4 June 2012
Last week, I looked at the work of Mary Jane West-Eberhard on the contribution that developmental plasticity makes to evolution.
In particular, I reviewed examples like Slijper’s goat and Faith the Dog, in which a severe perturbation at the genetic level results in a truly stunning compensation at the level of the adult phenotype.
Such cases provide a vivid illustration of the general principle that no novel phenotype that is presented to selection is ever truly random, because in between the genetic variation (which may be random, though not necessarily so) and the new viable adult phenotype there always intervenes a decidedly non-random, goal-directed developmental process.
This fact, in turn, means that the theory of natural selection derives whatever plausibility it has largely from the unspoken assumption that when you perturb the living organism, it will compensate as best it can.
This power of spontaneous compensation—which West-Eberhard refers to as “developmental (or phenotypic) plasticity”—distinguishes organisms from machines. It is presupposed by natural selection—in its absence, evolution is inconceivable—and so natural selection cannot explain it.
West-Eberhard is an important theorist who has used the 70-year-old Slijper’s goat example for her own illustrative purposes. However, there is a great deal of more-recent experimental work that points in the same direction.
In today’s column, I will review work by four different experimental teams that reinforces the lessons that West-Eberhard has derived from the classic case of Slijper’s goat.
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Paul Bach-y-Rita was born in New York City in 1934. He received his M.D. from the Universidad Nacional Autónoma de México in Mexico City. He practiced as a physician in Mexico for several years before relocating to the U.S. He taught medicine and bioengineering at the University of Wisconsin from 1983 until his death in 2006.
Bach-y-Rita was a pioneer in the study of neuroplasticity—the ability of the brain to reorganize itself in order to compensate for various types of unprecedented challenges, both from within and without.
During his long career, he both pursued pure research and developed therapeutic applications for the treatment of patients with various sensory disabilities. He is particularly known for his work on sensory substitution, which involves the substitution of one sense modality for another that has been lost.
For example, Bach-y-Rita demonstrated that blind patients could learn to “see” objects at a distance using only their sense of touch. In the original experiments, an object in the vicinity of a patient was scanned by a video camera attached to the patient’s head and the output from the camera was converted into an electrical stimulation of the skin on the patient’s back. The blind patient was able to identify the object by feeling its shape in the pattern of stimulation on his skin.
Later on, Bach-y-Rita’s team came to favor the tongue for the purpose of sensory substitution—although their experiments and therapies using the tongue relied upon that organ’s sense of touch, not its sense of taste. The reason why the tongue works best is the density of the nerve endings found there.
Bach-y-Rita and his team also developed similar treatments for patients who had lost their sense of balance due to damage to the vestibular system of the inner ear. In this case, the tongue was stimulated by the electrical output from accelerometers worn upon the body. In these experiments, the researchers found that the brain reorganization which occurred eventually enabled patients to dispense with the sensory-substitution system. In other words, the patients’ sense of balance was restored by retraining their brains.
Bach-y-Rita summed up his path-breaking research as follows:
We see with the brain, not with the eyes. You can lose your retina but you do not lose the ability to see as long as your brain is intact.
Bach-y-Rita, Paul, “Tactile Sensory Substitution Studies,” in Mihail C. Roco and Carlo D. Montemagno, eds., The Coevolution of Human Potential and Converging Technologies (=Annals of the New York Academy of Sciences, vol. 1013). New York: New York Academy of Sciences, 2004, pp. 83–91.
Ptito, Maurice, Solvey M. Moesgaard, Albert Giedde, and Ron Kupers, “Cross-Modal Plasticity Revealed by Electrotactile Stimulation of the Tongue of the Congenitally Blind,” Brain, 2005, 128: 606–614.
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Martin Heisenberg was born in Munich in 1940. He is the son Werner Heisenberg, one of the founding father of quantum mechanics. He was trained in both genetics and neurobiology, and has taught for many years at the University of Würzburg.
If anyone is tempted to dismiss the theoretical significance of Bach-y-Rita’s work described above, on the grounds that human beings have special abilities far beyond those of other animals, Heisenberg’s work provides the refutation.
It has been known for more than a century that if an experimental subject wears special glasses that invert his visual field, such that he initially experiences the world as upside down, within a few days the subject adapts and comes to experience the world as rightside up again while still wearing the glasses.
