(Continued from the previous page)
Charles Lyell, the father of uniformitarianism, limited his explanations of geological change to “the known or possible operations of existing causes” because he believed the history of science showed this method has always put scientists “on the road to truth—suggesting views which, although imperfect at first, have been found capable of improvement, until at last adopted by universal consent” (Lyell 1835: vol. III, 331). He also felt theories based on unobserved forces “relieve men of the painful necessity of renouncing preconceived notions” (Lyell 1835: vol. I, 113). The production of new types of organisms via stabilization processes is a “known operation of an existing cause”—an existing phenomenon having a known effect. In contrast, the gradual evolution of one type of organism into another via "numerous, fine, intermediate varieties" is supported by little, if any, observational data. Therefore, in denying the efficacy of known forces (stabilization processes) to produce new types of organisms, and in embracing an unobserved, theoretical force (gradual "speciation" in reproductive isolation), gradualists have actually abandoned their own professed faith in the explanatory sufficiency of ordinary, known phenomena.
Similarly, gradualists claim macroevolution can be entirely understood as the gradual accumulation of mutations in genes. Supposedly, as soon as two populations (descended from a common, ancestral population) differ with respect to a sufficient number of genes, they become distinct "species" and are no longer able to interbreed. But this process has never been observed. Only the origin of new types of organisms through stabilization processes has actually been seen. Of course, gradual changes in populations have in fact been observed under conditions of artificial selection. But, so far as we know, physiologically isolated populations, producing hybrids of low fertility, have not been produced by this means. Nor have we seen forms treated as species arise in such a manner. So far as we know, they have been produced only via stabilization processes. Nor do the mechanisms of inheritance discussed in neo-Darwinian theory explain the origin of new chromosets. They do not apply to karyotypic evolution. True, in a population of individuals sharing the same karyotype, various versions (alleles) of a gene might occur at a particular locus. With the passage of generations, these alleles might become more or less common in the population. But at no time would these statistical shifts in allele frequency bring about a change in the karyotype. Moreover, point mutations are excessively rare. Even those that do occur are either detrimental or without effect in the vast majority of cases. So the production of new types of organisms by the gradual accumulation of such mutations would be such a slow process that it could never be directly observed. It could only be observed, even potentially, in the fossil record. But, as we have seen (Section 6), it has not been observed even there (or, if it has been observed, it is at best an extremely rare phenomenon). For all these reasons, the gradualists' claim, that the typical form arises gradually from an ancestral form, is inconsistent with true uniformitarian doctrine.
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology