Earlier, I quoted from Eugene Koonin’s recent paper, “Darwinian evolution in the light of genomics” (Nucleic Acids Research, 2009, 1–24), as he outlines two of the excerpts from the principal concepts of the Modern Synthesis:
Evolution proceeds by fixation of the rare beneficial variations and elimination of deleterious variations: this is the process of natural selection that, along with random variation, is the principal driving force of evolution according to Darwin and the Modern Synthesis. Natural selection which is, obviously, akin to and inspired by the ‘invisible hand’ (of the market) that ruled economy according to Adam Smith, was the first mechanism of evolution ever proposed that was simple, plausible, and did not require any mysterious innate trends. As such, this was Darwin’s second key insight. The founders of population genetics, in particular, Sewall Wright, emphasized that chance could play a substantial role in the fixation of changes during evolution not only in their emergence, via the phenomenon of genetic drift that entails random fixation of neutral or even deleterious changes. Population-genetic theory indicates that drift is particularly important in small populations that go through bottlenecks (6,16). However, the Modern Synthesis, in its ‘hardened’ form (13), effectively, rejected drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution.
The beneficial changes that are fixed by natural selection are ‘infinitesimally’ small, so that evolution proceeds via the gradual accumulation of these tiny modifications. Darwin insisted on strict gradualism as an essential staple of his theory: ‘Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being . . . If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.’ [(1), chapter 6]. Even some contemporaries of Darwin believed that was an unnecessary stricture on the theory. In particular, the early objections of Thomas Huxley are well known: even before the publication of the Origin Huxley wrote to Darwin ‘‘You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly’”
I then asked two questions.
Why did Darwin insist on “strict gradualism as an essential staple of his theory”?
Why did most proponents of the Modern synthesis “reject drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution”?
Let me take a stab at these questions.
The second question is actually easier to answer as this answer comes from Gert Korthof:
In the beginning orthodox Darwinists did not exactly like Kimura’s theory, because he was telling the scientific community that all-powerful Natural Selection was not so powerful after all. Natural selection had its limitations. On the molecular level the power of Natural Selection was greatly minimised, if not banished at all. Randomness took its place. Molecular variation in proteins and DNA was uncovered that had no influence on the fitness of the individual organism: in other words: is selectively neutral. One could even doubt if Natural Selection was of any importance in the traditional areas of morphology and anatomy.
The proponents of the modern synthesis sought to emphasize, not understate, the important role of natural selection. This makes sense in that such scientists were attempting to unite the findings of Mendelian genetics with the theories of Charles Darwin:
What is now known as the modern synthesis (or synthetic theory or evolutionary synthesis) is the eventual marriage of neo-Darwinism, with its support of natural selection and rejection of the inheritance of acquired characteristics, to Mendelian genetics, with its particulate inheritance. The modern synthesis is the fundamental basis for all current work in evolutionary biology.
If you are looking to develop a theory that seamlessly blends and unites Darwin with Mendel, of course you will be skeptical of any explanation that minimizes the role of Darwin’s primary contribution – natural selection.
So in the end, we must return to Darwin, as he is the one who established the mental template that would shape thinking in the subsequent decades. And as Koonin noted, Darwin insisted on a “strict gradualism.” In fact, Darwin explicitly lays it out in his own words (as quoted by Koonin):
Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being . . . If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.’
Darwin considered it absolutely essential that all variations would exist as infinitesimally small inherited modifications, each profitable to the preserved being Revisionists might have us sweep the ‘infinitesimal’ adjective under the rug, but Darwin chose this word when making an important point. Infinitesimal is defined as 1) indefinitely or exceedingly small; minute and 2) immeasurably small; less than an assignable quantity. Synonyms of ‘infinitesimal’ are: atomic, imperceptible, inappreciable, inconsiderable, insignificant, little, microscopic, miniature, minuscule, minute, negligible, teeny*, tiny, unnoticeable
In other words, Darwin insisted that the variations used by natural selection must be very, very, very small
Now, I found an interesting paper/chapter on the internet:
It’s by Jeffrey Schwartz and it recounts the dispute between Darwin and his gradualism and Mivart and his saltationism. Darwin’s insistence on strict gradualism permeated his thought. After invoking artificial selection to advance his theory, Darwin would later turn and rationalize away “monstrosities” because they were products of artificial selection. Darwin’s embrace of gradualism appeared to cause him to, in turn, embrace blending inheritance and propose an elaborate mechanism of inheritance that was similar to Lamarkianism. And most important, Darwin is quoted as doubting whether a LOSS of an organ would ever be tolerated by natural selection. Since the loss of an organ would not count as an infinitesimally small inherited modification, Darwin could not see how a deeply adapted organism could get by with such a radical elimination of an adaptation.
