Degeneracy and natural selection

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Chapter 2 endnote 22, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

The one-to-many principle [of degeneracy] ​— ​any single neuron can contribute to more than one outcome ​— ​is metabolically efficient and increases the computational power of the brain. This kind of brain creates a flexible mind without fingerprints.

Chapter 13 endnote 8, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

Such a brain [with degeneracy] therefore makes you more likely to survive and pass your genes to the next generation. [...] Degeneracy accompanies natural selection. It makes the brain more resilient to injury, which is why natural selection favors a brain built with degeneracy. The variation that degeneracy provides is a prerequisite for natural selection in the first place.

Degeneracy, you may recall, occurs when different sets of elements are interchangeable in a given context, performing the same function or producing the same output in that context. Degeneracy therefore makes a system more complex, which increases its robustness to damage.[1][2] An example is lesion-induced adaptive plasticity.

Degeneracy is a prerequisite for — and product of — natural selection.[3] When the context changes, its different sets of elements might perform different functions or produce different outcomes. In other words, when you’re surrounded by a variable world, degeneracy pays. If some disaster were to alter the human environment dramatically, a species built with degeneracy (in genes, proteins, antibodies, and brain networks) has a better chance to survive because natural selection has more variation to act on immediately, without waiting for reproduction and mutation.[1][4][5] Degeneracy creates a hidden reservoir of genetic variation and therefore ensures the survival and evolvability of a species, because it promotes evolutionary innovation when it is most needed. Evolution not only depends on degeneracy but also brings it about (i.e., selects for it).[3]

Therefore, degeneracy allows for:

  • Phenotypic variety and change
  • New functions to improve or maintain fitness in a new environment
  • New opportunities for innovation

So, degeneracy allows natural selection to happen quickly. Such a view is consistent with the evidence that natural selection proceeds very quickly; it only takes a handful of generations for natural selection to adjust the beak shape of Darwin’s finches to changes in the environment.[6] But this runs contrary to the assumptions of evolutionary psychology, which proposes that the human mind remains adapted to the conditions of the Pleistocene era, because it takes eons for natural selection to sculpt naturally available mutations.

The fact that degeneracy and natural selection are linked also runs counter to the assumption that natural selection prefers systems of independent, single-function modules, such as a brain filled with mental organs. Natural selection rarely (if ever) produces systems of independent modules. Computer simulations show that modularity evolves only when evolutionary goals themselves are modular.[7]

Evolution must select for the genes that produce our kind of brain. And a human brain has a genotype (sequences of DNA) that itself is degenerate and complex. The more important a system is to the survival of a species, the more degeneracy and complexity will exist in the genes that support that system.

Degeneracy is not equipotentiality

Equipotentiality is the idea, developed by the psychologist Karl Spencer Lashley, that any part of the brain can stand in for any other part. Lashley observed in a rat brain that destroying gray matter and white matter throughout the cortex did not disrupt functions. He erroneously interpreted this finding to mean that all neurons are equivalent, when in fact he was observing degeneracy in action.[5][1] Degeneracy makes functions more reliable.[5] This may be why it looks like large swaths of cortex can be damaged with no effect.


Notes on the Notes

  1. 1.0 1.1 1.2 Whitacre, James, and Axel Bender. 2010. "Degeneracy: a design principle for achieving robustness and evolvability." Journal of Theoretical Biology 263 (1): 143-153.
  2. Whitacre, James M. 2010. "Degeneracy: a link between evolvability, robustness and complexity in biological systems." Theoretical Biology and Medical Modelling 7 (6): 1-17.
  3. 3.0 3.1 Edelman, Gerald M., and Joseph A. Gally. 2001. “Degeneracy and Complexity in Biological Systems.” Proceedings of the National Academy of Sciences 98 (24): 13763–13768.
  4. Wagner, 2005 [full reference to be provided]
  5. 5.0 5.1 5.2 Grashow et al., 2009 (Check) [full reference to be provided]
  6. Grant [full reference to be provided]
  7. Kashtan, Nadav, and Uri Alon. 2005. "Spontaneous evolution of modularity and network motifs." Proceedings of the National Academy of Sciences 102 (39): 13773-13778.