Heisenberg and coworkers developed an ingenious means of testing whether this sort of neuroplasticity was a uniquely human capacity. They devised a way of causing fruit flies to experience an inverted visual field, just as though they were wearing the special inverted-field glasses.
What happened when fruit flies were subjected to the same experience as the human subjects? They underwent the very same sort of initial disorientation and subsequent adaptation. That is, they soon learned to fly about normally, which presumably meant that they, too, were now experiencing their world as rightside up.
In other words, the phenomenon of adaptation to an inverted visual field has nothing to do with any essentially human capacities. Heisenberg and his team have demonstrated that the fruit fly’s brain possesses a power of plasticity—of compensatory adaptation to unprecedented challenges—very similar to that which the human brain enjoys.
Heisenberg, Martin and Reinhard Wolf, Vision in Drosophila: Genetics of Microbehavior. Berlin: Springer-Verlag, 1984; pp. 194–204.
Heisenberg, Martin, Reinhard Wolf, and Björn Brembs, “Flexibility in a Single Behavioral Variable of Drosophila,” Learning and Memory, 2001, 8: 1–10.
Stratton, George M., “Vision without Inversion of the Retinal Image,” Psychological Review, 1897, 4: 341–360, 463–481.
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Mriganka Sur was born in India, in Uttar Pradesh state, in 1953. He was educated in India and at Vanderbilt University in the U.S. Since 1986, he has been a professor of neuroscience at MIT.
Sur’s team has demonstrated brain plasticity in non-human animals in a way that is even more striking than Heisenberg’s experiments with fruit flies.
In a nutshell, they have operated upon newborn ferrets in order to disconnect the optic nerve from its usual target are in the visual cortex of the brain, and reconnect it instead with the auditory cortex—the part of the brain that ordinarily supports hearing.
At first, as you might expect, the animals have difficulty getting around. But in a relatively short time, they learn to see nearly as well as normal control animals.
In other words, the brains of these animals are able to compensate for the massive injury that has been done to them, and to reorganize themselves in such a way as to compensate for the lost functionality.
In the case of Bach-y-Rita’s experiments, I put the word “see” in quotation marks. But in the case of Sur’s experiments, it seems unjustifiable to do so. If the animals are using their eyes to navigate their world quite successfully, it seems a mere prejudice to say that they are not “seeing” in the full sense of the word, just because they are using their auditory cortex to do so.
Newton, Jessica R. and Mriganka Sur, “Rewiring Cortex: Functional Plasticity of the Auditory Cortex during Development,” in Josef Syka and Michael M. Merzenich, eds., Plasticity and Signal Representation in the Auditory System. Berlin: Springer, 2005, pp. 127–138.
Sharma, Jitendra, Alessandra Angelucci, and Mriganka Sur, “Induction of Visual Orientation Modules in Auditory Cortex,” Nature, 2000, 404: 841–847.
Tropea, Daniela, Audra Van Wart, and Mriganka Sur, “Molecular Mechanisms of Experience-Dependent Plasticity in Visual Cortex,” Philosophical Transactions of the Royal Society, 2009, B 364: 341–355.
von Melchner, Laurie, Sarah L. Pallas, and Mriganka Sur, “Visual Behaviour Mediated by Retinal Projections Directed to the Auditory Pathway,” Nature, 2000, 404: 871–876.
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Toshiyuki Nakagaki was born in Japan in 1963. He is currently a professor in the Department of Intelligent Complex Systems at Future University—Hakodate, on the island of Hokkaido.
In case you are thinking that all the striking and unexpected adaptive abilities demonstrated so far are capacities of brains, and that therefore they cannot be of general biological significance since brains are a relatively late evolutionary invention, Professor Nakagaki’s work will cause you think again.
Nakagaki and his team work with the plasmodium of the slime mold, Physarum polycephalum. Physarum is an amoeboid, multinucleate protist—a single eukaryotic cell, but with thousands of nuclei. Its plasmodium is the feeding form of its life cycle.
The Physarum plasmodium consists of a complex network of veins filled with streaming cytoplasm. It is capable of extending and retracting these veins in all directions, as it probes its environment, exploring for food.
