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Punctuated Equilibrium

This post is in response to a question asked by a follower: What is punctuated equilibrium?

To begin, this isn’t a critique of the validity of punctuated equilibrium (also called punctuated equilibria). I do not support this hypothesis and a discussion of why it is incorrect can be reserved for another post. This post only aims to explain what punctuated equilibrium is and where it came from.

Most evolutionary biologists favour phyletic gradualism, the theory of evolution as a gradual process, which is supported by data found in the fossil record and by genetic studies that provide evidence for the slow transition from one species to descendent species. However, although we have evidence for the gradual evolution of many species such as that evidenced by molars of arvicolids (e.g., species of Mimomys) during the late Pliocene and Pleistocene*, there are nevertheless gaps in the fossil record for other species. The most common and most likely explanation for these gaps contends that the fossil record is inherently incomplete because of factors such as preservation and population size. Almost all individuals that die do not fossilise. Eldridge and Gould (see below) saw little evidence for speciation at the species level but saw a great deal of evidence for speciation between larger groups. This is important to note.

A minority of scientists prefers the controversial hypothesis referred to as punctuated equilibrium, which is less of a process and more of an event. Punctuated equilibrium was first proposed in the early 1970s by Niles Eldredge and Stephen Jay Gould. It is a hypothesis that asserts that there is a high rate of evolution at times of speciation but a low rate or nil rate of evolution between these speciation events (cladogenesis). It concerns the sudden appearance of closely related taxa but does not apply to higher taxa. It “refers to both a pattern of change in the fossil record and a hypothesis about evolutionary processes” (Futuyma 2009:93). 

Punctuated equilibrium sees stasis in the fossil record at points wherein taxa do not display gradual change, a period that is then punctuated by rapid evolution. Accordingly, phenotypes (and genotypes) evolve in tandem with speciation. That is to say, change does not occur slowly over a long span of time; changes in a species that lead to speciation event occur concomitantly with that speciation event. This is in direct contrast to the slow accumulation of changes that phyletic gradualism proposes. To boot, the hypothesis is yet more controversial. Punctuated equilibrium also asserts that barring populations that are undergoing speciation, phenotypes cannot evolve because of a genetic constraint. The reason this is controversial will be explained in the following paragraphs.

To understand why this hypothesis was proposed we need to understand where it came from. Punctuated equilibrium was based on Ernst Mayr’s model of “peripheral isolate speciation,” also called “founder-effect speciation,” and “peripatric speciation,” which is a kind of allopatric speciation that Mayr believed was the dominant manner of speciation. It also had roots in I. Michael Lerner’s hypotheses about developmental and genetic homeostasis. According to Mayr’s model, a new taxon (the peripheral isolate), which is at the edge of its species range, will abruptly emerge in the fossil record because the taxon evolved in a small population isolated from its ancestral species. When fully formed as a new species, it migrated to wherever it is the fossils were found. Because the speciating population was small, it is statistically less likely specimens will fossilise and furthermore, specimens are not found in the geographic range of the parent population. To be clear, founder effect, according to Mayr, is the:

“establishment of a new population by a few original founders (in an extreme case, by a single fertilized female) that carry only a small fraction of the total genetic variation of the parental population” (Mayr 1942:237).

This model maintains its foundation in phyletic gradualism but suggests that the appearance of a species in the fossil record is rapid because we simply do not have the fossils from their place of origin to prove it. In Futuyma’s words, the evolution occurred slowly “off stage” (2009:95).

Under punctuated equilibrium, speciation is an event in the geological sense. Here’s the trickier part: Presumed stasis for the duration of a taxon questions adaptation and variation in that taxon (intraspecifically) except when it occurs during the speciation event of the peripheral isolate (the descendent species). As stated above, this means that new morphologies will not evolve slowly because they are genetically constrained by gene flow**. This is a significant difference because it “decoupled intraspecific evolution from macroevolution, creating two levels of a new evolutionary hierarchy” (Erwin & Anstey 2004:188). 

Again, I hope this was helpful. In truth I find this topic difficult to understand and even more difficult to explain. If anyone offers construct criticism this post can be edited. I’m here to learn too!

*Molars of taxa in the genus Mimomys are useful for dating archaeological and hominid-fossil bearing sites.

**According to allopatric speciation large populations are maintained in part by their volume and gene flow. In contrast, small populations of the peripheral isolate are not subject to the effects of gene flow that homogenise the larger population thus stymying gradual evolution.

 Online references:

• Berkeley. Competing Hypotheses about the Pace of Evolution
• Etrilobite.com (first image)
• PBS. Punctuated Equilibrium (second image)
• Princeton. Punctuated Equilibrium
• Ridley, M. Online resource for Evolution (2004).
• Saylo, M.C., Escoton, C.C. and Saylo, M.M. 2011. “Punctuated Equilibrium vs. Phyletic Gradualism,” International Journal of Bio-Science and Bio-Technology 3(4):27-42.
• Templeton, A.R. 1980. “The Theory of Speciation via the Founder Principle,” Genetics 94:1011-1038.
• Tesakov, A.S. and van Kolfschoten, T. 2011. “The Early Pleistocene Mimomys hordijki (Arvicolinae, Rodentia) from Europe and the origin of modern nearctic sagebrush voles (Lemmiscus),” Paleontologia Electronica, article number: 14.3.39A (third image)

 Main references: 

• Erwin, D.H. and Anstey, R.L. 2004. “Speciation in the fossil record” in Mark Ridley (ed.), Evolution. Oxford: 185-197
• Futuyma, D.J. 2009. Evolution.  Sinauer Associates
• Mayr, E. 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist. Harvard University Press.