Following the previous post summarizing Gould (1980), I decided I should probably also cover Gould (1982). As you may recall, the point of the last review was to analyze the claim that Gould called for the overthrow of the Modern Synthesis or linked punctuated equilibria with saltationist mechanisms. We may ask the same things about the paper from 1982, though I hope that such will not be the only use of this post.

To begin with, here is the summary:

The essence of Darwinism lies in the claim that natural selection is a creative force, and in the reductionist assertion that selection upon individual organisms is the locus of evolutionary change. Critiques of adaptationism and gradualism call into doubt the traditional consequences of the argument for creativity, while a concept of hierarchy, with selection acting upon such higher-level “individuals” as demes and species, challenges the reductionist claim. An expanded hierarchical theory would not be Darwinism, as strictly defined, but it would capture, in abstract form, the fundamental feature of Darwin’s vision–direction of evolution by selection at each level.

So, the point, more or less as with Gould (1980), is to question what he views as the exclusivity of adaptationism, gradualism, and reductionism inherent in the Modern Synthesis and to make a claim that an expanded, hierarchical theoretical approach is required.

After a Gouldian (i.e., somewhat esoteric and not terribly relevant, though brief) introduction, the paper continues through eight different sections.

What is Darwinism?

The meaning and appropriateness of the term “Darwinism” remain the subject of debate. I side with Scott and Branch (2009), who argue that the term is misleading and easily misused, and should never be taken as synonymous with “modern evolutionary theory”. Others disagree, especially those who believe that not that much has changed since the Modern Synthesis — nor needs to. (Creationists are particularly enamored with this term because it allows them to portray evolution as an ideology based on the views of a single individual — another very good reason to avoid endorsing it if its use provides no countervailing benefit). If anything, the term should be used only in a specific context, namely in reference to Darwin’s own views. This is how many historians use the term. However, even this narrow usage is not without challenges. As Gould notes:

Although “Darwinism” has often been equated with evolution itself in popular literature, the term should be restricted to the body of thought allied with Darwin’s own theory of mechanism, his second goal. This decision does not provide an unambiguous definition, if only because Darwin himself was a pluralist who granted pride of place to natural selection, but also advocated an important role for Lamarckian and other nonselectionist factors.

Gould then defines “Darwinism”, which he will argue is incomplete and requires “expansion”, as follows:

If we agree, as our century generally has, that “Darwinism” should be restricted to the world view encompassed by the theory of natural selection itself, the problem of definition is still not easily resolved. Darwinism must be more than the bare bones of the mechanics: the principles of superfecundity and inherited variation, and the deduction of natural selection therefrom. It must, fundamentally, make a claim for wide scope and dominant frequency; natural selection must represent the primary directing force of evolutionary change.

Gould suggests that there are two central claims in this view: 1) Natural selection is a primarily creative process, and 2) selection occurs at the level of organisms within populations. He spends time discussing them in sequence.

1) The creativity of natural selection. Darwinians cannot simply claim that natural selection operates since everyone, including Paley and the natural theologians, advocated selection as a device for removing unfit individuals at both extremes and preserving, intact and forever, the created type. The essence of Darwinism lies in a claim that natural selection is the primary directing force of evolution, in that it creates fitter phenotypes by differentially preserving, generation by generation, the best adapted organisms from a pool of random variants that supply raw material only, not direction itself. Natural selection is a creator; it builds adaptation step by step.

Thus defined, Gould then points out what he sees as three assumptions made by Darwinism in terms of the nature of variation:

(i) It must be copious since selection makes nothing directly and requires a large pool of raw material. (ii) It must be small in scope. If new species characteristically arise all at once, then the fit are formed by the process of variation itself, and natural selection only plays the negative role of executioner for the unfit. True saltationist theories have always been considered anti-Darwinian on this basis. (iii) It must be undirected. If new environments can elicit heritable, adaptive variation, then creativity lies in the process of variation, and selection only eliminates the unfit. Lamarckism is an anti-Darwinian theory because it advocates directed variation; organisms perceive felt needs, adapt their bodies accordingly, and pass these modifications directly to offspring. [Emphasis added].

He also points out two additional points that he considers part of classical Darwinian evolution, though these “are not absolute prerequisites or necessary deductive consequences”:

(i) Gradualism. If creativity resides in a step-by-step process of selection from a pool of random variants, then evolutionary change must be dominantly continuous and descendants must be linked to ancestors by a long chain of smoothly intermediate phenotypes. … (ii) The adaptationist program. If selection becomes creative by superintending, generation by generation, the continuous incorporation of favorable variation into altered forms, then evolutionary change must be fundamentally adaptive. If evolution were saltational, or driven by internally generated biases in the direction of variation, adaptation would not be a necessary attribute of evolutionary change.

So, here again Gould mentions saltationism, specifically as a challenge to adaptationism (and, obviously, gradualism). There is no doubt that he was interested in this as a possible mechanism, though in 1980 this was largely in reference to the origin of basic forms of new innovations — for example, the origin of jaws from gill arches through developmental mutations. He does not go into details at this point in the 1982 paper, but there is no mention that saltationism is part of punctuated equilibria here.

As in 1980, though much more briefly, he points out that proponents of narrow-sense Darwinism do not rule out mechanisms other than gradual adaptation but that they relegate these to exceptions of minor importance.

Then on to the second central claim:

2) Selection operates through the differential reproductive success of individual organisms (the “struggle for existence” in Darwin’s terminology). Selection is an interaction among individuals; there are no higher-order laws in nature, no statements about the “good” of species or ecosystems. If species survive longer, or if ecosystems appear to display harmony and balance, these features arise as a by-product of selection among individuals for reproductive success.

…

As a primary consequence, this focus upon individual organisms leads to reductionism, not to ultimate atoms and molecules of course, but of higher-order, or macroevolutionary, processes to the accumulated struggles of individuals. Extrapolationism is the other side of the same coin–the claim that natural selection within local populations is the source of all important evolutionary change.

To summarize, Gould defines a specific intepretation of evolution based the primacy of natural selection, and identifies what he considers its main claims and assumptions. These include the creative role of selection; the occurrence of ubiquitous, undirected, and incremental variation; gradual transformation over time, the main outcome of which is adaptation; and the emphasis on organisms in populations as the only important level of selection.

Darwinism and the Modern Synthesis

This section recounts Gould’s view that the Modern Synthesis of the 1930s and 1940s (which united Mendelian inheritance and Darwinian natural selection) began as relatively pluralistic but later hardened around an adaptationist core (see Gould 1983 for a full discussion). In this paper, as in 1980, he quotes a summary of the synthesis by Mayr (this time Mayr 1980):

The term “evolutionary synthesis” was introduced by Julian Huxley … to designate the general acceptance of two conclusions: gradual evolution can be explained in terms of small genetic changes (“mutations”) and recombination, and the ordering of this genetic variation by natural selection; and the observed evolutionary phenomena, particularly macroevolutionary processes and speciation, can be explained in a manner that is consistent with the known genetic mechanisms. [Mayr 1980, p.1].

