Wednesday, 16 December 2009

Punctuated Equilibria Explained (or PEE if you like...)

Note: there is something bizarre going on in the references which I can't seem to alter.

In 1972 a landmark paper was published in ‘Models in Paleobiology’ titled, “Punctuated Equilibria: an alternative to phyletic gradualism.” The paper, by Stephen Jay Gould and Niles Eldredge, was in many ways nothing new, yet at the same time purported to challenge many long cherished ideas in evolutionary biology. Over the years it has simultaneously been embraced and reviled by scientists, and consistently distorted by creationists.

In many ways Eldredge and Gould had simply connected the dots between different lines of evidence and come to conclusions. They took common knowledge from biostratigraphy and combined it with known models of speciation used by neontologists (those that study living organisms). Here they saw that species appeared in a geologically abrupt time and persisted unchanged for most of their duration. At the time biology and palaeontology were only beginning to overlap, so Eldredge and Gould were the first to realise that the pattern they perceived in the fossil record was exactly what should be expected if Ernst Mayr’s peripatric speciation model were applied to the fossil record.

During peripatric speciation a small group becomes peripherally isolated from the main population. Gene flow between the two populations stops, allowing the two to accumulate separate mutations. Smaller populations can evolve more rapidly, and as they are small they are unlikely to yield fossils. When the isolated group is reintroduced to he larger population, if sufficient time has passed they will no longer be able to interbreed – they have become a new species. Over geologic time this appears sudden.

So why the fuss if these were well established views? Creationist distortions aside, Gould and Eldredge were interested in some of the implications of the theory. They changed ideas of tempo and mode in evolution; they challenged the way we think of natural selection; they raised the possibility of unknown mechanisms; and often claimed to separate micro- and macro-evolution. These were strong boasts which led to years of feuding and bickering among scientists.

The punctuational aspect of ‘punk eek’ (or evolution by jerks as some used to call it) has received the most attention from detractors. It was also one of the main focuses (at first) of Gould and Eldredge, as they loudly proclaimed that Darwinian orthodoxy had been challenged. The first to protest often misunderstood. Many biologists interpreted rapid to mean saltation, where a new species is born instantly from an old one. Creationists also made this mistake and believed they had new evidence of instantaneous creation. Both were mistaken; rapid on a geological timescale means at least tens of thousands of years.

Many ‘phyletic gradualists’ rightly pointed out that a straw man of gradualism had been erected and defeated. No gradualist believes that evolution occurs at a strict pace (Dawkins 1986). Even Darwin had made comments that sound a lot like punctuated equilibria; discussing his tree diagram he said, “But I must here remark that I did not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification.”

Many biologists tried to ignore punctuated equilibria, focussing on the tempo aspects and dismissing them. An analogy using gears is apt, as evolution can change gears, even becoming so slow as to allow stasis (evolutionary biologists have words like bradytely, horotely and tachytely to describe pace). Is this dismissal valid? Not completely, but to dismiss only the attacks on gradualism evidence is needed. Gould himself (1993) acknowledged that it is a ‘complement to phyletic gradualism’ as gradualism has been documented in groups from microfossils (Macleod 1991) to mammals (Gingerich 1976, Chaline & Laurin 1986).

One of the best examples of punctuations as gradualistic was found by Stephen Jay Gould (1996). He was fortunate to find numerous shells of the Bahamian land snail Cerion, in a single mudflat, the equivalent to a single bedding plane in strata. Using geochemical methods he was able to date the shells; when put in order they showed a gradual, microevolutionary trend spanning 20,000 years.

The other main aspect of punk eek is stasis, the equilibrium aspect of the model. The authors had developed a little motto, “stasis is data” to remind themselves of the importance of this observation. One of Gould’s harshest critics, Jeffrey S. Levinton, agreed with this observation, stating, “This is…the issue of stasis, which I believe to be the legitimate problem spawned by the punctuated equilibrium model.” (1988).

