On The Origin of Stasis by Means of Natural Selection


Charles Darwin made the concept of major evolutionary change plausible by convincing scientists that natural selection could account for the appearance of design in nature (Horan, 1979). He would never have considered evolution to be a fact without a plausible theory of how it could occur. The very title of his book reflects the importance of an evolutionary mechanism. Although much evidence has been cited in favor of macroevolution, as it had been prior to 1859, such evidence in Darwin's own opinion would be unsatisfactory without a mechanism:

In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and such other facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration (Darwin, 1859, p. 66).

In considering the origin of phyla and the stability of the major body plans, it is quite conceivable that scientists, reflecting upon the empirical evidence of the history of life on earth might come to the conclusion that mechanisms exist which prevent major evolutionary change from occurring on a gradual step-by-step basis. Stasis is a fact of life and mechanisms must exist which account for it. Ironically, natural selection itself may play a major role in the phenomenon of higher taxon-level stasis.

Darwin was not without his critics. In his book, Darwinism: The Refutation of a Myth, Soren Lovtrup points out that

"some critics turned against Darwin's teachings for religious reasons, but they were a minority; most of his opponents ... argued on a completely scientific basis."

He goes on to explain:

...the reasons for rejecting Darwin's proposal were many, but first of all that many innovations cannot possibly come into existence through accumulation of many small steps, and even if they can, natural selection cannot accomplish it, because incipient and intermediate stages are not advantageous (Lovtrup, 1987).

Perhaps the most formidable of Darwin's critics was St. George Mivart. His major book, On the Genesis of Species, took aim at the notion that natural selection could account for the accumulation of the incipient stages of useful structures (Mivart, 1871). Stephen Jay Gould notes that

Darwin offered strong, if grudging, praise and took Mivart far more seriously than any other critic...Mivart gathered, and illustrated "with admirable art and force" (Darwin's words), all objections to the theory of natural selection---"a formidable array" (Darwin's words again). Yet one particular theme, urged with special attention by Mivart, stood out as the centerpiece of his criticism. It remains today the primary stumbling block among thoughtful and friendly scrutinizers of Darwinism. No other criticism seems so troubling, so obviously and evidently "right" (against a Darwinian claim that seems intuitively paradoxical and improbable).

Mivart awarded this criticism a separate chapter in his book, right after the introduction. He also gave it a name, remembered ever since. He called it "The Incompetency of 'Natural Selection' to account for the Incipient Stages of Useful Structures." If this phrase sounds like a mouthful, consider the easy translation: we can readily understand how complex and full developed structures work and owe their maintenance and preservation to natural selection---a wing, an eye, the resemblance of a bittern to a branch or of an insect to a stick or dead leaf. But how do you get from nothing to such an elaborate something if evolution must proceed through a long sequence of intermediate stages, each favored by natural selection? You can't fly with 2% of a wing or gain much protection from an iota's similarity with a potentially concealing piece of vegetation. How, in other words, can natural selection explain these incipient stages of structures that can only be used (as we now observe them) in much more elaborated form (Gould, 1985)?

Gould goes on to point out that among the difficulties of Darwinian theory

"one point stands high above the rest: the dilemma of incipient stages. Mivart identified this problem as primary and it remains so today."

There are numerous examples of organisms with systems of highly specialized, interdependent components, all of which must be integrated before they are functional and offer any selective advantage. Nature abounds in such systems of "irreducible complexity." Although there may be some examples where certain components take on some preadaptive function, these cases may be considered as exceptions to a more general rule. As a rule, any subset of the components would prove to be a burden to an organism and thus be eliminated by natural selection. Take for example the sensory and motor mechanism of the common bacterium, Escherichia coli, a relatively simple unicellular prokaryotic organism.

The sensory and motor mechanism consists of a number of receptors which initially detect the concentrations of a variety of chemicals. Secondary components extract information from these sensors which in turn is used as input to a gradient sensing mechanism. The output of this mechanism is used to drive a set of constant torque proton-powered reversible rotary motors which transfer their energy through a microscopic drive train and propel helical flagella (see Figure 7). This highly integrated system allows the bacterium to migrate at the rate of approximately ten body lengths per second. Dr. Robert Macnab of Yale University concluded a major 50 page review of this mechanism with these remarks:

As a final comment, one can only marvel at the intricacy in a simple bacterium, of the total motor and sensory system which has been the subject of this review and remark that our concept of evolution by selective advantage must surely be an oversimplification. What advantage could derive, for example, from a "preflagellum" (meaning a subset of its components), and yet what is the probability of "simultaneous" development of the organelle at a level where it becomes advantageous (Macnab, 1978)?


Figure 7: Conceptual block diagram of the sensory and motor mechanism of the bacterium, E. coli.


Each component in the flagellum alone (see Figure 8) is highly specialized in its function and it is highly unlikely that many of the components, if any at all, would have any preadaptive value. A subset of components would constitute a collection of superfluous parts which according to Darwinian theory should be eliminated by natural selection. Darwin pointed out in The Descent of Man (Darwin, 1871) that natural selection would act to preserve those individuals which were least encumbered with a superfluous part. Although this is one of the simplest organisms on our planet, it serves to illustrate how natural selection contributes to the phenomenon of stasis and helps explain the fact that most species do not arise gradually by the steady transformation of their ancestors.


Figure 8: Conceptual diagram of the motor mechanism of E. coli.