Re: Cost of natural selection revisited


"Andrew Lang" <fireforeg@xxxxxxxxx> wrote in message
news:gpojgg$1f22$1@xxxxxxxxxxxxxxxxxxxxxx
Graham Jones wrote:
I am not clear about your definitions here. It could be that most
females are producing .5 offspring, a few fit ones are producing 3 and
the
selection intensity would be log(3/.5) = log(6). It seems this could be
larger than what you call 'Nunney's maximum selection intensity' when
R=10.

All females have the same number of offspring (except to the extent that
number is non-integer, so if fecundity is 2.33, most have 2 children but
every third female has 3). Not all children survive, but this is based
on genetic fitness and is the point of the exercise.

Selection intensity is the log of (children born) / (children that
survive). For a species to maintain its numbers (as the simulated ones
do) two children must survive per female on average, in the long term.
The genes of females that only have 0.5 surviving children will die out.
Those that have 3 surviving children will become the norm and population
pressure will reduce the number to 2.


OK, that's clear enough, but I don't think you are using Haldane's
definition of 'selection intensity'. He defines it as log(optimum survival
rate/some other survival rate). This number directly affects how quickly
substitution happens.

I'm not sure how much sense it makes to compare Haldane's substitution rate
with Nunney's anyway. They are looking at quite different situations.


How are you implementing 'typically the fittest individuals mate with
each
other'?

Sort all the males by fitness; sort all the females by fitness. Match
them one-one all the way down. There will be some leftovers of one sex
or the other; these fail to find mates. Just like real life.


That's not like real life at all. You are giving the organisms supernatural
powers to determine fitness in their mates, and it is not surprising that
this speeds up substitution a lot.

I don't mean to sound too critical. You seem to have done a lot of useful
work!

Graham


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