Nakagaki and his team have demonstrated that if a complicated maze is cut into agar on a Petri dish, a plasmodium is placed inside the maze, and food is placed at two entrances to the maze, then the plasmodium will spontaneously expand its veins throughout the maze in such as way as to connect the two entrances by the shortest route possible.
Of course, you may say that Physarum is simply adjusting itself to the most efficient configuration. That is no doubt true. But in order to do so, it must “solve” the maze in the sense that it must determine what the shortest path through the maze is. And somehow it “knows” to do this even though it possess neither brain nor nervous tissue of any sort.
In more recent experiments, the Nakagaki team has shown that if pieces of a plasmodium are placed at positions on a flat surface corresponding to cities on a map, they will spontaneously create the minimal network of links necessary to connect to each other in the most efficient way possible—and these plasmodial links will correspond fairly accurately to the highway system that people have devised to connect the corresponding cities.
Finally, Physarum has been shown to have the ability to learn to anticipate novel types of periodic behavior.
What human civil engineers and physicists require long training and advanced mathematics to achieve, Physarum achieves—once again—without a nervous system of any sort.
Nakagaki, Toshiyuki, Hiroyasu Yamada, and Ágotha Tóth, “Maze-Solving by an Amoeboid Organism,” Nature, 2000, 407: 470.
Nakagaki, Toshiyuki, Atsushi Tero, Ryo Kobayashi, Isamu Onishi, and Tomoyuki Miyaji, “Computational Ability of Cells based on Cell Dynamics and Adaptability,” New Generation Computing, 2009, 27: 57–81.
Nakagaki, Toshiyuki and Robert D. Guy, “Intelligent Behaviors of Amoeboid Movement Based on Complex Dynamics of Soft Matter,” Soft Matter, 2008, 4: 57–67.
Saigusa, Tetsu, Atsushi Tero, Toshiyuki Nakagaki, and Yoshiki Kuramoto, “Amoebae Anticipate Periodic Events,” Physical Review Letters, 2008, 100: number 018101.
Tero, Atsushi, Ryo Kobayashi, and Toshiyuki Nakagaki, “A Coupled-Oscillator Model with a Conservation Law for the Rhythmic Amoeboid Movements of Plasmodial Slime Molds,” Physica D, 2005, 205: 125–135.
Tero, Atsushi, Seiji Takagi, Tetsu Saigusa, Kentaro Ito, Dan P. Bebber, Mark D. Fricker, Kenji Yumiki, Ryo Kobayashi, and Toshiyuki Nakagaki, “Rules for Biologically Inspired Adaptive Network Design,” Science , 2010, 327: 439–442.
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What is the bearing of these experiments on the controversy surrounding evolution?
The point is simply this: Organisms of all sorts are capable of intelligent, goal-directed, adaptive behavior that cannot possibly be accounted for on the basis of the theory of natural selection.
Never in the evolutionary history of human beings was there selection for “seeing” with the tongue.
Never in the evolutionary history of fruit flies was there selection for adaptation to an inverted visual field.
Never in the evolutionary history of ferrets was there selection for the brain reorganization necessary to see with the auditory cortex.
And never in the evolutionary history of the slime mold was there selection for solving mazes.
Of course, the Darwinist will say that there is no need to posit past selection for plasticity. Instead, we will be invited to view plasticity as a “spandrel”—an accidental side effect of other abilities that were selected for.
But that would be entirely ad hoc. There is absolutely no evidence to support such a claim.
Moreover, it would be absurd, in terms of the relative significance of cause and effect.
To say that the massive reorganization exhibited by the brain in the first three experiments is a side effect of selection for some specific neural trait like vision would be like saying that binocular vision in all its complexity is a side effect of selection for the retina or selection for the lens. It would be to confuse the tail with the dog.
Another strategy that the desperate Darwinist might adopt would be to posit selection for an entirely general capacity for intelligent, goal-directed plasticity.
That is certainly a more promising way to go. However, such a strategy would still be tantamount to admitting defeat.
Why? Because it would be to acknowledge the existence of a fundamental, inherent, and quite general biological principle of what we might call “adaptivity.”
Why would that matter? Because the main task of Darwinian theory is to “reduce” teleology and normativity to mechanism.
Therefore, as soon as the Darwinist admits the reality of a general capacity for adaptivity extending throughout all of the living world, he has already given away the whole ballgame.
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