Across the gap

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RAPID EVOLUTIONARY ESCAPE BY LARGE POPULATIONS FROM LOCAL FITNESS PEAKS IS LIKELY IN NATURE:

Fitness interactions between loci in the genome, or epistasis, can result in mutations that are individually deleterious but jointly beneficial. Such epistasis gives rise to multiple peaks on the genotypic fitness landscape…Here we develop an analytic expression for Ncrit, the critical population size that defines the boundary between these regimes, which shows that both are likely to operate in nature. Frequent recombination may disrupt high-fitness escape genotypes produced in populations larger than Ncrit before they reach fixation, defining a third regime whose rate again slows with increasing population size. We develop a novel expression for this critical recombination rate, which shows that in large populations the simultaneous fixation of mutations that are beneficial only jointly is unlikely to be disrupted by genetic recombination if their map distance is on the order of the size of single genes. Thus, counterintuitively, mass selection alone offers a biologically realistic resolution to the problem of evolutionary escape from local fitness peaks in natural populations.

Please note I’m not making assertions about the ubiquity of these novel processes. I suspect they play a role in speciation, but I won’t wager any guesses beyond that.

4 Comments

  1. Is there any mathematical difference between the fitness of a gene being a function of other genes (its genetic environment) and its fitness being determined by external factors? I’m thinking, for example of Darwin’s finches (if I remember correctly): big bill + dry weather = fit, small bill + wet weather = fit, but not the other way around. 
     
    Of course, with genetic environment, you can turn the telescope around and look from the other end, but from the point of view of a single gene, it’s always the case that its fitness depends on its environment. 
     
    Also, the genetic environment is itself subject to evolution, but so is the external environment as far as it’s determined by other life forms. Maybe we should think of epistasis like Red Queen/Co-evolution?

  2. I’m sure this is very important and interesting, but why the f**k can’t biologists write in English any more? Reading this stuff make my brain hurt.

  3. Sounds interesting, but since I don’t know precisely what they mean by ‘map distance’ nor ‘mass selection’ I guess I’m still in the dark. 
     
    One reason for not writing in English is to dress up trivial results… like the ‘sign epistasis’ stuff posted not long ago.

  4. ?Is there any mathematical difference between the fitness of a gene being a function of other genes (its genetic environment) and its fitness being determined by external factors?? 
     
    My guess is that the environment is unusually unaffected by a gene. Thus the environment?s dependence on that gene can be safely ignored. (Clearly humans do affect the environment so there is some dependence of the environment on the gene.) So a system of linear differential equations can model the gene/environment interaction. 
     
    I suspect genes are strongly affected by other genes. So the dependence of one gene on a different gene cannot be ignored. So the gene/gene interactions require a system of non-linear differential equations for modeling. 
     
    Non-linear differential equations are much harder to handle mathematically. (Linear differential equations are a subset of non-linear differential equations. Non-linear equations are usually solved by local approximation with linear solutions.) 
     
    ?One reason for not writing in English is to dress up trivial results… like the ‘sign epistasis’ stuff posted not long ago.? 
     
    (I agree that jargon is used to dress up papers. I disagree that the result was trivial.) 
     
    The result that a gene?s fitness depends on other genes and can change signs was obvious. Successfully building a mathematical model that captures that genetic interaction isn?t trivial. In this case the meat is in the details, not the abstract. (I haven?t read the paper so I?m just guessing.)

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