Mutationism Myth (5): The Restoration

The Mutationism Myth, part 5. The Restoration

In the Mutationism Myth (see part 1), the Modern Synthesis (MS) rescues evolutionary biology from the Mendelian heresy, by showing that genetics is consistent with selection. In reality, the Mendelians had already synthesized genetics and selection (part 3), but rejected Darwin’s errant views of heredity (part 2) and rejected, to varying degrees, the Darwinian doctrines that subordinated the role of variation so as to render selection the ruling principle in evolution. How, then, did the Modern Synthesis restore Darwinism?

The response to Mendelian heterodoxies

As recounted previously (part 4), the Mendelians recognized mutation as a source of discontinuity, initiative, direction and creativity in evolution. In this section, we will look at the MS (Modern Synthesis) as a Darwinian response to these Mendelian heterodoxies ¹.

Initiative

Darwin knew that spontaneously arising “sports” (mutants) occurred in nature, but denied them any meaningful role in evolution. Hereditary fluctuation, induced by altered “conditions of life”, would produce subtle variations in bulk, providing suitable material for selection. Evolution, in this view, is a process of automatic adjustment to altered “conditions of life”.

The Mendelians frequently invoked a quite different view of evolution as a causal sequence initiated by a new mutation. This made the impetus for change internal rather than external, and suggested that, to understand evolution, it would be important to understand the rates and propensities of mutations.

The architects of the MS rejected this view and asserted that evolution begins, not with a new mutation, but with the abundance of variation in the “gene pool”

“evolution is not primarily a genetic event. Mutation merely supplies the gene pool with genetic variation; it is selection that induces evolutionary change” (Mayr 1963, p. 613).

Rates and propensities of mutation are rendered irrelevant, because the “gene pool” serves as a dynamic buffer that insulates evolution from mutational effects:

“mutations are rarely if ever the direct source of variation upon which evolutionary change is based. Instead, they replenish the supply of variability in the gene pool . . . . Consequently, we should not expect to find any relationship between rate of mutation and rate of evolution. There is no evidence that such a relationship exists.” (Stebbins, 1966, p. 29)

“The large number of variants arising in each generation by mutation represents only a small fraction of the total amount of genetic variability present in natural populations. … It follows that rates of evolution are not likely to be closely correlated with rates of mutation. Besides mutation, natural selection and migration help maintain high levels of genetic variation in natural populations. Even if mutation rates would increase by a factor of 10, newly introduced mutations would represent only a very small fraction of the variation present at any one time in populations of outcrossing, sexually reproducing organisms.” (Dobzhansky, et al., 1977, p. 72) ²

“Those authors who thought that mutations alone supplied the variability on which selection can act, often called natural selection a chance theory. They said that evolution had to wait for the lucky accident of a favorable mutation before natural selection could become active. This is now known to be completely wrong. Recombination provides in every generation abundant variation on which the selection of the relatively better adapted members of a population can work.” (Mayr, 1994, p. 38)

Thus, Darwin’s view of a process initiated by a change in conditions is restored: the source of initiative is not the occurrence of mutations, which are individually insignificant (Dobzhansky et al. 1977, p. 72) and merely “replenish the supply of variability in the gene pool” (Stebbins 1966, p. 29), but the change in conditions that brings on selection of variation already present (e.g., Dobzhansky 1955, p. 282; Dobzhansky et al. 1977, p. 6; e.g., Stebbins 1982, p. 160).

Infinitesimalism (“gradualism”)

Darwin believed that evolution always proceeds by “infinitesimal” or “insensible” steps, and he stated this clear commitment in various ways, e.g., he said that “Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications” (Origin of Species, Ch. 4) and that

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, Ch. 6, Origin)

The “saltationist” alternative typically offered to this doctrine, as stated by Huxley in his 1860 review of the Origin of Species (online source), is merely that evolution takes some jumps:

Mr. Darwin’s position might, we think, have been even stronger than it is if he had not embarrassed himself with the aphorism, “Natura non facit saltum,” which turns up so often in his pages. We believe, as we have said above, that Nature does make jumps now and then, and a recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of transmutation.

As we found out in part 4, the Mendelians sided with Huxley on this issue, and did not share Darwin’s commitment to infinitesimalism.