I might interject another (in my opinion, remarkable) quote of Mayr’s in the preface to the 1998 reprint of the same book:

In a way, the synthesis was nothing but a confirmation of Darwin’s original theory, even though Darwin had published prior to the development of genetics and cytology, and had been forced to treat the origin of variation as a black box. His basic theory, that evolutionary change is due to the combination of variation and selection, was, however, completely sound and is daily confirmed by every evolutionist. This basic principle of Darwinism is as solid at the end of the twentieth century as it was in 1850, untouched by all the advances made by evolutionary biology and other branches of biology in the intervening years. [Mayr 1998, p.xiii].

I say “remarkable” because it is indeed astounding that Darwin’s basic idea of natural selection has survived so well in the face of 150 years of research. With that I certainly agree. However, it would also be remarkable if, by this paragraph, Mayr meant to claim that not much else has been added to evolutionary theory beyond natural selection. (I notice that several years ago I wrote in the margin beside this quote, simply, “Wow”).

Back to Gould. After quoting Mayr’s assessment, he argues that he is not erecting a straw man and that it actually does represent the main thrust of the synthesis. Also of note, he argues against attempts to re-brand the synthesis to incorporate new findings such that they are not seen as challenges:

The modern synthesis has sometimes been so broadly construed, usually by defenders who wish to see it as fully adequate to meet and encompass current critiques, that it loses all meaning by including everything.

Obviously, defining what the Modern Synthesis actually is will be important in current discussions about whether it needs expanding. Specifically, it is important to determine whether the “Modern Synthesis” has already been transforming gradually to incorporate all new discoveries — and if so, whether or at what point it no longer warrants description by the same name. Even anagenetic change can, at some point, be said to have led to a novel species.

What is Happening to Darwinism

This is a brief section that lays out the crux of the claim being made. Here it is in full.

Current critics of Darwinism and the modern synthesis are proposing a good deal more than a comfortable extension of the theory, but much less than a revolution. In my partisan view, neither of Darwinism’s two central themes will survive in their strict formulation; in that sense, “the modern synthesis, as an exclusive proposition, has broken down on both of its fundamental claims”. However, I believe that a restructured evolutionary theory will embody the essence of the Darwinian argument in a more abstract, and hierarchically extended form. The modern synthesis is incomplete, not incorrect. [Emphasis added].

Critique of Creativity: Gradualism

In this section, Gould clearly identifies the target of his criticism — it is not natural selection, but exclusive assumptions regarding gradualism and adaptationism:

At issue is not the general idea that natural selection can act as a creative force; the basic argument, in principle, is a sound one. Primary doubts center on the subsidiary claims–gradualism and the adaptationist program. If most evolutionary changes, particularly large-scale trends, include major nonadaptive components as primary directing or channeling features, and if they proceed more in an episodic than a smoothly continuous fashion, then we inhabit a different world from the one Darwin envisaged.

Critiques of gradualist thought proceed on different levels and have different import, but none are fundamentally opposed to natural selection. They are therefore not directed against the heart of Darwinian theory, but against a fundamental subsidiary aspect of Darwin’s own world view–one that he consistently conflated with natural selection, as in the following famous passage: “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”.

He then brings up saltationism for the second time:

At the levels of microevolution and speciation, the extreme saltationist claim that new species arise all at once, fully formed, by a fortunate macromutation would be anti-Darwinian, but no serious thinker now advances such a view, and neither did Richard Goldschmidt, the last major scholar to whom such an opinion is often attributed. Legitimate claims range from the saltational origin of key features by developmental shifts of dissociable segments of ontogeny to the origin of reproductive isolation (speciation) by major and rapidly incorporated genetic changes that precede the acquisition of adaptive, phenotypic differences. [Emphasis added].

Once again, Gould does not claim radical macromutations cause speciation (let alone that they are the mechanism of punctuated equilibria). As in 1980, he cites “saltationist” processes in two particular contexts: 1) developmental mutations of large impact that are involved in the emergence of new innovations, 2) chromosomal mutations causing reproductive isolation.

He goes on:

Are such styles of evolution anti-Darwinian? What can one say except “yes and no.” They do not deny a creative role to natural selection, but neither do they embody the constant superintending of each event, or the step-by-step construction of each major feature, that traditional views about natural selection have advocated. If new Bauplane often arise in an adaptive cascade following the saltational origin of a key feature, then part of the process is sequential and adaptive, and therefore Darwinian; but the initial step is not, since selection does not play a creative role in building the key feature. If reproductive isolation often precedes adaptation, then a major aspect of speciation is Darwinian (for the new species will not prosper unless it builds distinctive adaptations in the sequential mode), but its initiation, including the defining feature of reproductive isolation, is not.

The main point here is that “saltationism” (in the two contexts he invokes it) undermines standard interpretations because it allows non-adaptive changes to play a major role before adaptive processes take over.

Punctuated equilibria makes its first appearance in the next section. As with the 1980 article, it is mentioned in the context of discussions about macroevolutionary trends produced by species selection. Here is the full section:

At the macroevolutionary level of trends, the theory of punctuated equilibrium proposes that established species generally do not change substantially in phenotype over a lifetime that may encompass many million years (stasis), and that most evolutionary change is concentrated in geologically instantaneous events of branching speciation. These geological instants, resolvable in favorable stratigraphic circumstances (so that the theory can be tested for its proposed punctuations as well as for its evident periods of stasis), represent amounts of microevolutionary time fully consistent with orthodox views about speciation. Indeed, Eldredge and I originally proposed punctuated equilibrium as the expected geological consequence of Mayr’s theory of peripatric speciation. The non-Darwinian implications of punctuated equilibrium lie in its suggestions for the explanation of evolutionary trends (see below), not in the tempo of individual speciation events. Although punctuated equilibrium is a theory for a higher level of evolutionary change, and must therefore be agnostic with respect to the role of natural selection in speciation, the world that it proposes is quite different from that traditionally viewed by paleontologists (and by Darwin himself) as the proper geological extension of Darwinism.

Gould concludes this section by pointing out that the debate between gradualism and punctuationism is not about natural selection, but about the larger patterns of change. Specifically, in admittedly “ridiculously oversimplifed” terms, whether change is “constant” or “difficult” as a phenomenon.

