivocation—a cheap ploy to prove evolution while bypassing its massive scientific problems?
No, this is not an equivocation. To understand the evolution genre one must understand the history of thought behind it. Even evolutionists are often not completely aware of this history, but the equating of evolution with mere change over time is shorthand for a centuries old metaphysical claim that underwrites evolution. The claim is that if God created the species they would be fixed. Indeed, divine creation would produce a static, unchanging world.
This thinking is often associated with the great eighteenth century Swedish botanist Carl Von Linne, or Linnaeus. At one time he advocated the fixity of species concept and later was troubled when he discovered hybrids—species that are produced by the crossing of two related species.
Linnaeus softened his doctrine of fixity of species, but this was inconsequential. His system with its conception of species became deeply rooted, and the nineteenth century began with the notion of species as immutable still strongly in place. This notion was increasingly being challenged but it was nonetheless a major obstacle for Darwin to overcome.
It was therefore highly significant when Darwin became persuaded that related populations of birds he saw at the Galapagos were actually different species. If there was the slightest foundation for this idea, Darwin had anticipated in a famous notebook entry, it "would undermine the stability of species."
Today's readers often fail to understand the significance. After all, what can be so important about some different birds on some islands? Certainly the birds did not suddenly reveal to Darwin how fishes could change to amphibians, or how amphibians could change to reptiles, or how reptiles could change to mammals. Rather, the revelation was that the idea of divine creation was suddenly becoming untenable. The crucible for Darwin was not an abundance of positive evidence for evolution but rather negative evidence against creation.
Evolutionist Ernst Mayr has pointed out that Darwin's conversion from creationist to materialist was due to three key scientific findings and later reinforced by several additional findings. These scientific findings were all findings against creation. In other words, the key evidence that swayed Darwin was not direct evidence for evolution but rather evidence against creation that indirectly argued for evolution.
And as Mayr further points out, the doctrine of fixity of species was a key barrier to overcome in order if the concept of evolution was to flourish:
Darwin called his great work On the Origin of Species, for he was fully conscious of the fact that the change from one species into another was the most fundamental problem of evolution. The fixed, essentialistic species was the fortress to be stormed and destroyed; once this had been accomplished, evolutionary thinking rushed through the breach like a flood through a break in a dike.
The pre-Darwinian metaphysic was that species were fixed and essentialistic. Evidence for small-scale change argued against the old view and in so doing became an important proof text for evolution.
This is the story between the lines when evolutionists casually associate their theory with change over time. It is shorthand for a long-held tradition in the history of thought. If there is change, then divine creation is false, and if creation is false then evolution, in one form or another, is true.
Metaphysical claims such as these mandate evolution. They underwrite the fact of evolution. The rest is just research problems on how evolution occurred—the theory of evolution. As Lenski explains, evolution is both fact and theory. Religion drives science, and it matters.