SIUC Cognitive Science Colloquium

Evolution of Neural Circuits

David G. King, Ph.D., Associate Professor
Department of Anatomy and Department of Zoology

Footnotes for Evolution of Neural Circuits


". . . the single best idea anyone ever had."

I don't necessarily agree with Dennett's assertion that evolution by natural selection really is "the single best idea anybody ever had".  For example, I think the idea, "Do unto others as you would have others to do unto you," also deserves prominent billing.  Nevertheless, as a foil for the idea that evolution may be important for understanding cognition, Dennett will do nicely.


Behavior is controlled by the nervous system.  

Please don't take "controlled" too seriously.  

More precisely, Differences in the structural and functional organization of the nervous system can be causally associated with differences in behavior.  

Actually, of course, behavior is controlled / caused / determined by the nervous system only by way of interaction (including memory of past interactions) with the rest of the body and with the environment, and also (presumably) with some variables which are influenced by quantum randomness and/or deterministic chaos.  So "control" is problematic, except the ordinary, everyday sense in which the word is commonly used and understood.  

The nervous system consists of large numbers of individual cells, which are electrically active and which communicate with one another by secreting chemicals at sites of contact called synapses.  

In this venue I won't even touch upon the deep and murky concept of "communicate".  I know just enough about semiotics to appreciate what a tangled web that is. 

To rephrase the above statement, The electrical and chemical activity of nerve cells affects the electrical and chemical activity of other nerve cells.

Somehow, the properties of the cells and their connections determine behavior. 

Please ignore the dreadful implications of "determine" and focus on the "somehow".  This is not the place to worry about free will.  

Surely we can agree that differences in cognitive capacity which separate clams from cognitive scientists are due, at least in part, to differences in the structural and functional organization of these creatures' nerve cells.  And those differences are due, at least in part, to differences in the cells' DNA (in interaction, of course, with the rest of their bodies and with the environment, including stochastic factors).

These issues become much more fraught when we consider the effects of genetic information on behavior.

Nevertheless, because adaptive behaviors do evolve (and, presumably, neither by magic nor by "intelligent design"), genes must exert some significant influence on behavior.  To quote a recent (2013) news item in Science, "Animal behavior is determined to a large extent by genes."  I wince at that bald assertion.  But the associated research report in Nature describes behavioral differences between closely related mouse species which can be analyzed just like morphological traits. 

For futher discussion, see below.

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Neural circuits.  

The phrase "neural circuit" is common shorthand (neurobiological jargon) for "a set of nerve cells and their mutual interconnections".  Although many "circuits" do contain loops (i.e., feedback or feed forward connections), the concept of a closed loop is not an essential feature of this rather casual usage.  The term is used most commonly by invertebrate cellular neurobiologists who work with rather small sets of nerve cells (a few to dozens) which can be more-or-less isolated from the rest of the animal (often literally, by dissection).  Such isolated pieces of "neural circuitry" may remain functional in vitro (literally, in a glass dish) and retain the ability to generate patterns of electrical activity quite similar to those that can be recorded from intact animals. [Example.]

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However the brain works, the behavior it produces is readily "reprogrammed" by evolution.  

Like the statement [above] that behavior is controlled by the nervous system, this assertion is simplistic.  Natural selection acts on differences in behavior (just as much as, if not more than, on differences in structure) only to the extent that such differences are statistically associated with differences in hereditary genetic material (DNA sequences).  

Although all behavior involves some interaction between organism and environment, there are nonetheless many aspects of many behaviors which seem to be "determined" or "programmed" by heredity -- that is to say,"by the genes".  Such aspects of behavior are often called instinctive, in recognition of this innate quality.  This does not imply that nothing but the genes is necessary to produce the behavior.  

The cleanest examples (least-compromised by complications related to learning) occur among animals which grow from egg to adult with no contact with other members of the species, including parents, but which nevertheless can perform complete and effectual behaviors (prey capture, nest building, courtship dance, etc.) at first try.  Such instincts can be quite elaborate among the so-called "lower" animals (e.g., spiders building webs).  Comparative evidence indicates that evolution of behavioral differences often precedes, or leads, divergence of more conspicuous morphological differences.  Indeed, many "cryptic species" can be distinguished in the field (as opposed to DNA sequence analysis) only by behavioral differences.

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I heartily recommend the following books by Douglas R. Hofstadter.  

Gödel, Escher, Bach: An Eternal Golden Braid (Basic Books, 1979).  A remarkable, and idiosyncratic, work about the nature of information and meaning, winner of the Pulitzer Prize (for General Nonfiction) and the American Book Award (for Science Hardback) in 1980.

The Mind's I: Fantasies and Reflections on Self and Soul (Basic Books, 1981), together with Daniel C. Dennett (editors).  An anthology, with commentary, of writings about the problem of consciousness.

Metamagical Themas: Questing for the Essence of Mind and Pattern (Basic Books, 1985). The title is an anagram of "Mathematical Games", Martin Gardner's column in Scientific American which Hofstadter inherited from Gardner.  This volume is a collection of those essays, with additional material.  The topics are quite various.

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Comments and questions: dgking@siu.edu
Department of Zoology e-mail: zoology@zoology.siu.edu
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SIUC / College of Science / Zoology / Faculty / David King
URL: http://www.zoology.siu.edu/king/cs01note.htm
Last updated:  18 January 2013/ dgk