Critique of Creativity: Adaptation

The message of this part of the essay will be familiar to Gould’s readers: not everything is an adaptation. (If I may, I would argue that “Not everything is an adaptation” should be one of the most commonly emphasized lines in evolution courses, up there with “Organisms do not evolve, populations do”). Gould cites neutral molecular evolution and developmental constraints as counterpoints to adaptationism. He also notes that current function may not provide much information regarding historical origin, for example because of co-option. He recognizes that these can be interpreted in adaptationist terms, for example if past adaptations determine constraints and if co-option merely involves a shift from one adaptative function to another. However, as Gould and Vrba (1982) subsequently pointed out (he cites that paper here as in press), non-adaptive features can also be exapted. As usual, it is a question of relative frequency. As he says:

Evolutionists admit, of course, that all selection yields by-products and incidental consequences, but we tend to think of these nonadaptations as a sort of evolutionary frill, a set of small and incidental modifications with no major consequences. I dispute this assessment and claim that the pool of nonadaptations must be far greater in extent than the direct adaptations that engender them.

Gould closes this section by repeating that he is not challenging natural selection, only strict adaptationism:

I do not claim that a new force of evolutionary change has been discovered. Selection may supply all immediate direction, but if highly constraining channels are built of nonadaptations, and if evolutionary versatility resides primarily in the nature and extent of nonadaptive pools, then “internal” factors of organic design are an equal partner with selection. We say that mutation is the ultimate source of variation, yet we grant a fundamental role to recombination and the evolution of sexuality–often as a prerequisite to multicellularity, the Cambrian explosion and, ultimately, us. Likewise, selection may be the ultimate source of evolutionary change, but most actual events may owe more of their shape to its nonadaptive sequelae.

Is Evolution a Product of Selection Among Individuals?

As noted in the abstract, one of Gould’s major arguments is that evolutionary theory should undergo an expansion to include a hierarchical outlook. This includes natural selection as a primary mechanism, but one that takes place at several levels:

I believe that the traditional Darwinian focus on individual bodies, and the attendant reductionist account of macroevolution, will be supplanted by a hierarchical approach recognizing legitimate Darwinian individuals at several levels of a structural hierarchy, including genes, bodies, demes, species, and clades.

He briefly mentions the idea of “selfish DNA”, which provides a convincing example of selection within genomes. I agree (Gregory 2004, 2005). Species selection is more complex, but one of the basic requirements is that it must be possible to view species as “individuals” with births, lifespans, differential reproduction, and deaths. It is here that punctuated equilibria comes in, because it provides species with a comparatively short “gestation” and birth, a potentially long lifespan of relative stasis, differential production of daughter species by cladogenesis, and finally extinction. As Gould states:

If new species usually arose by the smooth transformation of an entire ancestral species, and then changed continuously toward a descendant form, they would lack the stability and coherence required for defining evolutionary individuals. The theory of punctuated equilibrium allows us to individuate species in both time and space; this property (rather than the debate about evolutionary tempo) may emerge as its primary contribution to evolutionary theory.

He then points out that having the properties of “individuals” does not guarantee species selection. Indeed, there was substantial debate among macroevolutionary theorists about what would count as species selection and what could be reduced to organism-level selection (e.g., Vrba 1983, 1989; Lloyd and Gould 1993). Nevertheless, he argues:

We need not, however, confine ourselves to the simple fact of individuation as an argument against Darwinian reductionism. For the strong claim that higher-level individuals act as units of selection in their own right can often be made. Many evolutionary trends, for example, are driven by differential frequency of speciation (the analog of birth) rather than by differential extinction (the more usual style of selection by death). Features that enhance the frequency of speciation are often properties of populations, not of individual organisms, for example, dependence of dispersal (and resultant possibilities for isolation and speciation) on size and density of populations.

There has been quite a bit written about population- and species-level properties, including their heritability, since this paper (e.g., Jablonski 1987; Hunt et al. 2005). Still, the main point is clear enough. There are also shades of the distinction between “sorting” (a pattern of differential survival and reproduction) and “selection” (one cause of sorting), which was introduced more explicitly later (Gould and Vrba 1986). Finally, Gould responds to some common criticisms of species selection as an important phenomenon.

Evolutionary Pattern by Interaction Between Levels

Gould notes that a hierarchical approach would not imply that all levels are disconnected from one another. Rather, interactions among levels might be responsible for some of the patterns that are seen:

The hierarchical model, with its assertion that selection works simultaneously and differently upon individuals at a variety of levels, suggests a revised interpretation for many phenomena that have puzzled people where they implicitly assumed causation by selection upon organisms. In particular, it suggests that negative interaction between levels might be an important principle in maintaining stability or holding rates of change within reasonable bounds.

He once again mentions “selfish DNA”, which shows interaction between intragenomic and organism-level selection. He also brings up “overspecialization”, in which adaptation that results from organism-level selection can lead to shorter species longevity. (For a recent example, see Van Valkenburgh et al. 2004). In addition to antagonism between levels, Gould proposes that sometimes selection on more than one level can lead to, and enhance, the same result. Finally, he suggests that sometimes a feature can be subject to selection at a higher level that is not impeded at the lower level, thereby making it widespread among extant lineages (such as sexual reproduction, which delays extinction and accelerates speciation).

He concludes:

We live in a world with reductionist traditions, and do not react comfortably to notions of hierarchy. Hierarchical theories permit us to retain the value of traditional ideas, while adding substantially to them. They traffic in accretion, not substitution. If we abandoned the “either-or” mentality that has characterized arguments about units of selection, we would not only reduce fruitless and often acrimonious debate, but we would also gain a deeper understanding of nature’s complexity through the concept of hierarchy.

A Higher Darwinism?

The final section of the essay highlights Gould’s notion of a hierarchical theory, which he says is neither a minor extension of standard theory nor a revolution. This is because it is not a simple addition to selection at the organism level, but neither does it invoke any new general mechanisms. It is a matter expanding the application of well-understood processes:

What would a fully elaborated, hierarchically based evolutionary theory be called? It would neither be Darwinism, as usually understood, nor a smoothly continuous extension of Darwinism, for it violates directly the fundamental reductionist tradition embodied in Darwin’s focus on organisms as units of selection.

…

… the hierarchically based theory would not be Darwinism as traditionally conceived; it would be both a richer and a different theory. But it would embody, in abstract form, the essence of Darwin’s argument expanded to work at each level. Each level generates variation among its individuals; evolution occurs at each level by a sorting out among individuals, with differential success of some and their progeny. The hierarchical theory would therefore represent a kind of “higher Darwinism,” with the substance of a claim for reduction to organisms lost, but the domain of the abstract “selectionist” style of argument extended.

