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Bob O'H

Posts: 2564
Joined: Oct. 2005

 Quote (Steve Schaffner @ June 18 2009,20:53) It would make more sense if it were described in terms of some other phenotype with an effect on fitness. The phenotype has a genetic component and an environmental (or random) component, i.e. has a heritability. The phenotype then confers a fitness, which is the probability of successful reproduction. The number of successful offspring is also drawn from a random distribution, which is what's being done in this bit of code (I guess treated as a binomial distribution).

The problem with describing the model in terms of a phenotype is that you then have to map the effect of the phenotype onto fitness.  It's easier just to leap in and model fitness directly.  Everything works fine if you define fitness as proportional to the expected number of offspring produced by an individual.

I haven't looked at the code, but if we assuming a constant population size and discrete generations, then (ignoring recombination and mutation), the way to model this is to assume that each parent has a fitness si.  The offspring are then drawn from a multinomial distribution with probability for the ith parent being

si/sum(si)

(this would reduce to a binomial distribution if there were only 2 parents).  The multinomial sampling is genetic drift.

You can treat log(si) as you would any standard trait: it's additive, so you can add the genetic and environmental effects.

 Quote As a model of selection that seems reasonable (apart from the way the noise scales), but expressing it in terms of the heritability of fitness I find hard to understand -- fitness isn't a phenotype, it's a measure of the success of a phenotype. And the whole thing is pretty convoluted, when the essence of the model could be captured simply by assigning a fitness to the genotype and then calculating the number of offspring. This is a model of evolution written by a breeder rather than by a population geneticist, I would say.

Not even a breeder: I would expect them to understand heritability a bit better than this!

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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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