Fisher (1930) and Dobzhansky (1937) played a significant role in restoring a “Darwinian” view, at least superficially. Fisher argued on theoretical grounds that evolution was more likely to take place by arbitrarily small modifications. He considered the evolutionary prospects of a phenotypic form represented by a point located somewhere near a point in geometric space that represents a global fitness optimum. Under this condition, the chance that a shift in location will be beneficial approaches an upper limit of 50 % as the size of the shift becomes arbitrarily small, i.e., infinitesimal. This argument was influential with MS authors in spite of the fact that the model was geometrical rather than biological (Orr and Coyne, 1992).

The architects of the MS developed a second line of argument based on the genetics of differences between species or sub-specific races, to the effect that the analysis of such differences showed a predominance of “small” or “slight” effects. Dobzhansky (1937) tied these arguments together:

“The high frequency of mutations producing small changes in the phenotype raises a strong presumption in favor of supposing that such mutations play a greater role in evolutionary processes than mutations with grosser effects. Fisher (1930) has given an interesting mathematical argument in favor of this view. These considerations agree very well with the results of the genetic analysis of the interracial and interspecific differences (Chapter III), showing these differences to be caused in a majority of cases by cooperation of numerous genes, each of which taken separately has only slight effects on the phenotype.” (p. 26)

I think this aspect of the MS is familiar and uncontroversial, so I won’t bore the reader with more quotations from original sources, several of which are given in the review by Orr and Coyne (1992). In general, the architects of the MS emphasized the importance of “slight” or “small” differences, and they claimed that this position was borne out by theoretical considerations, as well as by experimental studies.

Orr and Coyne took issue— rather audaciously— with the latter claim. They argue that the architects of the MS “seem to have based their support of micromutationism on almost no data at all”, claiming that the studies reviewed by Dobzhansky above did not justify Dobzhansky’s own stated conclusions.

Another interpretation of this apparent discrepancy is that Orr and Coyne are taking Darwin literally, while the MS architects did not.³ That is, the architects of the MS aligned themselves rhetorically with Darwin, and against the mythic saltationists-who-believe-in-evolution-by-large-jumps-without-selection. By doing so, they leveraged loyalty to the Darwin brand, but in reality, their scientific product was not the same as Darwin’s. Darwin says that the fraction of saltations in evolution is zero, whereas Dobzhansky, in the passage quoted above, says merely that mutations producing “small changes” play a “greater role” than those with “grosser effects”, and that evidence for this view is seen, not universally, but merely in “the majority of cases”. Orr and Coyne (1992) conclude that large effects are common, which contradicts Darwin’s extreme view but not the fuzzier claims made by MS advocates.

Creativity (novelty)

The Darwinian claim that natural selection is “creative” ⁴ was disputed initially (see Beatty, 2010) and continues to be questioned by critics (e.g., Kirschner and Gerhardt, 2005) and defended by Darwin’s followers (e.g., Charlesworth, 2005).

The critics always make the same objection, namely that creativity resides somehow in the process of variation, which accounts for “the arrival of the fittest”, rather than selection (“the survival of the fittest”), which merely decides what will live and what will die. The Mendelians, in particular, argued that the discovery of genetics had clarified the previously fuzzy line between selection and hereditary variation, which is not induced but arises spontaneously by mutation, making clear that, in Punnett’s words, “the function of natural selection is selection and not creation” (see part 4).

The architects of the MS responded by renewing the claim for the creativity of selection. However, its also interesting to note that, early in the “Synthesis” period, some of the architects (to my knowledge, only Huxley and Dobzhansky) appeared to offer a compromise, to the effect that creativity was shared by selection and mutation, e.g., Huxley wrote that “Neither mutation nor selection alone is creative of anything important in evolution; but the two in conjunction are creative” (Huxley, 1942, p. 28).

The more prominent argument, which eventually became orthodox, was to attribute creativity to selection by depicting it as a craftsman or artist using “raw materials” supplied by variation. Gould (1977) has analyzed this pattern extensively, and I will simply repeat a passage that he offers in explanation:

“But why was natural selection compared to a composer by Dobzhansky; to a poet by Simpson; to a sculptor by Mayr; and to, of all people, Mr. Shakespeare by Julian Huxley? I won’t defend the choice of metaphors, but I will uphold the intent, namely, to illustrate the essence of Darwinism— the creativity of natural selection. Natural selection has a place in all anti-Darwinian theories that I know. It is cast in a negative role as an executioner, a headsman for the unfit . . . The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies the raw material only. Natural selection directs the course of evolutionary change. It preserves favorable variants and builds fitness gradually. In fact, since artists fashion their creations from the raw material of notes, words, and stone, the metaphors do not strike me as inappropriate.”

