The big mistake in Origin that Darwinists won’t admit is gradualism. Darwin explained that according to his theory we should expect to observe a continuum of living species each with only the slightest of variations between them. He postulated that we don’t observe this because the fittest species take over and the insensibly slight variants die off leaving species that are fully characteristic of their kind which then makes possible taxonomic classification by those characters. It’s in the full title in the latter half “The Preservation of Favored Races”.
That left Darwin with explaining the fossil record which is indisputably a record of saltation. Species in the fossil record appear abruptly fully characteristic of their kind, persist unchanged for an average of about 10 million years, then disappear as abruptly as they appeared. Darwin explained this away by saying the fossil record was incomplete and that when it was more fully explored the insensibly small variations that cumulatively led to the emergence of new species would be apparent. One hundred fifty years of fossil hunting later has not revealed what Darwin thought it would reveal. Some still say the fossil record in incomplete. Stephen Gould’s candid admission (“the trade secret of paleontology” is that it fails to support the very theory it is based upon) and formation of the theory of punctuated equilibrium is perhaps the most famous attempt to salvage gradualism.
No Darwinists I know or read give saltation any credence. The reason why is because saltation implies front loading. How would one species change in just a few generations to something taxonomically different? All the new characters that distinguish the new species must have been present in the predecessor if they were expressed that quickly. Random mutation & natural selection, through a tedious trial and error process, takes a very long time to generate novel characters. Indeed this insufficiency is at the very core of Intelligent Design. Haldane’s Dilemma is alive and well. Only an intelligent agent has the capacity to plan for the future. Intelligent agency is proactive and that proactivity is what distinguishes it from RM+NS. RM+NS is reactive in that it can “learn” from past experience but it can’t plan for future contingencies which have not been experienced in the past.
My position, which has remained unchanged for several years, is that phylogenesis was a planned sequence. Common descent from one or a few ancestors beginning a few billion years ago has overwhelming evidence in support of it. Gradualism however does not have overwhelming evidence. Gradualism in evolution survives to this day because the only alternative to it is intelligent design. Gradualism doesn’t survive by the weight of the evidence but rather by the tightly held belief in philosophic naturalism held by an overwhelming number of the practioners of evolutionary biology. As Richard Dawkins famously wrote “Although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist.” These people are clinging to gradualism like religious dogma because to say it’s wrong is tantamount to giving up their religion.