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Bob O'H

Posts: 2564
Joined: Oct. 2005

Quote (Henry J @ June 19 2009,16:16)

 Quote Of course, there's a big difference between using models to analyze how particular aspects of evolution work and trying to model the entire process well enough to say whether it can occur.

Well, that's just being picky! :p

But yeah, if populate size is allowed to go way up, then the simulation would have to deal with food shortages (i.e., fitness would drop across the board if population gets too large for the food supply).

Henry

That's not too difficult to implement.  You scale the fitness so that it is above 1 at low density, and then apply some density dependence to it, which affects all genotypes equally.  Then you draw the number of offspring from a Poisson distribution with that mean.

i.e. if fitness is si, and the population size is N, then the number of offspring is Poisson distributed with mean si exp(-N/K), for example (exp(-N/K) is the discount for density).

I think doing it this way is nicer because (a) it's easier to programme (there's no independence problems, or multinomials with huge N's), and (b) you're modelling absolute fitness, so it can go below 1, and you might get extinctions.

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It is fun to dip into the various threads to watch cluelessness at work in the hands of the confident exponent. - Soapy Sam (so say we all)

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