Stasis had previously been dismissed as a lack of data, a situation which has changed a lot since 1972. Stasis is not a lack of evolution; it is a ‘wobble’ or fluctuation around means, with no substantial change (no broader than the range of geographic variation in modern species) and no directional evolution.

There are some issues with empirical verification of stasis, though it is widely acknowledged as occurring. Fossil species are morphospecies, that is they are identified by morphology and only that which fossilises. Substantial phenotypic change can occur without being detectable in the fossil record. Similarly, neontologists often discover new species through genetic testing; practically impossible with fossils. Conversely, intraspecific variation, functional polymorphisms and ontogenic variation may all be wrongly identified as separate species. Despite these possibilities, comparisons suggest that it is not too big an issue for interpreting stasis (Jackson and Cheetham 1994) as the bias is against punk eek.

Some biologists tried to explain stasis away using stabilising selection in unchanging environments. Stabilising selection removes the extremes of a population, keeping it centred around the mean. However, this could not explain stasis through climatic change (Cronin 1985, Prothero and Heaton 1996, Prothero 1999). Many possible explanations for stasis including a homeostatic mechanism resisting selection (a controversial view of Gould’s which fit with some of his other views), habitat tracking (Eldredge), constraints (Lieberman), normalising clade selection (G. L. Williams) and turnover pulses (Vrba) have all been suggested.

Gould and Eldredge originally tried claiming that all change was focussed around speciation events, a position they later changed. Douglas Futuyma (1987) gave strong insight into what may be occurring, “In the absence of isolation, differentiation is broken down by recombination. Given reproductive isolation, however, a species can retain its distinctive complex of characters as its spatial distribution changes along with that of its habitat or niche… Although speciation does not accelerate evolution within populations, it provides morphological changes with enough permanence to be registered in the fossil record. Thus, it is plausible to expect many evolutionary changes in the fossil record to be associated with speciation.”

Palaeontologists in recent years acknowledge punctuated equilibrium as a valid model of long term occurrences, one among many including phyletic gradualism and punctuated anagenesis (Jackson and Cheetham 1999). The main ‘controversial aspects focussed on are the potential decoupling of macro and microevolution; changes in understanding of levels of selection; and the prevalence of selection.

From Futuyma’s insight it is hard to see how punctuated equilibrium could potentially split micro and macro evolution, though this was a genuine early problem. It all depends on whether speciation is caused by more than simple isolation. Punctuated equilbirum presents the possibility of species selection and species sorting, and as most change is ‘tied up’ during speciation, this form of selection gains more prominence, therefore meaning that micro changes cannot be easily extrapolated as such selection would be ignored. Modern focus on speciation is on whether natural selection or genetic drift is more dominant (recent research though is favouring natural selection) therefore raising the possibility that natural selection is not responsible for all diversity (a less common view favoured by Gould).

Gould’s statement in 1993 is still a worthy interpretation, saying, “[Punctuated equilibria’s] most important implications remain the recognition of stasis as a meaningful and predominant pattern within the history of species, and in the recasting of macroevolution as the differential success of certain species (and their descendants) within clades.”

I have hopefully presented a brief and accessible explanation of punctuated equilibria, whilst clearing up any misconceptions. ‘Punk eek’ (or ‘eck’ to some) has changed a lot in the past 3 decades, yet still manages to provoke interesting debate (which is sadly misunderstood by the lay public). Gould believed it to be a “useful extension of evolutionary theory” which can clearly be seen once understood.


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Chaline, J. & Laurin, B. (1986) Phyletic gradualism in a European Plio-Pleistocene Mimomys lineade (Arvicolidae, Rodentia). Paleobiology. 12(2), 203-216.

Chaline, J., Laurin, B., Brunet-Lecomte, P. & Viriot, L. (1993) Morphological trends and rates of evolution in arvicolids (arvicolidae, rodentia): Towards a punctuated equilibria/disequilibria model. Quaternary International. 19, 27-39.

Cheetham, A.H. (2001) Evolutionary stasis vs. change. In: Briggs D.E.G. & Crowther, P.R. (eds) Palaeobiology II. pp. 137-142. Blackwell Publishing, Oxford.

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