Dobzhansky also invokes the same “raw materials” metaphor, but in the context of a factory rather than an artist:

“the objection [that natural selection cannot be the guiding agent in evolution because it produces nothing new] became invalid in the light of modern biological knowledge . . .We should clearly distinguish the two basic evolutionary processes: that of the origin of the raw materials from which evolutionary changes can be constructed, and that of building and perfecting the organic form and function. Evolution can be compared to a factory: any factory needs a supply of raw materials to work with, but when the materials are available they must be transformed into a finished product by means of some manufacturing process. ” (Dobzhansky, 1955, p. 131)

That is, selection is understood to be the builder or artist or manufacturing process, while mutation supplies “raw” materials. Note that Gould ultimately endorses the creativity claim precisely on the grounds that variation supplies raw material only. It might seem surprising that the metaphor of “raw materials” should play such an important role in evolutionary reasoning.

However, as we will learn later, metaphors are vital for causal reasoning, filling a gap that mathematics (which is not itself a language of causation) cannot cover. In the context of an argument about causation, to designate something as “raw material” is to reference the classic distinction ⁵ between material causes (stuff, material) and efficient causes (forces, agents). The architects of the MS had a theory of “forces”, and they believed that population genetics had proven that mutation (variation), though it contributes stuff or material to evolution, is not an effective force. This claim is explained further below.

Directionality

Many authors have suggested that the essence of Darwin’s “Natural Selection” theory is that evolution emerges by combining a random process of variation and a non-random process of selection, with selection directing the outcome, adding a previously absent component of direction:

“Darwin assumed in the Origin of Species that the evolution of living organisms depended on the origin of new forms which varied from old forms by continuous differences in no constant or predictable direction. Crossed together the new and the old showed blending inheritance. To these variations direction was given by a process of natural selection which, like artificial selection, preserved some while it destroyed others. A direction, an adaptive direction, was thus given to variations after they arose. This view was intended by Darwin to supplant the alternative view that direction was given to variations before they arose.” (Darlington, 1958, p. 231)

“The idea that evolution comes about from the interaction of a stochastic and a directed process was the essence of Darwin’s theory. The stochastic process that he invoked was the occurrence of small random variations which he supposed, provided the raw material for natural selection, a process directed by the requirements of the environment and one that builds up, step by step, changes that would be inconceivably improbable at a single step . . .The meaning of ‘random’ . . . is that the variations are, as a group, not correlated with the course subsequently taken by evolution (which is determined by selection).” (Wright, 1967, p. 117)

As noted in part 4, the Mendelians were ready to challenge this assumption, though their alternative view was not well developed. Some Mendelians noted that the repeated occurrence of a mutation improved its chance of being established in evolution (part 4), and that “in the deal out of mutations, the cards are stacked” (Shull, 1936). Vavilov (1922) applied this line of thinking at length in his explication of a possible role for parallel variations in parallel evolution.

The architects of the Modern Synthesis returned to the Darwinian view expressed in the statements of Wright and Darlington above. Frequently in MS writings, mutation is contrasted with selection and described as a “random” process. ⁶ In Mayr’s 1980 reflection on the Modern Synthesis, he defines “Darwinism” as “the theory that selection is the only direction-giving factor in evolution.” In the passage below, Dobzhansky refers to selection as “the directional force of evolution”:

“Each unitary random variation is therefore of little consequence, and may be compared to random movements of molecules within a gas or liquid. Directional movements of air or water can be produced only by forces that act at a much broader level than the movements of individual molecules, e.g., differences in air pressure, which produce wind, or differences in slope, which produce stream currents. In an analogous fashion, the directional force of evolution, natural selection, acts on the basis of conditions existing at the broad level of the environment as it affects populations.” (Dobzhansky, et al., 1977, p. 6)

Note that Dobzhansky uses an analogy with statistical physics to argue that selection’s unique role is due to its status as a high-level “force”, whereas a “random variation” is not a force, but is like the movement of a single particle.

