…By Any Other Name – Why the Concept of Punctuated Equilibrium is More a Matter of Skilled Branding than Scientific Revolution

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Branding is typically thought of as a concept whose applicability is largely confined to the world of business and marketing. However, branding can be important even within the cloistered halls of academia. Nowhere is this more obvious than in the remarkable stamina displayed by Stephen Jay Gould and Niles Eldredge’s concept of punctuated equilibrium. The product of the parochial perspective of professional paleontologists, punctuated equilibrium does two things: restates a point evolutionary biologists had been aware of long before Gould and Eldredge made up a catchy new name for it, and leads to some altogether outlandish ideas concerning the nature of selection.

Gould and Eldredge came up with the idea of punctuated equilibrium in the late 1970s after observing that the fossil record displayed long periods of stasis, interrupted by relatively rapid sequences of change. Specimens from single species recovered from rock formations spanning millions of years often display the same basic range of variation. Then, in the blink of a geological eye, observable changes measurably shift the range of variation, suggesting a rather rapid bout of evolution.

This pattern seems to contradict the standard view outlined by Darwinian or “phyletic” gradualism. In this view, evolutionary change occurs at a more or less constant and more or less glacial pace. The transition from bony fish with fins adapted to swimming to bony fish with fins adapted to swimming and crawling occurred as a result of tens of millions of years of steady evolutionary change. This type of thinking seems a natural byproduct of a perspective that holds that evolutionary change occurs as a result of changes in the frequency of genes within a given population produced by random mutations forced through a sieve of selective pressures.

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Sequence of tetrapod evolution.

As phrased, it is easy to see why Gould and Eldredge thought the standard view might be flawed. Empirical evidence derived from the fossil record seems to unambiguously contradict the gradualist position. What Gould and Eldredge missed in formulating their ideas is that this view of gradualism is at best a caricature of the modern understanding of evolutionary processes. True, evolution is sometimes – almost dogmatically – viewed as a sluggish process. But evolutionary biologist had become aware of the fact that evolutionary change occurs at varying rates long before Gould and Eldredge put forward the idea of punctuated equilibrium.

Evolutionary change, as understood in the modern synthesis, is a product of four fundamental processes: natural selection, gene flow, genetic drift, and mutation. The rate of evolutionary change varies in light of the frequency and intensity of those processes. Consider for example two populations, whimsically named Population A and Population B. Populations A and B experience the same selective pressures, but Population A lives a rather monastic lifestyle, isolated high in some mountain valley. Population B, on the other hand, is rather promiscuous – its members spend a lot of time mating with members of neighboring populations. According to the principles of the modern synthesis, Population A will evolve more rapidly than Population B because Population A accumulates mutations without the buffering effects of gene flow. In principle, one can tune the four dials of natural selection, gene flow, genetic drift, and mutation up or down for any given hypothetical population and achieve differing rates of evolutionary change.

This is all rather humdrum, boilerplate evolutionary biology. It’s dogma today and was at the time Gould and Eldredge came up with the notion of punctuated equilibrium. Indeed, Sewell Wright had laid bare these very principles in his shifting balance theorem, formulated some forty-five years prior to the publication Gould and Eldredge’s seminal papers on punctuated equilibrium. Elsewhere, the idea of long term evolutionary stasis had been explored through John Maynard Smith’s forays into game theory, resulting in the concept of evolutionary stable strategies. As elaborated by Dawkins, evolutionary stable strategies implicitly involve statistically stagnant gene complexes – and therefore stable populations – because mutations are actively penalized by selection (1976; 1982).

Really, a lot of the fuss over Gould and Eldredge’s ideas boils down to marketing. Punctuated equilibrium is a beautifully coined term, at once fluid, memorable, and imbued with the electric hum of scientific novelty. It’s a lot more gratifying to say or write “punctuated equilibrium” than it is to say or write “evolutionary change can occur at a variety of rates depending on the strength of the underlying processes”. Punctuated equilibrium, with its inherent suggestion that generations of Darwinists had gotten things fundamentally wrong, provided excellent fodder for headline-hungry popular periodicals.

