The Quiet Passing of Punctuated Equilibrium, Finally! - Evolution News & Views

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The Quiet Passing of Punctuated Equilibrium, Finally!

DebatingDD.jpegIn 1984, I was the primary co-author of a book titled The Natural Limits to Biological Change, with Lane Lester. One of my observations was that we had ample evidence of biological change in both plants and animals but there always seemed to be a limit. Only so much protein content in corn or sugar content in beets, or so many types of roses and dogs, or people three feet tall and some even eight feet tall but no-two foot dwarves or ten-foot goliaths.

Of course the Darwinian evolutionary solution was new variation added through the process of mutation. Neo-Darwinism had reigned supreme for over fifty years but there was a new kid on the block, what Stephen Gould and Niles Eldredge termed "punctuated equilibrium." In Natural Limits we critiqued both neo-Darwinism and punctuated equilibrium. Therefore I read with interest Stephen Meyer’s discussion of "Punk Eek" in Chapter 7 of Darwin’s Doubt.

From the beginning, punctuated equilibrium was primarily an observation from paleontology that most species seemed to stay the same over millions of years (stasis or equilibrium) and suddenly a new species appeared with no smooth transition (punctuation) from the previous species. Gould, Eldredge, and Stephen Stanley wrote numerous articles and books over twenty years summarizing and applying their ideas to different types of organisms in numerous geological time frames.

This was all fine and good as a paleontological observation, but how was this rapid punctuation supposed to happen biologically? Gould, in particular, was careful to point out that punctuated equilibrium was a descriptive theory of large-scale patterns over geological time, not a theory of genetic process. But if genetic process could not accomplish large-scale patterns, the observation becomes mute.

One of the candidates for explaining rapid change through speciation was some kind of developmental change. At the time it was called a bifurcation. A bifurcation is defined as a series of small changes that suddenly reveals a major shift. A child’s metal cricket or frog is a good example. These toys consist of a short piece of metal, grounded at one end. As the opposite end is bent, it eventually buckles emitting the desired “click.” So perhaps with populations, mutations in a particular adaptation could accumulate over time and suddenly result in a major morphologic shift.

But even in 1984 (when evolutionary developmental biology "evo-devo" was in its infancy) nearly all discussions of developmental gene mutations were discussions of lethal or near lethal changes. As we said in Natural Limits, “Mutations of genes intricately involved in development are what produce hopeless, not hopeful monsters.” In Darwin’s Doubt, following a helpful discussion of punctuated equilibrium along with its many difficulties, Meyer quotes Gould from 2002, “I recognize that we know no mechanisms for the origin of such organismal features other than conventional natural selection at the organismic level” (p. 149).

Indeed, in 1993 Gould and Eldredge published a review paper in Nature announcing that punctuated equilibrium had come of age. But towards the end of the paper under the heading of Difficulties and Prospects, they highlight the questions raised by evolutionary biologists. After dismissing a few of those objections, they admit “But continuing unhappiness, justified this time, focuses upon claims that speciation causes significant morphological change, for no validation of such a position has emerged.”

As Meyer spells out, Eldredge and Gould postulated that the speciation event itself was involved in bringing about abrupt morphological change. But as Meyer makes clear, there was no biology behind the claim, just an observation of the fossil record. Using development as the location of such change has proven extremely difficult, as Meyer also points out, especially when discussing the origin of the major body plans that appear in the Cambrian explosion. Body plans are set early in development and mutations in these early processes are universally disadvantageous. I came across this observation from an author Meyer mentions a few times, Wallace Arthur.

Arthur is a population ecologist who became intensely interested in the process of major morphologic changes. His 1997 book, The Origin of Animal Body Plans: A Study in Evolutionary Developmental Biology, is a wide-ranging discussion of the problems of achieving major morphological shifts but also an attempt at a solution. Arthur addresses the major difficulty on p. 14: “There is, however, a problem. Those genes that control early developmental processes are involved in the establishment of the basic body plan. Mutations in these genes will usually be extremely disadvantageous, and it is conceivable that they are always so” (italics in original). But on the following page Arthur states his faith position that “The genes involved have evolved” (italics in original). So if I may paraphrase: “Early developmental genes produce the differences in body plans, but these genes can’t be mutated without disastrous results. But we know they have evolved despite this evidence because evolution happened.”

Later, Arthur addresses the primary problem for a neo-Darwinian approach of accumulating small mutations to produce a major morphological shift (italics in original):

In a developmentally explicit approach it is clear that many late changes can not accumulate to give an early one. Thus if taxonomically distant organisms differ right back to their early embryogenesis, as is often the case, the mutations involved in their evolutionary divergence did not involve the same genes as those involved in a typical speciation event, where usually the early embryogeneses of the daughter species are virtually identical. [p. 22]

Punk Eek was dead over twenty years ago but persisted on the coattails of Stephen Gould’s considerable and deserved celebrity. But with him gone, his and Elderdege’s unique contribution to evolutionary theory is finally passing quietly away.