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Question's context

In the context of a presentation on mathematical models for antibiotic-resistance bacteria research, someone mentioned that wild type strain (WT) are expected to be more fit than mutant strains.

Assume that there is one WT introduced to an environment E (no drugs present), and consider the WT mutates, yielding a certain number of fit-enough mutant strains.

Question

Is WT expected to be the ONLY strain to persist in the long run?

How common is this scenario?

What about co-existence?

Examples and references for beginners will be very appreciated.

My context: Please note that this may be a very naive question, and I may even be using the wrong terminology, given my background on mathematical modeling and a very, very basic knowledge on evolution.

Thinking about dynamical systems, I would have guessed that coexistence would be possible, so I didn't understand how strong was such an assumption.

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  • $\begingroup$ There is no such thing as a "wild-type" strain. $\endgroup$
    – user37894
    Apr 18, 2018 at 9:04

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I will assume the mutation is not under some kind of balancing selection such as negative frequency dependent selection. As fitness is conditional upon the environment, I will assume a constant environment (heterogeneous environment can also lead to balancing selection).

Is WT expected to be the ONLY strain to persist in the long run?

Yes. In fact, even if the mutation was beneficial, as it starts out at low frequency it still has a low probability to actually fix (fixation = reaching a frequency of one).

How common is this scenario?

Mutations happen constantly. For example, every new human baby carry something like 10-100 new mutations that were not present in any parent. Most mutations are neutral or deleterious, only very few are beneficial. New mutations have a low probability to reach fixation even if the mutation is beneficial. The exact probability depends upon the selection coefficient $s$, the dominance coefficient $h$ and the effective population size $N_e$.

What about co-existence?

In absence of balancing selection (or frequent recurrent mutations) and assuming that ressources are limited, then one strain will necessarily end up fixing. Note that this is also true under genetic drift only and does not even require any selection.

Examples and references for beginners will be very appreciated.

Evo101 is a very short, very easy introduction to evolutionary biology. You should probably have a look at it.

Then, for something a bit more advanced, and because your background in mathematical modelling, I recommend having a look at the book Population Genetics: A Concise Guide by Gillespie

For more book recommendations, please have a look at the posts

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  • $\begingroup$ Perhaps you can add that fitness is conditional. $\endgroup$
    – WYSIWYG
    Apr 20, 2018 at 12:09

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