This wouldn’t detract too much from Gould and Eldredge’s work, were it not for their clear attempts to paint their ideas as revolutionary. Their initial 1977 paper even goes so far as to cite Thomas Kuhn’s The Structure of Scientific Revolutions, suggesting they thought their ideas rather more auspicious than they appear through the sober lens of retrospection. In balance, what they did was quite beneficial. The then banal realization that evolutionary change was not ubiquitously gradual was widely known among evolutionary biologists, but frequently missed by the inexpert. Giving the general concept a name greatly increased its public visibility, spreading the word as it were to many who would have otherwise persisted in ignorance.

Phyletic gradualism (top) vs. punctuated equilibrium (bottom)

Phyletic gradualism (top) vs. punctuated equilibrium (bottom)

Gould and Eldredge’s greater sins were to completely abandon the potential for evolutionary change to occur along a gradual gradient, and, more severely, suggest that species level selection plays a prominent role in shaping long term evolutionary trends. Concerning the former, Gould and Eldredge pointed out that gradualism seemed to demand steady orthogenetic selection, such that traits were more or less guided in a steady direction by consistent – but very small – selection pressures over the course of millions of years. Selection pressures of these kind would be swamped by other factors, so it was unrealistic to presume they were meaningful, especially in light of fossil evidence to the contrary (Dawkins 1982). Problematic is that their assessment fails to recognize that gradual evolutionary change can be produced by a mix of countervailing forces. In no scenario is it actually realistic to presume a stable but mild selection pressure is the only force exerted on a biological population. A strong selection gradient may be counteracted by high levels of gene flow or low levels of mutation, resulting in a net rate of evolutionary change identical to what one would expect under conditions of weak selection alone. As Dawkins points out (1976, 1982), much evolution is a result of organisms evolving in response to pressures exerted by other organisms, resulting in evolutionary arms races. The selective dynamics underlying evolutionary arms races are precisely the kind that would produce steady directional change.

In making their case against gradualism, Gould and Eldredge also greatly oversimplified the nature of the evidence. Fossils provide an excellent record of long term change, cataloguing the results of evolutionary processes over the course of millions of years. Yet it’s worth remembering the fossil record is primarily one of morphological change in hard tissue. Soft tissues like skin and stomachs and brains are only occasionally preserved in the fossil record. Moreover, behavioral change – surely relevant to any claim about the nature of evolutionary processes – can only be studied indirectly. Gould and Eldredge’s reliance on the fossil record implicitly grants preference to morphology as the only meaningful stage for observing evolutionary change, ignoring the fact that a fossil sequence that shows little change in limb length over the course of millions of years might disguise important changes in soft tissue and, critically, behavior. Put simply, the long term evolutionary stasis Gould and Eldredge saw as a basis for punctuated equilibrium is largely a product of what kinds of information do and do not fossilize.

Which brings us to their gravest sin: the claim that punctuated equilibrium shines light on the fundamental role of species level selection in shaping evolutionary processes. This again seems a product of the parochial perspective of a person who spends most of their time looking at fossils. These are necessarily low resolution records, revealing trends that play out on the scale of tens of thousands to millions of years. Little wonder, then, that the most pronounced signal of evolutionary change in the fossil record will often be that left behind by speciation events. Gould and Eldredge seem to have mistaken the locus of evolution for the locus the selection. Populations evolve, individuals do not. That is precisely what we see in the fossil record. From here, it is easy to slip into the trap of thinking selection is operating on level of populations.

The problem here is that populations evolve as the result of differential selection operating on either the individuals that comprise the population or, more fundamentally, the individual alleles whose frequency provides the definitional basis of evolutionary change. Evolutionary processes can be abstracted to involve the differential proliferation of replicators, bits of information that have sufficiently high levels of longevity, fidelity, and fecundity to be sensible to selective forces (Hull 2001; Dawkins 1976 & 1982). Candidates for the unit of selection must meet those criteria. A sufficiently short nucleotide sequence passes muster. But does a species?

