Structuralism is based on the idea of intrinsic forms that severely limit evolutionary pathways. These forms are constrained, and defined, by the physics of matter and energy. Some creationists like this idea because they believe that god created the universe and fine-tuned it for life. According to their faith, once the original laws of physics and chemistry were set up it was just a matter of time before humans evolved. These creationists can make their belief in a creator god compatible with evolution as they see it.
(Let's not forget that there are many structuralists who are legitimate scientists and some of them are atheists. Structuralism is not a creationist invention.)
I call this experiment "replaying life's tape." You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original. If each replay strongly resembles life's actual pathway, then we must conclude that what really happened pretty much had to occur. But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life? What could we then say about the predictability of self-conscious intelligence? or of mammals? or of vertebrates? or of life on land? or simply multicellular persistence for 600 million years? (pp. 48-50)Gould strongly believes that if you replay the tape of life it will not turn out the same. He thinks that the history of life is unpredictable for several reasons such as ...
- mutations are "random" and the timing of mutations can greatly influence the path that a lineage follows
- not all changes are due to adaptation and natural selection—many are just accidents
- there's no evidence that the forms we see are the only possible forms or Baupläne
- there's more to the history of life than just evolution and these other events, such as asteroid impacts and plate tectonics, play a major and unpredictable role in the history of life
One of those possible endings is a planet with no vertebrates, or no mammals, or no humans. Indeed, if you start the replay early enough, there may never be any multicellular species, no photosynthesis, no endosymbiosis leading to eukaryotic cells.
Gould revisited this idea in The Structure of Evolutionary Theory devoting many pages to challenging the structuralist ideas of D'Arcy Thompson who wrote Growth and Form, the "Bible" of structuralism. He also critiques the more modern views of of Stuart Kauffman and Brian Goodwin. He devotes two long chapters1 to the problem.
Chapter 11: The Integration of Constraint and Adaptation (Structure and Function) in Ontogeny and Phylogeny: Structural Constraints, Spandrels, and the Centrality of Exaptation in Macroevolution.
Most Sandwalk readers will never delve into those chapters to extract the essence of Gould's idea so let me try and explain it. Keep in mind that this is a difficult topic and if you are really interested in criticizing Gould's view of life there's no excuse for avoiding the hard work needed to justify your opinion.
Here's an example of Gould's thoughts on the predictability of the history of life (page 1156)
If a group of Martian paleontologists had visited earth during the Eocene epoch, they would have encountered two coexisting, and scarcely distinguishable species of the genus Hyracotherium. If they had then followed the subsequent history of the lineages, they would have watched one species differentiate into the clade of rhinoceroses and the other into the clade of horses. But if a modern commentator then concluded that horses and rhinos had existed as distinct designs in their modern form (lithe runners vs. horned behemoths) since the Eocene, we would laugh at such a silly confusion ...
Xidazoon (right) as an example of a body plan that didn't leave any descendants and asks how that fits into structuralist notions.
But even more profound is the idea that the bilateran body plan itself may not have been the only possibility.
If the basic developmental patterns of bilaterans arose quickly, and have remained fixed in form since then, do these historical invariants represent a set of mechanistically limited and excellent, perhaps even optimal, designs that natural selection would have established in much the same way at any time and under any ecological of geological regime? Or do they represent just one possible solution among many numerous and entirely plausible alternatives of strikingly different form, each yielding a subsequent history of life entirely different from the outcome actually experienced on earth? ....What he means is that just because the basic body plans of modern animals all evolved from a bilateran ancestor doesn't mean that this pattern was preordained by the immutable laws of physics and chemistry. It could be a accident of history that was locked in by constraints on subsequent evolution (historical contingency) just as the genetic code may have been a "frozen accident."
