[R] Simulation - Natrual Selection

Mike Marchywka marchywka at hotmail.com
Wed Jan 5 17:37:04 CET 2011

> Date: Wed, 5 Jan 2011 15:48:46 +0000
> From: benjamin.ward at bathspa.org
> To: r-help at r-project.org
> Subject: [R] Simulation - Natrual Selection
> Hi,
> I've been modelling some data over the past few days, of my work,
> repeatedly challenging microbes to a certain concentration of cleaner,
> until the required concentration to inhibit or kill them increaces, at
> which point they are challenged to a slightly higher concentration each
> day. I'm doing ths for two different cleaners and I'm collecting the
> required concentration to kill them as a percentage, the challenge
> number, the cleaner as a two level variable, and the lineage theyre in,
> because I have several different lineages. I'm expecting the values to
> rise for one cleaner but not the other as they aqquire resistance for
> one but not the other. Which has happened, but I have wide variation
> because one linage aqquired a very dramatic change which has made it
> immune to 50%, whereas the others, have exhibited a much more gradual
> increace, and so I have very weak p values for the cleaner variable,
> because it is secondary to the challenge vector, which has the most
> explanatory power, because without time and these challenges, the
> selection would no happen. I was using two bacterium species, but one
> was keen on giving hight erratic results, and insisted on becoming cross
> contaminated, BUT if I include it's data, It shoves cleaner over the
> p0.05 threshold, so i may just be having a problem with lack of data. So
> I've been asking about bootstrapping, which I plan to do to my cases,
> and thenfit a model to see what the confidence is like then. I assume if
> I bootstrap then it will re-select whole cases, and not jumble
> everything up, otherwise a microbe (totake the most extreme value as an
> example) with 50% concentration tolerance at the beginning, would make
> no sense at all. I'm also planning on doing models lineage by lineage,
> rather than putting them into one whole, just to have a look at what
> happens.
You can't really have a p-value without a specific hypothesis to test,
if you have that then all your other questions are probably easy to answer.
Generally you want to sample from things that are "iid" or maybe you
want to test the "identical" i. 

Generally you want to have done a lit search ahead of time and 
had some idea of likely evolution dynamics of your system given
your design and things like your forcing functions etc.
Most statisticians would not take seriously a posteriori designs and
indeed it can be hard to avoid rationalization and selection bias ( problems
that always and only effect people who disagree with me LOL) as being
anything other than exploratory or hypothesis generating- you are looking
for predictive value. While it is not always worthwhile doing blind tests,
it may be something worth considering ( do you know which group gets what thing?)

> But what I really wanted to know from this email, was if there's a
> package or function for natrual selection simulation I could make use
> of, to see if I can simulate the experiment. I want to start with a


but as implied above, R has lots of analysis stuff and maybe you
would find something more useful that is not linked to the keywords
you suggest. You may find, for whatever reason, you could write a differential
equation to express your results but that isn't often used with "natural selection."

> distribution of concentration tolerance values, taken from th

> inhibitory concentration values from my first lot of microbes, back when
> term began. Draw 3000 from this. Then values in that draw that fall
> below the exposure concentration I did in my experiment, are removed, or
> have a high chance of being removed. Then, from what is left, a draw is
> made again - or perhaps a copy operation (rather than a random draw)
> until I have 3000 again, rather than have all exactly the same
> concentration, then a value can be added to some of them, that increaces
> their concentration tolerance slightly, but not by a great deal, except
> in a few individuals, where it may be increaced dramatically(some sort
> of exponential dstribution perhaps). Then when the distribution of this
> simulated population of microbes has reached the next concentration
> (possibly the mean or mode of the distribution) (I have a series of 1 in
> 2 dilutions, so 100% 50%, 25% and so on), then they move on to the next
> concentration.
> I know it's probably quite a heavy thing, it was just a thought that
> came to me, if anybody has any experience in this area of R or knows of
> something that allows this to be done, please let me know.
> Thanks,
> Ben.
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