Moreover, selection will work differently on the objects of diverse levels. The phenomena of one level have analogs on others, but not identical operation. For example, we usually deny the effectiveness of mutation pressure at the level of organisms. Populations contain so many individuals that small biases in mutation rate can rarely establish a feature if it is under selection at all. But the analog of mutation pressure at the species level, directed speciation (directional bias toward certain phenotypes in derived species), may be a powerful agent of evolutionary trends (as a macroevolutionary alternative to species selection). Directed speciation can be effective (where mutation pressure is not) for two reasons: first, because its effects are not so easily swamped (given the restricted number of species within a clade) by differential extinction; second, because such phenomena as ontogenetic channeling in phyletic size increase suggest that biases in the production of species may be more prevalent than biases in the genesis of mutations.

Each level must be approached on its own, and appreciated for the special emphasis it places upon common phenomena, but the selectionist style of argument regulates all levels and the Darwinian vision is extended and generalized, not defeated, even though Darwinism, strictly constructed, may be superseded.

He closes by quoting and praising Darwin.

So, what can we take away from the essay? In my reading, Gould is arguing for a revision to what he considers a narrow view of Darwinian processes. There is no call for revolution, though his hierarchical view is more than a simple tweaking of the synthesis.

We are in a similar position today, I think. There are many mechanisms that extend beyond the borders of the theory developed 70 years ago. However, the basics of the synthesis are solid.

The question is whether these new phenomena can be included in something that would still reasonably be called the “Modern Synthesis”, or whether we should recognize another major transition (and probably not the final one) in the evolution of evolutionary theory. Like Gould, I believe most who favour the idea of an “Extended Synthesis” view it as a major step — but not a giant leap.

_______________

References

Scott, E.C. and G. Branch (2009). Don’t call it “Darwinism”. Evolution: Education and Outreach 2: 90-94.

Gould, S.J. (1980). Is a new and general theory of evolution emerging? Paleobiology 6: 119-130.

Gould, S.J. (1982). Darwinism and the expansion of evolutionary theory. Science 216: 380-387.

Gould, S. J. (1983). The hardening of the modern synthesis. In Dimensions of Darwinism , ed. by M. Grene, Cambridge University Press, Cambridge, pp. 71-93.

Gould, S.J. and E. Vrba (1982). Exaptation–a missing term in the science of form. Paleobiology 8: 4-15.

Gregory, T.R. (2004). Macroevolution, hierarchy theory, and the C-value enigma. Paleobiology 30: 179-202.

Gregory, T.R. (2005). Macroevolution and the genome. In The Evolution of the Genome ed. by T.R. Gregory, Elsevier, San Diego, pp. 679-729.

Hunt, G., K. Roy, and D. Jablonski (2005). Heritability of geographic range sizes revisited. American Naturalist 166: 129-135.

Jablonski, D. (1987). Heritability at the species level: analysis of geographic ranges of Cretaceous mollusks. Science 238: 360-363.

Lloyd, E.A. and Gould, S.J. (1993). Species selection on variability. Proceedings of the National Academy of Sciences USA 90: 595-599.

Mayr, E. (1980). Prologue: Some thoughts on the history of the evolutionary synthesis. In The Evolutionary Synthesis, ed. by E. Mayr and W.B. Provine. Harvard University Press, Cambridge, pp. 1-48.

Mayr, E. (1998). Preface, 1998. In The Evolutionary Synthesis, ed. by E. Mayr and W.B. Provine. Harvard University Press, Cambridge, pp. ix-xiii.

Van Valkenburgh, B., X. Wang, and J. Damuth (2004). Cope’s Rule, hypercarnivory, and extinction in North American canids. Science 306: 101-104.

Vrba, E.S. (1983). Macroevolutionary trends: new perspectives on the roles of adaptation and incidental effect. Science 221: 387-389.

Vrba, E.S. (1989). Levels of selection and sorting with special reference to the species level. Oxford Surveys in Evolutionary Biology 6: 111-168.

Vrba, E.S. and Gould, S.J. (1986). The hierarchical expansion of sorting and selection: sorting and selection cannot be equated. Paleobiology 12: 217-228.

As a follow up to the discussion of Gould (1980), and perhaps as a prelude to a similar post on Gould (1982) (if I get time and feel inclined), here is a convenient excerpt from Gould (2002) on this subject:

Punctuated equilibrium’s threefold history

The “Urban Legend” of Punctuated Equilibrium’s Threefold History: The opponents of punctuated equilibrium have constructed a fictional history of the theory, primarily (I suppose) as a largely unconscious expression of their hope for its minor importance […] This supposed threefold history of punctuated equilibrium also ranks about as close to pure fiction as any recent commentary by scientists has ever generated. In stage one, the story goes, we were properly modest, obedient to the theoretical hegemony of the Modern Synthesis, and merely trying to bring paleontology into the fold. But the prospect of worldly fame beguiled us, so we broke our ties of fealty and tried, in stage two, to usurp power by painting punctuated equilibrium as a revolutionary doctrine that would dethrone the Synthesis, resurrect the memory of the exiled martyr (Richard Goldschmidt), and reign over a reconstructed realm of theory. But we were too big for our breeches, and the old guard still retained some life. They fought back mightily and effectively, exposing our bombast and emptiness. We began to hedge, retreat, and apologize, and have been doing so ever since in an effort to regain grace and, chastened in stage three, to sit again, in heaven or Valhalla, with the evolutionary elite.

…

In particular, and most offensive to me, the urban legend rests on the false belief that radical, “middle-period” punctuated equilibrium became a saltational theory wedded to Goldschmidt’s hopeful monsters as a mechanism. I have labored to refute this nonsensical charge from the day I first heard it. But my efforts are doomed within the self-affirming structure of the urban legend. We all know, for so the legend proclaims, that I once took the Goldschmidtian plunge. So if I ever deny the link, I can only be retreating from an embarrassing error. And if I, continue to deny the link with force and gusto, well, then I am only backtracking even harder (into stage 3) and apologizing (or obfuscating) all the more. How about the obvious (and accurate) alternative: that we never made the Goldschmidtian link; that this common error embodies a false construction; and that our efforts at correction have always represented an honorable attempt to relieve the confusion of others.

[Read the rest here].

Based on several recent discussions in the blogosphere, I decided to re-read Stephen Jay Gould’s paper entitled “Is a new and general theory of evolution emerging?”, published in 1980. This is one of Gould’s most (in)famous papers, but one that I am convinced most people have not read carefully, or at least have not understood. It’s the one that is usually cited when Gould is accused of calling for a rejection of natural selection or population genetics. Apparently it is also the one that caused many people to think that punctuated equilibria was a saltationist theory. In this post, I will quote extensively from the paper, with some thoughts and opinions interjected throughout.