Historical constraint based on developmental homology assumes great importance in either case, but if the particular constraints that actually set the channels of bilateran diversity could only have arisen within a narrow range of basically similar and workable states, then much of life's pageant unfolds by predictable regularities of natural selection. If, however, the developmental plans actually established during the Cambrian explosion—albeit eminently workable, and therefore exploited by natural selection to build the particulars of life's later successes and failures—represent only one contingently-achieved set among a broad range of alternatives (each "equally pleasing" to natural selection), then life's actual pageant on earth becomes highly unpredictable, and the happenstance of a realized beginning (the historical constraints of bilateran developmental homology) assumes a far more prominent role in shaping the subsequent history of life.
thought experiment) of replaying the tape of life is an important way of focusing attention on divergent views about the history of life. As we have seen, Gould doesn't believe that the replay would look anything like the original.
It's worth reminding readers of the play on words in the title Wonderful Life. It not only refers to the wonderful life of the Burgess Shale but also the well-known movie "It's a Wonderful Life starring James Stewart and directed by Frank Capra. In the movie, an angel takes the hero back in time to show him how different life would have been if he had never been born. It's an example of replaying the tape of life and getting a different result.
Gould's analogy is presented in the form of a thought experiment but that doesn't mean the conclusion is based on thought alone. In fact, there's a great deal of scientific evidence to support the idea that the history of life is not the entirely result of predictable laws of form.
Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts (USA) and the resulting model was known as the MBL Model. E.O. Wilson's student, Dan Simberloff was part of the original group.
Later on the group was joined by
The work culminated in a number of papers showing that the actual fossil record looked very similar to simulations where the survival and extinction of clades was essentially random (Gould et. al., 1977
Raup, D.M., Gould, S.J., Schopf, T.J., and Simberloff, D.S. (1973) Stochastic models of phylogeny and the evolution of diversity. The Journal of Geology, 525-542. [PDF]The results are nicely summarized in the abstract of Gould et. al. (1977)
Raup, D.M., and Gould, S.J. (1974) Stochastic simulation and evolution of morphology-towards a nomothetic paleontology. Systematic Biology, 23(3), 305-322. [doi: 10.1093/sysbio/23.3.305]
Schopf, T.J., Raup, D.M., Gould, S.J., and Simberloff, D.S. (1975) Genomic versus morphologic rates of evolution: influence of morphologic complexity. Paleobiology, 63-70. [PDF]
Gould, S.J., Raup, D.M., Sepkoski, J.J., Schopf, T.J., and Simberloff, D.S. (1977) The shape of evolution: a comparison of real and random clades. Paleobiology, 3(01), 23-40. [doi: 10.1017/S009483730000508X]
The history of life is replete with apparent order. Much of this order may reflect the deterministic causes conventionally invoked, but we cannot be sure until we measure and subtract the order that arises in simple random systems. Consequently, we have constructed a random model that builds evolutionary trees by allowing lineages to branch and become extinct at equal probabilities. We proceed by dividing our simulated tree into clades and by comparing their sizes and shapes with the patterns exhibited by “real” clades as recorded by fossils.This can be simplified by saying that the actual history of life looks random and unpredictable except for some effects due to historical contingency.
We regard the similarity of real and random clades as the outstanding result of this comparison. In both real and random systems, extinct clades arising after an “ecological barrel” had been filled have their maximum diversity at the midpoint of their duration; clades arising during the initial “filling” reach an earlier climax during this preequilibrial period of rapid diversification. However, some potential differences also emerge. Clades still living are much larger than extinct clades. We may attribute this to the morphological superiority of survivors, but we can also simulate it in a model that chooses the originators of clades at random. Real clades undergo greater fluctuations in diversity than do random clades, but the effect is not marked.
The idea of replaying the tape of life dates back to a letter Gould published in response to Niles Eldredge in 1976 after Eldredge had criticized the totally random interpretation of the fossil record (Eldgredge, 1976; Gould, 1976). Gould agreed with the criticism and that's where historical contingency met with randomness to make replaying the tap of life the better analogy.