Species do seem to last a long time, so perhaps we can tick one box in favor the species as replicator position. But it’s difficult to see how a species can display either fidelity or fecundity. Species do not reproduce – the individuals of which a species is comprised do. Whatever fidelity or fecundity is exhibited by a species is a product of processes that occur at the level of individuals. Speciation events are not good candidates for instances of species reproduction, because – by definition – they involve a species changing into something else. In that case fidelity seems compromised. When we turn to fecundity, the argument for species level replication seems just as dubious: parent species don’t sire lots of copies of themselves.

On geological time scales, speciation seems to occur in the blink of an eye. But selection operates on timescales that make most speciation events appear gradual. Even if we grant the already suspicious claim that species can coherently serve as replicators, the fact nonetheless remains that selection operating on replicators with a faster turnover rate will swamp the effects of species level selection (Dawkins 1982). That is, individuals reproduce and introduce novel mutations into the gene pool at the rate of generations. Depending on the species, that can be anywhere from days to decades. Speciation, by comparison, occurs at a relatively glacial pace. Populations become reproductively isolated and evolutionarily distinct on a scale that must be measured in anywhere from millennia to millions of years. The idea that some selective pressure operates on the species as a whole, when all evolutionary change is a product of the differential reproduction of the individuals within that species, is far-fetched at best.

As Dawkins points out (1982), species level selection also finds itself tripped up by one of the very arguments Gould and Eldredge leveled against gradualism. Gradualism seems to demand slight but consistent directional selection. We’ve already discussed the problems with this, but consider the idea when turned to species level selection for complex adaptations. A demand is placed not only on directional selection for a trait, but directional selection on many traits that might not be genetically intertwined. Species level selection falls into the same orthogenetic trap Gould and Eldredge had laid for gradualism, but does so far more deeply and devastatingly.

Species level selection is a chimera. Any given instance of speciation marks a point at which all the interesting change has already occurred at the level of individuals and genes. None of which is to say speciation and extinction aren’t evolutionarily important. They most certainly are. Rather, the crucial point is that selection can’t operate on the level of the species because selection pressures can’t make it that far up the chain. By the time a selection pressure becomes sensible at the level of the population or species, it has already been taken care of by adaptations expressed on the level of the individual. If we picture selection as hierarchical process, the most navigable of selection pressures will never even been sensed by genes. Behavioral plasticity and learning will take care of them. If an organism proves too developmentally inflexible, a beneficial mutation resulting in a slight adaptive advantage (these produced at the rate of generations) will take care of the problem. By the time a selection pressure made it the level of the species, individuals within the population will have had tens of thousands of chances to deal with it, and chances are, they already will have. Populations and species evolve as a result of the aggregate effects of selection on individuals and the genes they carry.

In the final analysis, punctuated equilibrium is a concept well worth keeping. It seems to make the notion that evolution is not merely a steady trudge into the future considerably more digestible by taking the concept embodied by the phrase “evolutionary change can occur at a variety of rates depending on the strength of the underlying processes” and compressing it into a simple, memorable term – “punctuated equilibrium”. But it’s worth remembering that punctuated equilibrium – as formulated by Gould and Eldredge – overstates the case against gradualism, misrepresents the evidence presented by the fossil record, and makes a grossly misleading – and flatly incorrect – argument about the nature of selection. Let’s use punctuated equilibrium to remember that sometimes evolution can happen very fast and discard the rest.


References and Further Reading:

Dawkins, R. 1976. The Selfish Gene. Oxford University Press

Dawkins, R. 1982. The Extended Phenotype. Oxford University Press

Hull, D. 2001. Science and Selection. Cambridge University Press

Gould, S. J. & N. Eldredge. 1993. Punctuated equilibrium comes of age. Nature. 366

Gould, S.J. & N. Eldredge. 1977. Punctuated equilibria: the tempo and mode of evolution reconsidered. Paleobiology 3 (2): 115-151

Kuhn, T. 1962. The Structure of Scientific Revolutions. University of Chicago Press

Maynard Smith, J. & G. R. Price. 1973. The logic of animal conflict. Nature 246 (5427): 15–8.

Wright, S. 1932. The roles of mutation, inbreeding, crossbreeding and selection in evolution. Proceedings of the 6th International Congress of Genetics: 356-366.

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