To start, here is the abstract:

The modern synthesis, as an exclusive proposition, has broken down on both of its fundamental claims: extrapolationism (gradual allelic substitution as a model for all evolutionary change) and nearly exclusive reliance on selection leading to adaptation. Evolution is a hierarchical process with complementary, but different, modes of change at its three major levels: variation within populations, speciation, and patterns of macroevolution. Speciation is not always an extension of gradual, adaptive change to greater effect, but may represent, as Goldschmidt argued, a different style of genetic change–rapid reorganization of the genome, perhaps non-adaptive. Macroevolutionary trends do not arise from the gradual, adaptive transformation of populations, but usually from higher-order selection operating upon groups of species, while the individual species themselves generally do not change following their geologically instantaneous origin. I refer to these two discontinuities in the evolutionary hierarchy as the Goldschmidt break (between change in populations and speciation) and the Wright break (between speciation and trends as differential success among species).
A new and general evolutionary theory will embody this notion of hierarchy and stress a variety of themes either ignored or explicitly rejected by the modern synthesis: punctuational change at all levels, important non-adaptive change at all levels, control of evolution not only by selection, but equally by constraints of history, development and architecture–thus restoring to evolutionary theory a concept of organism. [Emphasis added]

The paper proper is divided into six parts. I will work through each in sequence, noting areas where I think confusion or misquotation have distorted the intended arguments.

I. The Modern Synthesis

Gould begins by quoting what he calls “one of the last skeptical books written before the Darwinian tide of the modern synthesis asserted its hegemony”, by Robson and Richards (1936):

The theory of Natural Selection … postulates that the evolutionary process is unitary, and that not only are groups formed by the multiplication of single variants having survival value, but also that such divergences are amplified to produce adaptations (both specializations and organization). It has been customary to admit that certain ancillary processes are operative (isolation, correlation), but the importance of these, as active principles, is subordinate to selection. [Robson and Richards 1936, p.370-371].

He then summarizes the core of the Modern Synthesis as follows:

Its foundation rests upon two major premises: (1) Point mutations (micromutations) are the ultimate source of variability. Evolutionary change is a process of gradual allelic substitution within a population. Events at broader scale, from the origin of new species to long-ranging evolutionary trends, represent the same process, extended in time and effect–large numbers of allelic substitutions incorporated sequentially over long periods of time. In short, gradualism, continuity, and evolutionary change by the transformation of populations. (2) Genetic variation is the raw material only. Natural selection directs evolutionary change. Rates and directions of change are controlled by selection with little constraint exerted by material (slow rates are due to weak selection, not insufficient variation). All genetic change is adaptive (though some phenotypic effects, due to pleiotropy, etc., may not be). In short, selection leading to adaptation. [Gould 1980, p.119-120.]

As anyone reading this surely knows, there are exceptions to all of these assumptions. Of course, proponents of the Modern Synthesis have always known this too. But, as Gould points out:

All the synthesists recognized exceptions and “ancillary processes,” but they attempted to both prescribe a low relative frequency for them and to limit their application to domains of little evolutionary importance. Thus, genetic drift certainly occurs–but only in populations so small and so near the brink that their rapid extinction will almost certainly ensue. And phenotypes include many non-adaptive features by allometry and pleiotropy, but all are epiphenomena of primarily adaptive genetic changes and none can have any marked effect upon the organism (for, if inadaptive, they will lead to negative selection and elimination and, if adaptive, will enter the model in their own right). Thus, a synthesist could always deny a charge of rigidity by invoking these official exceptions, even though their circumscription, both in frequency and effect, actually guaranteed the hegemony of the two cardinal principles. [Gould 1980, p.120].

Recognizing that he might be accused of erecting a straw man, he then quotes directly from Mayr (1963), one of the architects of the Modern Synthesis:

The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species. [Mayr 1963, p.586, emphasis added].

Referring specifically to this characterization of the synthesis, Gould wrote the following paragraph which, perhaps more than any of his other writings, raised the ire of defenders of the synthesis:

I well remember how the synthetic theory beguiled me with its unifying power when I was a graduate student in the mid-1960’s. Since then I have been watching it slowly unravel as a universal description of evolution. The molecular assault came first, followed quickly by renewed attention to unorthodox theories of speciation and by challenges at the level of macroevolution itself. I have been reluctant to admit it–since beguiling is often forever–but if Mayr’s characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy. [Gould 1980, p.120].

So, was it textbook orthodoxy, or was Gould making this up? I don’t have any textbooks from 1980, but here’s what Freeman and Herron (2007, p.96) state in one of the most widely used evolution textbooks:

…the Modern Synthesis, or the Evolutionary Synthesis, was a consensus grounded in two propositions:

• Gradual evolution results from small genetic changes that are acted upon by natural selection.
• The origin of species and higher taxa, or macroevolution, can be explained in terms of natural selection acting on individuals, or microevolution.

Nevertheless, critics construed Gould’s statement about the death of the Modern Synthesis as characterized by Mayr to mean that he was advocating a total overthrow of evolutionary theory. Here is how Charlesworth et al. (1982) quoted the passage above:

I have been watching it [neo-Darwinism] slowly unravel as a universal description of evolution … I have been reluctant to admit it … but … that theory, as a general proposition, is effectively dead, despite its persistence as a text-book orthodoxy.

I suspect that this is the version most people remember. But here is the full quote again, with the cherry picked bits indicated:

I well remember how the synthetic theory beguiled me with its unifying power when I was a graduate student in the mid-1960’s. Since then I have been watching it slowly unravel as a universal description of evolution. The molecular assault came first, followed quickly by renewed attention to unorthodox theories of speciation and by challenges at the level of macroevolution itself. I have been reluctant to admit it–since beguiling is often forever–but if Mayr’s characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy.

What Gould argues in Part I is simply that the Modern Synthesis, as it had developed by 1980, had a rather narrow focus on small-scale variation and natural selection, and assumed that most major evolutionary patterns could be explained by extrapolating processes within populations into deep time. What he was rejecting was this very narrow interpretation, though later critics gave the impression that he was challenging all aspects of the synthesis and calling for a revolution in evolutionary theory.

II. Reduction and Hierarchy

In this section, Gould highlights the inherent conflict between reductionism/extrapolationism and a theory involving hierarchy and emergence. As he put it:

The general alternative to such reductionism is a concept of hierarchy–a world constructed not as a smooth and seamless continuum, permitting simple extrapolation from the lowest level to the highest, but as a series of ascending levels, each bound to the one below it in some ways and independent in others. Discontinuities and seams characterize the transitions; “emergent” features not implicit in the operation of proceses at lower levels, may control events at higher levels. The basic processes–mutation, selection, etc.–may enter into explanations at all scales (and in that sense we may still hope for a general theory of evolution), but they work in different ways on the characteristic material at divers[e] levels [Gould 1980, p.121].

He then gives an example from the molecular level, including the notion of gene regulation (versus purely reductionistic genetics), repetitive DNA, transposable elements, and developmental regulatory genes of large effect. Thus, he says:

We may find, for example, that structural gene substitutions control most small-scale, adaptive variation within local populations, while disruption of regulation lies behind most key innovations in macroevolution. [Gould 1980, p.121].

