There may be an unknown factor that will cause quite as great a surprise as Darwin’s.
—Henry Fairfield Osborn
The Hereditary Mechanism and the Search
for the Unknown Factors of Evolution (1895)
The distinction between the saltationist and gradualist perspectives is more than a question of temporal pattern. It is also more than a simple disagreement over the size of the steps in evolution. After all, "big" and "small" are always relative. Gradualists say evolutionary changes are "minor" and "continuous." Saltationists say they are "major" and "discontinuous." But what do these words really mean? At some level, evolution has to be discontinuous, because genes themselves are particulate. An organism either receives a given gene from its parents or it doesn't. It can't receive 32.4 percent of a gene. Thus, in a diploid organism, the offspring receive one copy of a particular version (allele) of a gene, or two copies, or no copy at all. There are no other possibilities. So, even with simple intrachromoset meiotic recombination, there is a jump: zero to one, one to two, two to one, …
At the heart of the gradualist perspective is the idea that the forces creating new types of organisms are “ordinary." The genetic processes that acted in the past to create new forms of life are supposed to be the same as those acting at the present day in the ordinary production of offspring. The same process acts not only in the short run to produce offspring, but also in the long run to produce types quite distinct. As Gould (1980b: xiii) remarks, this gradualist evolutionary perspective, which has dominated biological thought since the 1940s, has
In short, gradualistic accounts of evolution are strongly associated with the uniformitarian perspective. Darwin himself was a uniformitarian, an outlook he took from Lyell. This uniformitarian argument is eminently reasonable when properly applied, but it is often misinterpreted.
Scientists often shy away from admitting that some natural forces are still unknown or that forces active in the past are no longer in operation. So long as known forces can reasonably explain the phenomenon in question, this attitude is entirely justifiable. But sometimes the previous existence of a force can be inferred from its effects, even though the nature of the force itself is unknown. Cuvier believed the house-sized boulders scattered over Europe proved vast, violent floods had overwhelmed the land in bygone eras (Cuvier 1827: 21-23, 344-354). Because he could not fathom the cause of these catastrophes, he vaguely attributed them to "the operation of problematical causes".1 The nature of the force that could wreak such devastation was, for Cuvier, a burning question. Thus, regarding his own attempts to resolve this issue, he confessed that "these ideas have haunted, I may almost say have tormented me".2
Although he knew nothing of past ice ages, Cuvier had been right in assuming some unknown, "problematical cause" had been at work; for he had never realized such great stones ("glacial erratics") could be carried from the place of origin to their current positions by glaciers or that much of Europe had once been buried beneath a massive layer of ice. This fact remained unknown during his lifetime. His student, Louis Agassiz, first proposed the radical idea that periodic ice ages had occurred in prehistoric times and that glaciers had carried erratics to their present positions. As a good uniformitarian geologist, the usually astute Lyell opposed Agassiz’s ice age theory.3 No such forces are active today, Lyell reasoned; therefore we should deny they were active in the past. Of course, he was mistaken. What known force did Lyell think it was that had moved boulders weighing thousands of tons, many miles from their known origins? Not gradual sedimentation, surely.
The reader might suppose the ideas expressed in stabilization theory represent a radical rejection of traditional biological thought. But this alternative explanation of how evolution occurs can in fact be interpreted as a modern manifestation of an alternative scientific tradition in which many naturalists have claimed evolutionary change is saltatory. Therefore stabilization theory is neither radical, nor indeed is it even novel with respect to many of its tenets. It merely supplies an explicit explanation for the discontinuities long emphasized by saltationists. In the past, adherents of this intellectual faction were at a disadvantage because they lacked well-documented examples of processes producing saltatory change. Scientists of this school believed evolution was saltatory, but they knew of no force that might produce such change. And yet, they were convinced some such force must exist; for in their view, forms treated as species seemed to be discrete, stable types unconnected by morphological intermediates.
A recurrent theme in saltationist thought was the idea that the forces involved in ordinary reproduction are distinct from those producing forms sufficiently distinct to be treated as a species. Saltationists were not satisfied with the idea that the production of new types of organisms is simply a matter of the gradual accumulation of minor variation over time. Like Cuvier with his glacial erratics, they sought an unknown force. As Sedgwick (1860) put it: "a new phenomenon unaccounted for by the operation of any known law of Nature" (italics are Sedgwick’s). In his Address to the British Association (1858), Richard Owen pointed out that
This attitude is exemplified in the writings of Henri Milne-Edwards (1800-1885). As did virtually all French naturalists of the nineteenth and early twentieth centuries,5 Milne-Edwards rejected Darwin’s theory altogether because he saw no connection between everyday individual variation and the production of forms treated as distinct species. He thought the action of the environment would never be able to generate new forms sufficiently distinct to warrant treatment as distinct species. Instead, he believed they were "created" -- but not by God. Milne-Edwards emphasized that, when a saltationist speaks of the "creation" of a new type of organism, there is no intention to imply it
1. Cuvier (1827: 10).
2. Cuvier (1827: 242).
3. Milner (1993: 6).
4. Quoted in Darwin (1872: xvii-xviii).
5. Mayr (1982: 536).
6. Milne-Edwards (1867: 429), translated in Russell (1982: 245).
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On the Origins of New Forms of Life
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Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology