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In Per Bak's "How Nature Works" [1] he describes an simulation he did in which he tried to replicate the series of evolutionary discontinuities and mass extinct

In Per Bak's "How Nature Works" [1] he describes an simulation he did in which he tried to replicate the series of evolutionary discontinuities and mass extinctions we see in Earth's history. The reason this article brought that to mind was that Bak was not able to get the complex behavior by selecting only the fittest to survive. Instead, he ended up having to kill off the least fit of the species and leave the rest to proliferate and develop interdependencies. His result doing that was some very interesting behavior in which extinctions of all sizes happened in the population of species in a way evocative of natural history on Earth. His idea was that in order for an evolutionary system to reach the self-organized critical state in which big changes happen, the species had to develop interdependencies so that the success or failure of one could affect that of the others. It would probably be a much more difficult experimental procedure because he would have to keep the yeast strains in a community and somehow eliminate rather than select a variety, but it would be really interesting to see if the result of a common convergence point would survive with the model of "death of the least-fit" rather than survival of the fittest. 1. http://amzn.com/038798738X http://amzn.com/038798738X

icefox 12y ago
> ...not able to get the complex behavior by selecting only the fittest to survive. Instead, he ended up having to kill off the least fit of the species and leave the rest to proliferate and develop interdependencies. How is "selecting only the fittest to survive" aka killing the least fit different from "kill off the least fit of the species" aka selecting only the fittest to survive they sound like exactly the same thing thing.
- aethertap 12y ago
  I probably could have stated it more clearly like this: "selecting only the fittest ONE to survive," and "killing off the least fit ONE." The difference was in what the composition of the surviving population was - in the extreme case where only the fittest one survives, there's only one species that makes it to the next round. In the case where the least fit one is killed off, there are n-1 species still in the mix with their relationships intact. You can of course extend it to keeping more than one of the fittest, or killing more than one of the least fit. I was just re-reading that section of the book to try to give you a better answer and I think I must have mis-remembered his original experiment a bit (sorry about that). The original evolution model that failed to exhibit the behavior was more complex than just a survival of the fittest (he was playing with mutations and connections between species in a kind of complicated way). The part about killing off the least-fit species really was the key to getting complex behavior though, so at least that part of my original comment was right. The eventual solution was to simplify the model so that the least-fit single species would go extinct and be replaced by a new randomly generated one each round. The fitness of a species was related to its connections, so if one species was to go extinct it could trigger others to cross the fitness threshold and go extinct as well. This led to cascades of extinctions with a power law distribution (like the pattern observed in real extinctions on Earth). The mass extinctions would eventually stabilize at a point where all species were above the fitness threshold, but the "gene pool" had been seeded with lots of new randomly generated species at that time, so there were periods of rapid mutation punctuating long stable periods where not much changed. His book is pretty interesting, although it's sometimes a little over the top with self-congratulation. I'm reading another one now called "Self-Organized Criticality"[1] which is a much more rigorous treatment of the idea that I think will be a bit less sensational and more applicable to real problems. Bak's book is excellent if you want to get excited about making simple models do complex things though. 1. http://amzn.com/0521483719 http://amzn.com/0521483719

icefox 12y ago

> ...not able to get the complex behavior by selecting only the fittest to survive. Instead, he ended up having to kill off the least fit of the species and leave the rest to proliferate and develop interdependencies. How is "selecting only the fittest to survive" aka killing the least fit different from "kill off the least fit of the species" aka selecting only the fittest to survive they sound like exactly the same thing thing.

aethertap 12y ago
I probably could have stated it more clearly like this: "selecting only the fittest ONE to survive," and "killing off the least fit ONE." The difference was in what the composition of the surviving population was - in the extreme case where only the fittest one survives, there's only one species that makes it to the next round. In the case where the least fit one is killed off, there are n-1 species still in the mix with their relationships intact. You can of course extend it to keeping more than one of the fittest, or killing more than one of the least fit. I was just re-reading that section of the book to try to give you a better answer and I think I must have mis-remembered his original experiment a bit (sorry about that). The original evolution model that failed to exhibit the behavior was more complex than just a survival of the fittest (he was playing with mutations and connections between species in a kind of complicated way). The part about killing off the least-fit species really was the key to getting complex behavior though, so at least that part of my original comment was right. The eventual solution was to simplify the model so that the least-fit single species would go extinct and be replaced by a new randomly generated one each round. The fitness of a species was related to its connections, so if one species was to go extinct it could trigger others to cross the fitness threshold and go extinct as well. This led to cascades of extinctions with a power law distribution (like the pattern observed in real extinctions on Earth). The mass extinctions would eventually stabilize at a point where all species were above the fitness threshold, but the "gene pool" had been seeded with lots of new randomly generated species at that time, so there were periods of rapid mutation punctuating long stable periods where not much changed. His book is pretty interesting, although it's sometimes a little over the top with self-congratulation. I'm reading another one now called "Self-Organized Criticality"[1] which is a much more rigorous treatment of the idea that I think will be a bit less sensational and more applicable to real problems. Bak's book is excellent if you want to get excited about making simple models do complex things though. 1. http://amzn.com/0521483719 http://amzn.com/0521483719