Dobzhansky’s comment suggests that the MS position on direction is tied to “gene pool” thinking. Indeed, in the passage below, Stebbins ⁷ makes clear that selection “directs evolution” by choosing from among the abundance of offerings in the “gene pool”:

“Natural selection directs evolution not by accepting or rejecting mutations as they occur, but by sorting new adaptive combinations out of a gene pool of variability which has been built up through the combined action of mutation, gene recombination, and selection over many generations” (p. 31 Stebbins, 1966, Processes of Organic Evolution)

Finally, given this position, its not surprising that Vavilov’s hypothesis about the role of parallel variation in parallel evolution was not taken seriously. Given the abundance of variation in the “gene pool”, and the ability of selection to shape this gene pool to fit circumstances, it was not safe to assume that shared characters had a shared genetic basis, as Mayr (1963) argued in one of his more famous erroneous claims ⁸:

“In the early days of Mendelism there was much search for homologous genes that would account for such similarities. Much that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives (Dobzhansky, 1955). If there is only one efficient solution for a certain functional demand, very different gene complexes will come up with the same solution, no matter how different the pathway by which it is achieved.” (p. 609)

Defining “forces” and re-defining “evolution”

The previous section suggested the central importance of the “gene pool” view, which argues that evolution begins, not with a new mutation, but with the abundance of variation “maintained” in the “gene pool”. On the basis of this view, “evolution” was redefined so as to exclude the mutationist alternative:

“The theory of evolution by natural selection is a theory that relates the variation between individuals within a population to variation of populations in time and space. The theory amounts, in short, to the realization that intrapopulation variation is converted into spatial and temporal differentiation. The process of this conversion is the process of evolution.” (Lewontin, 1965, p. 67)

That is, “the process of evolution” (“evolution by natural selection”) literally does not include the introduction of new alleles, but instead corresponds to the sorting out of available variation.

Given that “evolution” (redefined) is all about “shifting gene frequencies”, the causes of evolution may be presented as “pressures” or “forces” that shift frequencies. Lets suppose we have two alleles, A1 and A2, with frequencies f(A1) = f(A2) = 0.5. If A1 improves fitness relative to A2, then “selection” can be seen as a “force” that shifts its frequency over some period of time to f(A1) = 0.5 + d, and ultimately to fixation, f(A1) = 1. Likewise, there is some rate of mutation from A1 to A2, and another rate for the reverse pathway, and this rate can be understood to shift the relative frequencies to the point where f(A1) = 0.5 + d.

The key innovation in this view is its identification of mathematically continuous shifts in allele frequency as the common currency underlying a theory of causal agency. In classical physics, the displacement of a particle in continuous space (over continuous time) plays a similar role as the common currency of causal effects: multiple forces can cause such displacement, and their effects can be combined or decomposed. Likewise, in the MS, selection, drift, and mutation are identified as “forces” because they can cause shifts in frequencies.

Accordingly, the founders of population genetics looked at mutation as a “force” or “pressure” that, if powerful enough, could cause or “drive” evolution. From the mutation-selection balance equation and available data on mutation rates, Fisher (1930) and Haldane (1932) argue that, because mutation rates are so small, the opposing “force” of selection rules and mutation “pressure” cannot drive an allele to fixation. The conclusion of this “opposing pressures” argument ⁹ is that mutation is not an effective evolutionary force. Thus, in classical population-genetics modeling, its quite common to simply leave out mutation rates, on the grounds that they don’t have substantial effects on the behavior of the model (e.g., as in the seminal analysis of 2-locus models by Lewontin & Kojima, 1960).

Having ruled out mutation as a “force”, this left the idea of mutation as a source of “raw materials”. That is, the architects of the MS looked at mutation in two ways, as a material cause, and as an efficient cause (agent, force). As just noted, the schematization of mutation as a “force” led to its rejection as an important “force”, while its role as a material cause was addressed with the metaphor of “raw materials”, as in several quotation given above, and in the following:

“The process of mutation supplies the raw materials of evolution, but the tempo of evolution is determined at the populational levels, by natural selection in conjunction with the ecology and the reproductive biology of the group of organisms” (Dobzhansky, 1955, p. 282)

“Mutation as an evolutionary force. In the early days of genetics it was believed that evolutionary trends are directed by mutation, or, as Dobzhansky (1959) recently phrased this view, ‘that evolution is due to occasional lucky mutants which happen to be useful rather than harmful.’ In contrast, it is held by contemporary geneticists that mutation pressure as such is of small immediate evolutionary consequence in sexual organisms, in view of the relatively far greater contribution of recombination and gene flow to the production of new genotypes and of the overwhelming role of selection in determining the change in the genetic composition of populations from generation to generation.” (p. 101 of Mayr, 1963)

In this way, following the arguments of Fisher (1930), population genetics was believed to undermine any and all non-Darwinian theories of “evolution worked by mutation”.

The keys to the kingdom

As we have seen, the restoration of “Darwinism” depended on several key innovations.

