William H. Calvin
ISLAND BIOGEOGRAPHY IN CEREBRAL CORTEX: Shaping Up New Cerebral Codes in Functional Archipelagos. William H. Calvin*, Univ. of Washington, Seattle 98195-1800.
While a darwinian process has six essential features (a pattern that copies with variants which compete for a limited workspace biased by a multifaceted environment, with this selective survival to reproductive maturity skewing the next generation's variants), another half-dozen features affect the speed (or stuck-in-a-rut stability) of evolutionary processes. We are all familiar with what the French call l'esprit d'escalier, thinking of the right response only after leaving the party. If plans of quality are to shaped up within the situational "windows of opportunity," some accelerating and equilibrium-escaping features may prove essential for mental versions of a darwinian process.
A well-known feature of island biogeography is relative speed, as in the differentiation of Darwin's finches unbuffered by large continental gene pools. When rising sea level converted part of the European coastline into an island (Jersey), the red deer trapped there underwent a rapid dwarfing in only 10,000 years. A second accelerating feature of islands is that a species can go extinct locally. This creates an "empty niche" which, when rediscovered, provides enough resources for even the rarer variants of the species for awhile. Should disease or climate change then shrink this boomtime population, some of those rarer variants may prove to have the right combination of features to survive the bottleneck and repopulate the island. Archipelagos allow for many parallel experiments. Episodes that recombine the islands (as when sea level temporarily lowers) create winner-take-most tournaments.
Triangular mosaics of synchronized cortical neurons not only provide the six essentials but form temporary "islands" with finchlike variant patterns; changes in cortical excitability may play a parcellation-then-tournament role similar to sea level in speeding the evolution of novel cerebral codes. (See W. H. Calvin, Soc. Neurosci. Abstr. 1992, 1993, 1994, and Scientific American 10/94).
University of Washington
Department of Psychiatry & Behavioral Sciences
Seattle, Washington 98195-1800 USA
In the present Darwin Machine model, insights from the periodic nature of the horizontal connectivity of the superficial pyramidal neurons are used to predict a triangular mosaic of synchronized neurons at 0.5 mm spacings. Like the unit pattern of mosaic wallpaper, the largest unique spatial pattern would be contained in a 0.5 mm hexagon - despite initial separations of the activated feature detectors that span much longer distances. The initial representation appears to be collapsed by end-around overwriting into a compact one that has an inherent error-correction feature. This candidate seems worthy of the name Cerebral Code.
A darwinian process, whether species evolution or the immune response, has six essential features:
Fluctuating environments (seasons, climate changes, diseases) change the name of the game, shaping up more complex patterns capable of doing well in several environments. For such jack-of-all-trades selection to occur, the climate must change much faster than efficiency adaptations can track it - or "lean mean machine" specialists will dominate the expensive generalists. Islands amplify the effects of climate:
Parcellation, as when rising sea level converts the hilltops of one large island into an archipelago, typically speeds evolution. In part, this is because individuals mostly live on the margins of the habitat where selection pressure is greater; in part, because there is no large continental population to buffer change. When rising sea level converted part of the coastline of France into the island of Jersey, the red deer trapped there in the last interglaciation underwent a considerable dwarfing in only a few thousand years. Local extinctions (as when an island population becomes too small to sustain itself) speed evolution later because they create empty niches. When subsequent pioneers rediscover the unused resources, their descendants go through a series of generations where there is enough food even for the more extreme variations that arise, ones that would ordinarily lose out in the competition with the more optimally endowed. When the environment again changes, some of those more extreme variants may be able to cope better with the third environment than the narrower range of variants that would reach reproductive age under the regime of a long-occupied niche.There are also "catalysts" acting at several removes, such as Darwin's example of what introducing cats to an English village would do for the bee-dependent flowers, via reducing the rodent populations that disrupt bee hives.
An example of how these catalysts work together is island biogeography (patchy resource distribution is much the same). Archipelagos allow for many parallel experiments. Episodes that recombine the islands (as when sea level falls during an ice age) create winner-take-most tournaments. Most evolutionary change may occur in such isolation, in remote valleys or offshore islands, with major continental populations serving as slowly changing reservoirs that provide pioneers to the chancy periphery.
Triangular mosaics of synchronized cortical neurons not only provide the six essentials but form temporary "islands" with finchlike variant patterns; changes in cortical excitability may play a parcellation-then-tournament role similar to sea level in speeding the evolution of novel cerebral codes. (See W. H. Calvin, Scientific American, October 1994).
A cerebral code consists of N neurons which, for geometric reasons, cover an area no larger than a 0.5mm hexagon (any larger and another member of a triangular array would be encountered). This compact pattern becomes standardized across the mosaic (a variant must overcome six simultaneous EPSPs); such pattern "crystallization" is a form of error correction. It's as if there were a honeycomb of clones of this standardized pattern; different clones may compete for the workspace, the size of the territory presumably important in gating thought into action.
When a territory's excitability falls below the requirements for maintaining triangular arrays, it becomes a barrier. Error correction can be escaped at narrow gateways through such barriers, allowing variants to arise that compete with the parental patterns.
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