He concludes this section by noting:

The modern synthesis drew most of its direct conclusions from studies of local populations and their immediate adaptations. It then extrapolated the postulated mechanism of these adaptations–gradual, allelic substitution–to encompass all larger-scale events. The synthesis is now breaking down on both sides of this argument. Many evolutionists now doubt exclusive control by selection upon genetic change within local populations. Moreover, even if local populations alter as the synthesis maintains, we now doubt that the same style of change controls events at the two major higher levels: speciation and patterns of macroevolution. [Gould 1980, p.121].

The importance of structural mutations versus regulatory mutations is a topic of substantial debate in current evolutionary biology, so I think Gould was on the mark here. We also have learned a lot more about the nature of eukaryote genomes, which are far more complex than Gould imagined nearly 30 years ago.

III. A Note on Local Populations and Neutrality

In this section, Gould takes a brief detour to emphasize the importance of neutral change at the molecular level–something that was largely lost from the Modern Synthesis as it grew to strongly emphasize selection. He begins by noting that there is more genetic variation in populations than was expected by the Modern Synthesis. One reason is that frequency dependent selection or selection in response to mildly fluctuating environments can maintain polymorphisms (versus directional or stabilizing selection, which both eliminate variation). More importantly, he argues that this reflects a much stronger than anticipated role for genetic drift and the predominance of neutral mutations. Again, he does not take this as a reason to overthrow all aspects of the synthesis:

None of this evidence, of course, negates the role of conventional selection and adaptation in molding parts of the phenotype with obvious importance for survival and reproduction. Still, it rather damps Mayr’s enthusiastic claim for “all evolution … guided by natural selection”. The question, as with so many issues in the complex science of natural history, becomes one of relative frequency. Are the Darwinian substitutions merely a surface skin on a sea of variation invisible to selection, or are the neutral substitutions merely a thin bottom layer underlying a Darwinian ocean above? Or where in between? [Gould 1980, p.122].

In summary, neutral processes are not a challenge to the importance of Darwinian adaptation, only to its exclusivity. Acknowledging a large role for neutral processes at the molecular and population levels would challenge the Modern Synthesis to the extent that it is based on an assumption that selection is the dominant mechanism at all scales and that chance enters only at the level of providing small mutations on which selection may act.

IV. The Level of Selection and the Goldschmidt Break

Gould begins this section by noting that speciation usually cannot be observed or replicated in the lab, and that therefore models of speciation are typically based on analogy and inference. He points out that Darwin drew an analogy with domestication, and considered subspecies as incipient species that gradually evolve into species through natural selection. Mayr’s allopatric speciation model challenged this, of course, but Gould suggests that it still maintained two major principles based on smooth extrapolationism:

(i) The accumulating changes that lead to speciation are adaptive. Reproductive isolation is a consequence of sufficient accumulation. (ii) Although aided by founder effects and even (possibly) by drift, although dependent upon isolation from gene flow, although proceeding more rapidly than local differentiation within large populations, successful speciation is still a cumulative and sequential process powered by selection through large numbers of generations. It is, if you will, Darwinism a little faster. [Gould 1980, p.122].

Gould then notes, “I do not doubt that many species originate in this way; but it now seems that many, perhaps most, do not.” He argues that several new (in 1980) models of speciation challenge the extrapolationist explanation. Some of these involve major chromosomal changes. I believe this is why he is often accused of proposing punctuated equilibria as a saltationist theory, but then of retreating when he was critcized for this view.

Gould’s list of challenges to “allopatric orthodoxy” relate to the amount of gene flow within species and especially to the plausibility of sympatric speciation. He argues that the assumption that gene flow homogenizes large populations (thereby making isolation necessary) may not be valid, and that instead local demes may be distinct enough for speciation. If that is the case, then the distinction between allopatric and sympatric speciation is oversimplified and sympatric speciation is much more likely than acknowledged. Strong selection or the rapid fixation of major chromosomal variations could underlie such processes. Here is where Gould first talks about rapid chromosome-level isolation:

The most exciting entry among punctuational models for speciation in ecological time is the emphasis, now coming from several quarters, on chromosomal alterations as isolating mechanisms–sometimes called chromosomal speciation. In certain population structures, particularly in very small and circumscribed groups with high degrees of inbreeding, major chromosomal changes can rise to fixation in less than a handful of generations [Gould 1980, p.123].

He uses the word “punctuational”, but he does not mention “punctuated equilibria” at all or cite any of his paper on the subject, and indeed what he is describing — mechanisms of rapid sympatric speciation — has little to do with punctuated equilibria, which is about peripheral isolates. As far as saltational mechanisms are concerned, their challenge to orthodoxy is that they undermine the emphasis on selection acting on small mutations as the dominant mechanism. He continues:

The control of evolution by selection leading to adaptation lies at the heart of the modern synthesis. Thus, reproductive isolation, the definition of speciation, is attained as a by-product of adaptation–that is, a population diverges by sequential adaptation and eventually becomes sufficiently different from its ancestor to foreclose interbreeding … But in saltational, chromosomal speciation, reproductive isolation comes first and cannot be considered as an adaptation at all. It is a stochastic event that establishes a species by the technical definition of reproductive isolation. To be sure, the later success of this species in competition may depend upon its subsequent acquisition of adaptations; but the origin itself may be non-adaptive. We can, in fact, reverse the conventional view and argue that speciation, by forming new entities stochastically, provides raw material for evolution. [Gould 1980, p.124].

Gould then notes that Richard Goldschmidt argued, while the synthesis was being constructed, that there is a break between what happens in populations and what happens when new species originate. He calls this the “Goldschmidt break”. Gould ends this section by quoting Goldschmidt (1940), though not in relation to any particular ideas about mutation or saltationism, only to say that extrapolation from short-term population processes is not enough to explain all speciation:

The characters of subspecies are of a gradient type, the species limit is characterized by a gap, an unbridged difference in many characters. This gap cannot be bridged by theoretically continuing the subspecific gradient or cline beyond its actually existing limits. The subspecies do not merge into the species either actually or ideally … Microevolution by accumulation of micromutations–we may also say neo-Darwinian evolution–is a process which leads to diversification strictly within the species, usually, if not exclusively, for the sake of adaptation of the species to specific conditions within the area which it is able to occupy … Subspecies are actually, therefore, neither incipient species nor models for the origin of species. They are more or less diversified blind alleys within the species. The decisive step in evolution, the first step towards macroevolution, the step from one species to another, requires another evolutionary method than that of the sheer accumulation of micromutations. [Goldschmidt 1940, p.183].