Eldredge, N. (1976) Differential evolutionary rates. Paleobiology, 2(02), 174-177. [doi: 10.1017/S0094837300003456]Here's part of Gould's letter. We see two themes here that come up again and again in Gould's writing. The first is that we must follow the data and not our intuition. The second is that we must have a clear null hypothesis and this null hypothesis cannot beg the question.
Gould, S.J. (1976) The genomic metronome as a null hypothesis. Paleobiology, 2(02), 177-179. [doi: 10.1017/S0094837300003468]
... if we could replay the tape of life, might coelacanths become the teleosts and beetles the monoplacophorans? In short, empirical differences in rate of speciation exist—how could it be otherwise in a non-typological world. But we must know whether these differences are an essentially random scattering about a mean rate (the metronome hypothesis) or a determined and predictable feature of certain morphologies and environment. ... (Frankly, I must confess that my own intuition leads me to Eldredge's conclusion—that the differences are predictable consequences of functional anatomy and environmental space. But I cannot vindicate this intuition with any satisfactory data. And scientists must learn to mortify their intuitions when they arise from the philosophical baggage that we all carry as products of Western intellectual traditions—i.e. a basic belief in determinism, mechanism, directionalism, etc.)Note that this letter was published two years before the Spandrels paper was presented (Gould and Lewontin, 1979) and it echoes some of the same ideas in that paper.
And it's not just historical contingency that makes evolution unpredictable ...
Another point in favor of the metronome and one not yet honestly faced by Darwinians—is the possibility that a relatively large amount of the enormous genetic variation in natural populations is not controlled by selection. It matters rather little whether this variation be strictly" neutral" (sensu Kimura and Ohta) or "slightly deleterious" (sensu Selander, so that mutation rate balances the small disfavor of selection to produce an equilibrium). In either case, the control of genetic change would lie with stochastic factors (genetic drift, mutation rate) unrelated to any deterministic controls on varying rates of speciation among higher taxa. These non-Darwinian factors almost surely act to decrease the among-group variance in genomic rates of evolution.The point here is that the concept of unpredictability in the history of life is based not on wild speculation but on solid evidence from the fossil record and computer simulations and a firm understanding of modern evolutionary theory and history.
You can believe that life was purposefully designed if you like but such a belief flies in the face of (i.e. conflicts with) science [see Full House: The Spread of Excellence from Plato to Darwin].
If you want to learn more about Gould's view of life you can't do better than to read Stephen Jay Gould: Reflections on His View of Life.
If you want something shorter try his essay Eight (or Fewer) Little Piggies in which he says,
Never apologize for an explanation that is "only" contingent and not ordained by invariant laws of nature--for contingent events have made our world and our lives. If you ever feel the slightest pull in that dubious direction, think of poor Heathcliff, who would have been spared so much agony if only he had stayed a few more minutes to eavesdrop upon the conversation of Catherine and Nelly (yes, the book wouldn't have been as good, but consider the poor man's soul). Think of Bill Buckner who would never again let Mookie Wilson's easy grounder go through his legs--if only he could have another chance. Think of the alternative descendants of Ichthyostega, with only four fingers on each hand. Think of arithmetic with base eight, the difficulty of playing triple fugues on the piano, and the conversion of this essay into an illegible Roman tombstone, for how could I separate words withoutathumbtopressthe spacebaronthistypewriter.
Photo Credit: The photo of Jack Sepkoski and David Raup is from the University of Chicago and was published in David M. Raup 1933-2015.
1. All of the chapters in that book are long. It took me the better part of a day just to re-read two chapters.
Gould, S.J., and Lewontin, R.C. (1979) The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London. Series B. Biological Sciences, 205(1161), 581-598. [doi: 10.1098/rspb.1979.0086]
Sepkoski, D. (2016) “Replaying Life's Tape”: Simulations, metaphors, and historicity in Stephen Jay Gould's view of life. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences. [doi: 10.1016/j_shpsc.2015.12.009]