So why was Darwin so deeply invested in strict gradualism, despite the objections from his strongest ally, Thomas Huxley?
My guess is that Darwin, like so many other great scientists, was greatly influenced by Newton’s success in finding a small number of simple laws that explained just about everything. In other words, Darwin, and the architects of the Modern Synthesis, sought a simple, elegant explanation for all of evolution. And in this Explanation, natural selection was supposed to be ubiquitous because then natural selection would exist almost as a Law.
So Darwin would insist on strict gradualism because it meant that natural selection was effectively omnipresent, always scrutinizing every bit of variation that popped into existence. This would mean that all those infinitesimally small inherited modifications were nothing more than boring brute givens and it was Natural Selection that was the Author of all biological features. In fact, one might argue that the smaller the variation, the greater the role for natural selection. The “purely adaptationist model of evolution” championed by the Modern Synthesis, comes from viewing natural selection as omnipresent.
But perhaps there is more. Consider some quotes from Darwin where he defends his strict gradualism against other scientists open to evolutionary leaps:
It may be doubted whether sudden and considerable deviations of structure such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature. Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by man in a perfect state.
He who believes that some ancient form was transformed suddenly through an internal force or tendency into, for instance, one furnished with wings, will be almost compelled to assume, in opposition to all analogy, that many individuals varied simultaneously. It cannot be denied that such abrupt and great changes of structure are widely different from those which most species apparently have undergone. He will further be compelled to believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions, have been suddenly produced; and of such complex and wonderful co-adaptations, he will not be able to assign a shadow of an explanation. He will be forced to admit that these great and sudden transformations have left no trace of their action on the embryo. To admit all this is, as it seems to me, to enter into the realms of miracle, and to leave those of Science.
It would seem to me that Darwin thus insisted on strict gradualism because he recognized that any appearance of design (“every part of every organic being is so beautifully related to its complex conditions of life” and “many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions”) could not reasonably be attributed to chance. If we remove strict gradualism, then we are removing the constant oversight of natural selection. This is just the flip side of wanting natural selection to behave as a Law. Without the “law,” there is no explanation for any appearance of design.
Thus, the stubborn insistence on strict gradualism and the initial rejection of Neural Theory could be the result of wanting natural selection to mimic natural law coupled with the realization that chance without natural selection cannot mimic a designer. As long as natural selection was omnipresent, we could be confident that all appearances of design were attributed to natural selection.
And this brings metaphysical undercurrent to the surface. Recently, Jerry Coyne provided an interesting quote from Darwin:
I entirely reject, as in my judgment quite unnecessary, any subsequent addition ‘of new powers and attributes and forces,’ or of any ‘principle of improvement’, except in so far as every character which is naturally selected or preserved is in some way an advantage or improvement, otherwise it would not have been selected. If I were convinced that I required such additions to the theory of natural selection, I would reject it as rubbish. . . I would give absolutely nothing for the theory of Natural Selection, if it requires miraculous additions at any one stage of descent.
By conferring omnipresence to natural selection (purchased with strict gradualism), by wanting it to function as a Law, it would crowd out any teleological force or divine intervention. Neutral theory was able to remove Darwin’s stranglehold because it cracked the door just enough to only let Chance through. That is tolerable from a non-telic perspective. This is why Coyne insists, “But any injection of teleology into evolutionary biology violates precisely the great advance of Darwin’s theory” and “If we’re to defend evolutionary biology, we must defend it as a science: a nonteleological theory in which the panoply of life results from the action of natural selection and genetic drift acting on random mutations.”
Whatever the reason for Darwin’s insistence of strict gradualism, one thing becomes clear. By insisting “Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being,” Darwin was formulating the non-teleological position in its strongest possible form. This is because natural selection is the only non-teleological mechanism that can account for the appearance of design. If natural selection is not omnipresent, and an appearance of design emerges when natural selection is not there, then the non-teleologist must explain it in terms of chance or natural law. But even Dawkins recognizes chance is not a good explanation for the appearance of design, and law would easily fit into a teleological perspective.