One can certainly disagree with Goldschmidt that subspecies are never incipient species, or with Gould for quoting this, but one cannot argue that Gould endorsed Goldschmidt’s particular genetic views nor linked these to punctuated equilibria in any way in this section.

V. Macroevolution and the Wright Break

In this section, Gould moves from mechanisms of speciation to large-scale macroevolutionary trends. He opens by summarizing the interpretation of macroevolutionary patterns given by the Modern Synthesis, namely that they occur gradually and with branching as an afterthought (i.e., cladogenesis subordinate to anagenesis). Moreover, most of this gradual transformation is an adaptive outcome of population-level selection.

Gould presents several counterpoints to this extrapolationist model. First, he notes that models of speciation emphasizing branching and stasis suggest that higher-level trends may actually result from sorting at the species level rather than as an extension of selection at the population level. This is where punctuated equilibria comes in, and I think it is worth noting that he does not link this to saltational mechanisms. In fact, he explicitly notes that any model of speciation that emphasized comparatively rapid speciation followed by stasis would have the same implications:

We regard stasis and discontinuity as an expression of how evolution works when translated into geological time. Large, successful central populations undergo minor adaptive modifications of fluctuating effect through time (Goldschmidt’s “diversified blind alleys”), but they will rarely transform in toto to somethign fundamentally new. Gradual change is not the normal state of a species. Speciation, the basis of macroevolution, is a process of branching. And this branching, under any current model of speciation–conventional allopatry to chromosomal saltation–is so rapid in geological translation (thousands of years at most compared with millions for the duration of most fossil species) that its results should generally lie on a bedding plane, not through the thick sedimentary sequence of a long hillslope. (The expectation of gradualism emerges as a kind of double illusion. It represents, first of all, an incorrect translation of conventional allopatry. Allopatric speciation seems so slow and gradual in ecological time that most paleontologists never recognized it as a challenge to the style of gradualism–steady change over millions of years–promulgated by custom as a model for the history of life. But it now appears that “slow” allopatry itself may be less important than a host of alternatives that yield new species rapidly even in ecological time). Thus, our model of “punctuated equilibria” holds that evolution is concentrated in events of speciation and that successful speciation is an infrequent event punctuating the statis of large populations that do not alter in fundamental ways during the millions of years that they endure. [Gould 1980, p.125].

Again, the main point is that trends at the macroevolutionary scale may represent the outcome of differential survival and reproduction (speciation) of species. This would make macroevolutionary trends at least partly distinct from microevolutionary processes. Gould notes that Sewall Wright emphasized this, which is why he calls this the “Wright break”. This is the one place that punctuated equilibria is discussed, specifically in light of its implications for species selection.

The second major issue that Gould then raises is that sometimes “saltational” changes probably do happen. Not at the level of normal speciation events, but in major innovations. (Again, this would have nothing to do with punctuated equilibria, which is strictly about the patterns of speciation). He also does not mean that whole new structures appear suddenly. As he explains:

I do not refer to the saltational origin of entire new designs, complete in all their complex and integrated features–a fantasy that would be truly anti-Darwinian in denying any creativity to selection and relegating it to the role of eliminating old models. Instead, I envisage a potential saltational origin for the essential features of key adaptations. [Gould 1980, p.127].

In general, this sort of “saltation” would involve major developmental changes that could be caused by small mutations of regulatory genes. Gould rejects Goldschmidt’s hypothesis of “systemic mutations”, but suggests that developmental regulatory mutations could sometimes lead to the saltational origin of basic forms of new features. This is a topic that is widely discussed today, so again, I think Gould was basically right to raise this point. In fact, the example he gives is of the origin of the jaw from gill arches, which he argues could have been “saltational” in the sense that it occurred quickly through a developmental mutation. Interestingly, a recent paper by Shigetani et al. (2002) suggests that this transition did indeed involve a developmental mutation, specifically a heterotopic shift in gene expression.

The third major challenge to extrapolationism in explaining macroevolutionary patterns that Gould presents has to do with non-adaptive processes. This follows from his classic paper with Richard Lewontin, published the year before (Gould and Lewontin 1979). Here, he revisits the points about there being more variation in populations than adaptationist models should expect, and that speciation by chromosomal mechanisms could be non-adaptive. He then moves on to discuss the relevance of large-scale trends that are not caused by population-level selection. As he says:

… if trends represent a higher-level process of differential origin and mortality among species, then a suite of potentially non-adaptive explanations must be considered. Trends, for example, may occur because some kinds of species tend to speciate more often than others. This tendency may reside in the character of environments or in attributes of behavior and population structure bearing no relationship to morphologies that spread through lineages as a result of higher speciation rates among some of their members. Or trends may arise from the greater longevity of certain kinds of species. Again, this greater persistence may have little to do with the morphologies that come to prevail as a result. [Gould 1980, p.128].

The point, yet again, is that the Modern Synthesis as described by Mayr — which emphasized gradual, adaptive change as the cause of macroevolution — is undermined by other processes at both higher and lower levels. Gould is not arguing that the synthesis is incorrect or that it needs to be overthrown, only that it is unduly narrow and needs to be expanded. Many of the issues he raises here are still under debate, though they are still dismissed by some who argue that the synthesis is more or less complete.

VI. Quo Vadis?

In this concluding section, Gould is clear about what he is arguing:

I think I can see what is breaking down in evolutionary theory–the strict construction of the modern synthesis with its belief in pervasive adaptation, gradualism, and extrapolation by smooth continuity from causes of change in local populations to major trends and transitions in the history of life. [Gould 1980, p.128].

Here is what he sees as the features of a revised evolutionary theory:

• It will be hierarchical in nature. This doesn’t mean that each level is totally disconnected, only that we will have to recognize the operation of (similar) mechanisms at each level.
• It will include punctuational patterns at all levels, not only gradual change.
• It will emphasize the role of constraints in shaping phenotypes.
• It will properly recognize the input of non-adaptive processes at all levels.
• It will link insights from paleontology, developmental genetics, and molecular biology with those derived from population studies.

Overall, the paper seems quite mild in retrospect. Gould did not call for a revolution in evolutionary biology, nor did he transform punctuated equilibria into a fundamentally radical mode of speciation. He merely argued against the features of the Modern Synthesis that its proponents themselves outlined. Many of the arguments he raised were ahead of their time but are now common in evolutionary discourse. This is especially true given the enormous growth of biological information in the nearly three decades since this paper was published.

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References

Charlesworth, B., R. Lande, and M. Slatkin. 1982. A neo-Darwinian commentary on macroevolution. Evolution 36: 474-498.

Freeman, S. and J.C. Herron. 2007. Evolutionary Analysis (Fourth Edn). Prentice Hall, Upper Saddle River, NJ.

Goldschmidt, R. 1940. The Material Basis of Evolution. Yale University Press, New Haven.

Gould, S.J. and R.C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London B 205: 581-598.