The first innovation was the change in attitude that inaugurated the process of redefining “Darwinism” that, for better or worse, continues today. For literally decades after the re-discovery of Mendelian genetics in 1900, the generality of Mendelian inheritance was disputed by biologists who, loyal to 19th century views of Darwin and Lamarck, reacted to genetics as though it must be some kind of misleading laboratory artefact, inapplicable in “nature”.

Nevertheless, by 1930, Mendelian inheritance had been shown in a wide variety of macroscopic organisms, while experimental support for the hereditary laws of Darwin and Lamarck was trivial in comparison. The architects of the MS took Mendelian genetics as a given and simply ignored Darwin’s laws of heredity, or treated them as an unimportant detail. In a sense, Darwin’s followers had switched their allegiance from Darwin’s specific theory₁ to a more abstract view, which we might call Darwinian selectionism, in which selection has a kind of causal pre-eminence, and all other issues are negotiable. Darwin’s followers quietly backed away from his risky position of natura non facit salta, and ignored the fact that their new idea of the maintenance of abundant variation in the gene pool was not isomorphic with Darwin’s concept of indefinite variability (hereditary fluctuation).

The second key innovation was this “gene pool”. Even if one deletes from the Origin of Species all the details that are obviously contrary to genetics, the resulting view still makes strenuous and non-arbitrary demands on a theory of variation, as Gould (2002) persuasively argues. Regardless of any other mechanistic details, Darwinian selectionism (the pre-eminence of selection) demands that variation be “copious, undirected, and small in effect” (in Gould’s formulation). Darwin’s view of evolution as an automatic process of smooth adaptation to altered conditions seemed to require abundant, uniform, and infinitesimal variation, yet the new science of Mendelian genetics seemed to suggest the kind of rare, idiosyncratic, and discrete variants that Darwin rejected.

The key to reconciling the two— at least, rhetorically— is the notional “gene pool”. According to an idea first articulated in 1926 by Chetverikov, every species has a “gene pool” that soaks up variation like a “sponge” (Chetverikov 1997), “maintaining” it for later use by selection, and ensuring an abundance of minute heritable variations in every trait, in every generation, as Darwin had conjectured. Thus, in Darwinism 1.0, fluctuation provides fuel “on demand” for selection, which is seen as the engine of evolution; in Darwinism 2.0, the engine has a tank of fuel, the “gene pool”, that automagically keeps itself full. Though not equivalent, both views represent variation as merely an abundant source of fuel, with no influence on where evolution goes.

The crowning innovation of the Modern Synthesis was to invoke “population genetics” as a framework of causation that excluded all alternatives to Darwinism (redefined). Viewing evolution from a distance, as a paleontologist or systematist, one sees patterns that might be explained by various modes of evolution: Darwinian, Lamarckian, Buffonian, orthogenetic, saltational, and so on. While Darwinism seems to have been the dominant interpretation, the range of interpretations remained wide at the turn of the century when Mendelism burst onto the scene. While Mendelism revolutionized the evolutionary thinking of those who accepted it, most doubted its relevance to evolution. As late as 1930, one could read in the pages of Nature the view that “a gene is germ damage of which the outward manifestation is a mutation . . In my opinion, mutations and adaptations have nothing to do with one another” (MacBride, 1930).

The architects of the MS, following the original arguments of Fisher (1930), claimed that they could reduce evolution to a causal mechanism based on population genetics, and that this causal mechanism ruled out all modes of evolution but the “Darwinian” one. By the Origin of Species centennial in 1959, the architects of the MS had declared that the debate over evolutionary theory was over, and that they— and Darwin— had won. “Mutationism” came to be seen as discredited:

The genetic work of the last four decades has refuted mutationism (saltationism) so thoroughly that it is not necessary to repeat once more all the genetic evidence against it. (Mayr 1960, p. 355)

if ever it could have been thought that mutation is important in the control of evolution, it is impossible to think so now (Ford 1971, p. 361)

As late as 1932 T.H. Morgan was asserting that ‘natural selection does not play the role of a creative principle in evolution’, but ten years later all but a very few biologists were agreed on an evolutionary theory based firmly on Darwin’s own ideas knitted with subsequent developments in genetics. (Berry 1982, p. 14)

for simplicity we speak of mutation as the first stage in the Darwinian process, natural selection as the second stage. But this is misleading if it suggests that natural selection hangs about waiting for a mutation which is then either rejected or snapped up and the waiting begins again. It could have been like that: natural selection of that kind would probably work, and maybe does work somewhere in the universe. But as a matter of fact on this planet it usually isn’t like that. (Dawkins 1996, p. 87)

Looking ahead

Thanks for your forbearance in plowing through all of this history. Ultimately, though, we want to move on to other things. The Curious Disconnect is not a blog about scientific history: its about the current muddle in evolutionary thinking. I’m only explaining the history so that we can take a critical look at the MS and the view of causation that we have inherited from it.

