Non-Random wrt Fitness Mutations?

Discussion in 'Biology & Genetics' started by contrarian, May 8, 2006.

  1. contrarian Registered Senior Member

    Messages:
    110
    From the quote above, it seems a reasonable definition of random with respect to fitness(R wrt F) mutations is mutations which take place at the same rate regardless of whether they are fit and nonfit *when everything else is equal*. One way of measuring this would be for the exact same (physical) mutation one would expect the rate of its occurence to be the same in environmental conditions where the mutation results in a fit phenotype as when environmental conditions cause the phenotype to be nonfit.

    Given this definition, I don't believe that 1. there is ANY scientific evidence for random wrt fitness mutations and 2 I believe there is SOME scientific evidence for nonrandom wrt fitness mutations.

    My argument for 2. is based on a simplified (for argument's sake) take on starvation induced hypermutation, whereby a starving bacteria increases its mutation rate when it is starving. Let's say that a hypothetical bacteria has 1000 possible (physically)mutations, each of which have the same probability of occuring as any other mutation at any given time.

    Let's say that while the organism is not starving it has the following possibilities for its mutations to affect fitness. 990 mutations will be neutral(no effect on fitness), 9 will be negative(reduce fitness) and 1 will be fit(increase fitness). Upon starvation, however, 998 mutations will be neutral and 2 will be fit. Since the bacteria is already starving, its fitness cannot decrease(it cannot reproduce at all) and I will say that a previously neutral mutation will improve digestion(possibly by allowing digestion of a different food source). If starvation causes a doubling of the mutation rate and the bacteria starves 1/2 the time, we can analyse the relationships that exist btw mutations and their fitness.

    Non starving 989 neutral 9 negative 1 positive
    Starving 1996 neutral 0 negative 4 positive

    If we measure the occurence of the mutation that changed from neutral to fit under starvation conditions, we see that it takes place twice as often when it is fit. Likewise a mutation that moves from negative to neutral under starvation will happen twice as often when its fitness value is higher than when it is lower.

    Now, as far as I can see, even this simplified(IMO uncontroversial) exercise shows that mutation is sometimes nonrandom wrt fitness, so it appears that random mutation is falsified.

    Any comments?
     
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  3. valich Registered Senior Member

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    3,501
    There is always random mutation in the genotype, but neutral theory suggests that most evolutionary change is the result of genetic drift acting on neutral alleles that express themselves in the phenotype. In the phenotype the genome still influences the phenotype and can lead to mutations.

    "In genetic drift, new alleles may decline and disappear, or in rare cases they may become "fixed"--meaning that the substitution they carry becomes a universal feature of the population or species. When an allele carrying one of these new substitutions becomes fixed, the effect is to add a substitution to the sequence of the previously fixed allele. In this way, neutral substitutions tend to accumulate, and genomes tend to evolve." http://en.wikipedia.org/wiki/Neutral_theory_of_evolution

    More tomorrow.
     
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  5. Maast AF E-7 Retired Registered Senior Member

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    101
    Man is that quote out of context.

    In the context of the quote, mutations arise non-randomly on a genome <b>due to their exposure to being mutated</b> that is to say that big portions of genomes are wrapped up and in-accessable to DNA changing elements. Basically the parts that are working and coding for proteins, etc are exposed to the random whack of a mutagen and mutate much more than other portions.

    In addition big portions are non-coding (junk DNA), you could change anything in it and (probably) nothing would happen. Mutations in the wrapped up portions of a genome usually happen only during transcription in cell division and so yes, portions of a genome do get mutated more than others, that is to say non-randomly.

    Heh, took a class once using that very same textbook, bloody expensive book too.

    I might be wrong, but the question sounded like the opening salvo of a "intelligent design" [pejorative deleted]
     
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  7. contrarian Registered Senior Member

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    110
    Hi Maast,

    Since it appears that we agree that some portions of a genome have a higher likelihood of mutating than others, I don't see how you feel that I am taking the quote out of context. IAC, the quote is not talking solely about the difference btw transcribed and non-transcibed sections of the genome. Mutational hotspots can be the result of other processes.

    http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=10631137&dopt=Citation
    "This nonhomogeneous distribution of variation suggests that multiple mutational hits at certain sites are common, an observation that challenges the fundamental assumption of the infinite-sites-mutation model. The nonrandom patterns of recombination and mutation suggest that randomly chosen single-nucleotide polymorphisms may not be optimal for disequilibrium mapping of this gene. Overall, these results indicate that both recombinational and mutational hotspots have played significant roles in shaping the haplotype variation at the LPL locus"

    IAC, that portion of my argument was only to try and establish what is meant by random wrt fitness(in my experience this is confusing concept). I am curious, do you agree that random wrt fitness means a situation where fit and nonfit mutations are equally likely, *where everything else is equal*? If you do, what do you think about the rest of my argument(that one can easily see how mutations could be nonrandom wrt fitness)?

    I am not making a pro-intelligent design argument, I just disagree with some elements of evolutionary theory.

    valich, I don't disagree that genetic drift is a part of evolution, my argument is addressed specifically towards the nature of mutation. As such, I don't disagree with what you posted, it just does not address what the nature of that mutation might be.

    Cheers,

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