Gould, S.J. 1980. Is a new and general theory of evolution emerging? Paleobiology 6: 119-130.

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

Robson, G.C. and O.W. Richards. 1936. The Variation of Animals in Nature. Longmans, Green, and Co., London.

Shigatani, Y., F. Sugahara, Y. Kawakami, Y. Murakami, S. Hirano, and S. Kuratani. 2002. Heterotopic shift of epithelial-mesenchymal interactions in vertebrate jaw evolution. Science 296: 1316-1319.

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Update — See also Gould (1982).

As we celebrate Darwin Year, we do well to remember that evolutionary theory has come a very long way since Darwin proposed some of the core ideas 150 years ago. For this reason, many biologists feel that the term “Darwinism” as shorthand for “modern evolutionary theory” is misleading historically and scientifically. Darwin got many things right, most notably the fact of common descent and the process of natural selection, but he did not — and could not — assemble a comprehensive theory of evolution based on what he knew at the time.

One glaring gap in Darwin’s brilliant writings, of course, was the total lack of information about the source of variation upon which natural selection acts. Later, when Mendelian genetics was (re)discovered, this was thought by many geneticists to pose a challenge to the operation of natural selection. Indeed, although Darwin had established the fact of evolution in his own time, his theory of natural selection did not take hold until decades later, and only after significant debate. Other possible mechanisms, including orthogenesis, mutationism, and neo-Lamarckism, were favoured by many evolutionists in the early 1900s, and it was only after the “Modern Synthesis” of the 1930s and 1940s that these were rejected and Mendelian genetics and Darwinian natural selection were unified.

Just as it is useful to consider what Darwin did not know, it is worth considering what was not yet known at the time of the “Modern Synthesis”. Here is a short list:

• DNA is the hereditary material.
• DNA has a double helix structure.
• Protein-coding genes use a triplet codon mechanism.
• Genes can be alternatively spliced to produce multiple protein products.
• Most of the DNA in eukaryotic genomes is not genes.
• Even complex organisms like humans have a relatively small number of genes (~20K).
• The majority of DNA in large genomes is transposable elements (genomic parasites).
• Genome duplications have occurred in many lineages.
• Much (if not most) evolution at the molecular level is neutral.
• Genes can be exchanged among even distantly related lineages.
• Development is regulated in part by a series of genes of major effect.
• Species that look similar may be very different genetically. Species that look very different may be very similar genetically.
• Differences in number of copies of genes may be an important source of variation.
• There have been several major mass extinctions over the past 600 million years.
• Survival through mass extinctions may be non-random, or at least may differ from survival of species during normal circumstances.
• Fossil data suggest that stasis is typical of many lineages, punctuated by relatively rapid speciation events.
• Natural selection operates at multiple levels, including within the genome.
• Non-genetic mechanisms (epigenetics) can influence development and be inherited.

There are two ways to consider this list. One is to say that fitting these into the Modern Synthesis is not difficult, and that what we have thought about evolution since the 1940s is still pretty much correct and comprehensive. The other is to have a conversation among experts from traditionally very disparate disciplines (genetics, genomics, developmental biology, paleontology) and ask: Do we need to expand our understanding of evolution to accommodate all of this new information? Jerry Coyne is one of the people who holds the first view, that everything is pretty much worked out — these are just dots on i’s and crosses on t’s. Not surprisingly, he is rather critical of those who are open to the second view. Most recently, he has gone after Carl Zimmer for writing this:

In the mid-1900s, biologists succeeded in merging the newest biological developments at the time into a new vision of evolution known as the Modern Synthesis. Today a number of biologists argue that it’s time for a new understanding of evolution, one that Pigliucci has called the Extended Evolutionary Synthesis. For now, they are fiercely debating every aspect of that synthesis–how important gene-swapping is to the course of evolution, for instance, and how gene networks get rewired to produce new traits.

Some researchers argue that many patterns of nature–such as the large number of species in the tropics–cannot be reduced to the effect of natural selection on individuals. They may be following rules of their own. “Which of these ideas is going to actually survive and prove fruitful is anybody’s guess,” says Pigliucci. “I don’t see things coalescing for at least a decade or more.”

Now, I tend to agree with Pigliucci on this point. I spoke at a conference called “Extending the Synthesis” in Leiden a number of years ago along with paleontologists, a developmental biologist, an ecologist, and an experimental evolutionary biologist. Pigliucci organized a similar symposium (with some of the same individuals) more recently. The point is that many people feel that we need to have a conversation about how well the “Modern Synthesis” covers these phenomena, and whether we need to expand it to include other components (say, non-genic sources of diversity, multi-level selection, or large-scale changes caused by developmental mutations).

To be fair to Coyne, he is right that declaring a revolution with each new discovery is nonsense. Much of the hype about epigenetics (especially mis-labeling it as Lamarckian) falls into this category, in my view. As he put it:

We have surely gone way beyond Darwin in our understanding of the pattern and process of evolution. But I am irritated by the constant appearance of what I call “BIS”–the Big Idea Syndrome. An evolutionist finds a new phenomenon, say transposable elements, or epigenetics, or “modularity,” and suddenly that one phenomenon becomes the centerpiece of a claim that modern evolutionary theory is ripe for a revolution. Yet when you look for the beef, it isn’t there. Where are all the examples of genetic assimilation, a phenomenon that was said to completely overturn our views?

The point is that it’s not one phenomenon. It’s phenomena from several very distant fields in evolutionary biology.

I once encountered a similar sentiment from a reviewer:

This paper exemplifies what might be called the “molecular geneticists’ fallacy” about the causes of evolution – that knowing the details of the molecular basis of mutational variation will radically change our view of how evolution works.

My response is that knowing the genetic details may have little bearing on how one understands microevolution — you can easily consider a genome duplication an “allele” — but it can have major implications for understanding macroevolutionary phenomena (did the diversification of teleost fishes depend on a genome duplication event?). That’s the whole point. We need to be willing to consider these phenomena from a variety of perspectives, not just population processes.

An extended synthesis would not involve an overthrow of current theory (hence, “extended”). It would represent an effort to incorporate as much of our new knowledge into existing theory as possible, but to expand any areas where mathematical models devised before the structure of DNA was known are not up to the job. It’s an exciting time, and I believe there really is a buzz in the air about where we will go in our next significant step in understanding how evolution operates.

In any case, Zimmer’s summary of the current state of evolutionary biology is, in my opinion, on the mark. Most people I know are interested in exploring what all of the new information we have come upon in the last couple of decades means for evolution. I know only a few individuals who see the “Modern Synthesis” as needing little or no extending.

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UPDATES:

• See Massimo Pigliucci‘s response.
• Coyne adds a bit in reply to Pigliucci (and seems to think “extending” and “revolution” are the same thing).