Here is an example of what I mean. Rates of evolution, including adaptive evolution as in Rokyta, et al, 2005, are sensitively dependent on rates of mutation, directly contradicting the MS doctrine that the buffering capacity of the “gene pool” insulates evolution from mutational dynamics ¹⁰.

Because the MS is a coherent and integrated view (not just an extendible list of stuff that happens in evolution), and because reality is cohesive as well, fixing the failure of the MS to recognize this dependence is not an arbitrary or isolated error. We can’t fix the MS by going through all the works of Mayr, Dobzhansky, Fisher, et al and 1) deleting every explicit claim that the rate or direction of evolution does not depend on mutation and 2) adding the words “And, we think the rate of evolution depends on the rate of mutation”. That would not be enough.

For instance, the Gillespie-Orr “mutational landscape” model underlying the analysis of Rokyta, et al, which accounts for the aforementioned rate dependence, is based on a mutationist conception of evolution as a 2-step mutation-fixatioon process. The author of the model writes (Orr, 2003):

Adaptation is a two-step process: (i) alleles having different effects on fitness arise by mutation and (ii) those alleles that improve fitness tend to increase in frequency by natural selection.

thus directly contradicting what Dawkins says (above) about evolution “on this planet” (which, I suppose, raises a question about where Dawkins was when he wrote that statement).

Thus, the problem is not just the specific issue of a rate correlation, but its also the 2-step mutationist view.

To accomodate the observation that the rate of evolution depends sensitively on the rate of mutation, then, we would need to fix the MS view of causation, changing the common currency of causation so that an effect of biases in the origination process can be recognized ¹¹. In turn, to allow a bias in the origination process to affect “evolution” would require us to put the origination process back into “evolution”, i.e., we would have to toss out the “shifting gene frequencies” definition. After that, we would have to delete all of the statements rejecting the 2-step “mutationist” view, including all the claims that “selection” never “waits” for a new mutation due to the magical maintenance of variation in the “gene pool”. In addition, the effect of mutation biases violates the Darwinian and MS rule that variation is “random” in the sense that the tendencies of variation are unrelated to the realized direction of evolutionary change (this is the sense used in the quotation from Wright given above; see Beatty, 2010 for the analysis of Darwin’s view).

The reason that so many changes would be required is that the exclusion of any important formative or directional role for mutation in the MS was not unforeseen or incidental, but utterly deliberate and essential. Real theories make commitments and take risks. Our analysis of the Mutationism Myth shows what these commitments were: the MS represents a commitment to the pre-eminence of selection and the subordination of variation, rejecting Mendelian ideas on mutation as a cause of discontinuity, initiative, creativity, and direction.

Because of this, its rather foolish to talk about “extending”, “finishing”, or “updating” the Modern Synthesis. We can’t “revise” the MS to repudiate Mayr’s understanding of evolutionary causation and endorse Morgan’s mutationist view instead. To “revise” the MS in that way would be to repudiate the MS itself. Real theories make commitments and take risks. What’s wrong about the Modern Synthesis is not a minor detail, but its essential and definitive subordination of the role of variation, and all that comes with it. The MS represents a wager, a bet, that reality would turn out to be a certain way. It didn’t. The architects of the MS bet on the wrong horse. End of story.

So, its time to place a new bet, but on what? Just tossing out the MS will not give us that new theory of causation, the one that allows us to compare causal effects across different types of causes. With that, we might be able to address some of the big questions of evolution and confront contemporary challenges relating to “evo-devo”. Our goal on the Curious Disconnect is to define issues like this, and to consider what are some of the possible bets. But thats getting ahead of ourselves.

Summary

Darwin’s 20th-century followers responded to the Mendelian threat— which (at least partially) called for a stochastic, non-infinitesimal, mutation-driven view of evolution and adaptation— by developing the Modern Synthesis (aka “modern neo-Darwinism” or the “New Synthesis”), a new theory that purported to be consistent both with genetics and with Darwin’s 19th-century view of evolution as a process of infinitesimal change controlled, initiated and directed by selection.

The development of this theory, which went on to dominate the 20th century, was based on 3 innovations. The first innovation was to redefine Darwinism. The version of “Darwinism” that the MS restored was not the one that the Mendelians rejected. Instead, Darwinism 2.0 was “Darwinian” in emphasizing the pre-eminence of selection, leaving out the Darwin’s non-Mendelian laws of heredity. The second innovation was the notional “gene pool”, a populational buffer that insulates «evolution» from effects of mutation by churning and mixing and “maintaining” abundant variation. The “gene pool” concept provided a foundation to reject the “lucky mutant” view and argue against Mendelian heterodoxies, e.g., given that evolution begins with the “gene pool”, selection (not mutation) initiates evolution, and chooses its direction from the abundance of possibilities.

Finally, the MS included an integrated view of causation in which continuous shifts in allele frequencies are seen as the common currency of causal effects. That is, a factor is identifiable as an evolutionary “force” to the extent that it is capable of causing mass-action shifts in allele frequencies. This view appeared to justify the claim that selection is the driving force in evolution, and that mutation is not a potent force, but merely serves to supply “raw materials” to the “gene pool”.

Thus, while the Mutationism Myth wrongly suggests that the MS reconciled genetics and selection (instead, the Mendelians accomplished this), it correctly suggests that the MS restored a “Darwinian” view, and that arguments from population genetics were the key to this restoration, though (as we’ll find out later) the crucial arguments from population genetics were based less on mathematics than on metaphors and metaphysics.

References

Beatty, J. 2010. Reconsidering the Importance of Chance Variation in M. Pigliucci, and G. Müller, eds. Evolution: The Extended Synthesis.

Berry, R. J. 1982. Neo-Darwinism. Edward Arnold, Ltd., London.

Charlesworth, B. 2005. On the Origins of Novelty and Variation. Science 310:1619-1620.

Chetverikov, S. S. 1997. On Certain Aspects of the Evolutionary Process from the Standpoint of Modern Genetics. Genetics Heritage Press, Placitas, New Mexico.

Darlington, C. D. 1958. The Evolution of Genetic Systems. Basic Books, New York.

Dawkins, R. 1996. Climbing Mount Improbable. W.W. Norton and Company, New York.

Dobzhansky, T. 1937. Genetics and the Origin of Species. Columbia University Press, New York.

Dobzhansky, T. 1955. Genetics and the Origin of Species. Wiley & Sons, Inc., New York.

Dobzhansky, T., F. J. Ayala, G. L. Stebbins, and J. W. Valentine. 1977. Evolution. W.H. Freeman.

Fisher, R. A. 1930. The Genetical Theory of Natural Selection. Oxford University Press, London.

Ford, E. B. 1971. Ecological Genetics. Chapman & Hall, London.

Gould, S. J. 2002. The Structure of Evolutionary Theory. Harvard University Press, Cambridge, Massachusetts.

Gould, S. J. 1977. Ever Since Darwin. W.W. Norton & Co., New York.

Huxley, J. S. 1942. Evolution: The Modern Synthesis. George Allen & Unwin, London.

Kirschner, M. W., and J. C. Gerhart. 2005. The Plausibility of Life: Resolving Darwin’s Dilemma. Yale University Press, New Haven.

Lewontin, R. C. 1965. The Gene and Evolution. Pp. 67-75 in R. M. Nardone, ed. Mendel Centenary: Genetics, Development and Evolution. Catholic University of America Press, Washington, DC.

Lewontin, R. C., and K. Kojima. 1960. The evolutionary dynamics of complex polymorphisms. Evolution 14:458-472.

MacBride, E. W. 1930. Embryology and Evolution. Nature 126:918-919.

Maynard Smith, J. 1976. What determines the rate of evolution? American Naturalist 110:331-338.

Mayr, E. 1963. Animal Species and Evolution. Harvard University Press, Cambridge, Massachusetts.

Mayr, E. 1980. Some Thoughts on the History of the Evolutionary Synthesis. Pp. 1-48 in E. Mayr, and W. Provine, eds. The Evolutionary Synthesis. Harvard University Mayr, E. 1994. The Resistance to Darwinism and the Misconceptions on which it was Based. Pp. 35-46 in J. H. Campbell, and J. W. Schopf, eds. Creative Evolution?! Jones & Bartlett, Inc., London.

Orr, H. A. 2003. The distribution of fitness effects among beneficial mutations. Genetics 163:1519-1526.

Orr, H. A., and J. A. Coyne. 1992. The Genetics of Adaptation: A Reassessment. American Naturalist 140:725-742.

Rokyta, D. R., P. Joyce, S. B. Caudle, and H. A. Wichman. 2005. An empirical test of the mutational landscape model of adaptation using a single-stranded DNA virus. Nat Genet 37:441-444.

Shull, A. F. 1936. Evolution. McGraw-Hill, New York.

Stebbins, G. L. 1966. Processes of Organic Evolution. Prentice Hall, Englewood Cliffs, NJ.

Stoltzfus, A., and L. Y. Yampolsky. 2009. Climbing mount probable: mutation as a cause of nonrandomness in evolution. J Hered 100:637-647.

Vavilov, N. I. 1922. The Law of Homologous Series in Variation. J. Heredity 12:47-89.

Wright, S. 1967. Comments on the preliminary working papers of Eden and Waddington. Pp. 117-120 in P. S. Moorehead, and M. M. Kaplan, eds. Mathematical challenges to the neo-Darwinian interpretation of evolution. Wistar Institutional Press, Philadelphia.

Yampolsky, L. Y., and A. Stoltzfus. 2001. Bias in the introduction of variation as an orienting factor in evolution. Evol Dev 3:73-83.

Notes

¹ The point of this comment is that I don’t claim to be presenting the MS in a comprehensive way. This view of the MS is one view. In particular, it represents a kind of dialectic perspective on the MS as a response to Mendelism, focusing on what seems to be a characteristically Darwinian view of the role of variation, and focusing on evolutionary causation.

² Note that Dobzhansky, in particular, started out as a bit of a heretic on the importance of mutation. In his 1937 book he speculated that different rates of mutation might explain different rates of evolution (p. 37), an idea that later was mocked by Simpson and others, lending credence to Gould’s idea of a “hardening” of the Synthesis.

³ Orr and Coyne write “the micromutational view of Darwin, Fisher and others is clear: adaptations arise by allelic substitutions of slight effect at many loci, and no single substitution constitutes a major portion of an adaptation.” I think they are right about Fisher and Darwin (ignoring the flagrant anachronism linking Darwin to a position on “allelic substitutions”), but who are the “others”? I can’t put Dobzhansky in the same category. He only emphasizes that “small” or “slight” differences predominate in “the majority of cases”.

⁴ Some present-day biologists have an adverse reaction to the term “creativity”. Perhaps this is similar to my own adverse reaction to “design”: I’d rather that biologists not use the term “design”, which smacks of teleology. A similar objection might be made to the term “creativity”. Nevertheless, in some sense, a theory of evolution must explain how new things come into existence (creativity) and how they appear to be adapted (design). So, if you are having an adverse reaction to “creativity”, then please bear in mind the possibility that there might be ways to re-frame the issues at stake, but that for now, we are going to continue to use the old language of “creativity” because that is what’s historically important.

⁵ This distinction is from Aristotle. His 4-fold taxonomy of causes includes material, efficient, formal (plans, archetypes), and final (goals, intentions) causes.

⁶ Its hardly ever clear what “random” means in such contexts. This is a topic that we will take up in a future post. The definition that is perhaps the most defensible historically is the one given by Wright, which also corresponds to Darwin’s view as discussed in Beatty, 2010. By this definition, “randomness” is not a property of mutation per se, but of its role in evolution.

⁷ In passing, note how this argument obscures where “new” things come from. In reality, new combinations of pre-existing alleles arise by sexual mixis, by the reassortment of chromosomes, and by intra-chromosomal recombination. These processes, and not future selection, bring the new combinations into existence (and may break it apart again).

⁸ This passage has been singled out by Gould, 2002 and others. Amundsen (2005) gives a brief explanation of the thinking that underlies this (in his Ch. 11).

⁹ The “opposing pressures” argument is analyzed in more detail in Yampolsky and Stoltzfus (2001).

¹⁰ In the genesis of the MS, this doctrine had no clear basis in theory or experiment. It was not considered in a rigorous way until Maynard Smith tried (and, in my opinion, failed) to justify it in 1976, long after it had become an established orthodoxy.

¹¹ This is explained in more detail in Stoltzfus (2009; see also Yampolsky and Stoltzfus, 2001).

Credits: The Curious Disconnect is the blog of evolutionary biologist Arlin Stoltzfus, available at www.molevol.org/cdblog. An updated version of the post below will be maintained at www.molevol.org/cdblog/mutationism_myth5 (Arlin Stoltzfus, ©2010)