quote:
Originally posted by Mister Pamboli:
Interesting stuff. Largely irrelevant, I suspect, but interesting.
I'd be interested in how the graph was arrived at - criteria for defining benefit, how mutations were observed, in what species, that sort of thing.
How would it class say a mutation to greater body weight by the increased adiposity in a mammal? Detrimental - making escape from predators more difficult? Beneficial - better able to survive a harsh winter? Or beneficial today but detrimental tomorrow?
I'm skeptical about graphing a qualitative judgement.
Mr. P: I agree, the graph is interesting. Like you, I would like to know how it was derived.
OTOH, I've always used the following "rule of thumb" in making a relatively objective determination of what constitutes a deleterious, neutral, or beneficial mutation at the gene level:
1. neutral mutation: A neutral mutation would be a change in nucleotide triplet(s) that: a) took place in an intron, and hence had no effect, b) eliminated or modified a redundant codon (again, no effect), or c) produced an alternative form of a gene that made no difference on the efficiency or form of the final product (ex: if the third base in the TCT codon for serine is changed to any one of the other three bases, serine will still be encoded). Since something on the order of 97% of the genome doesn't actually code for anything at all, neutral mutations would tend to be the rule, rather than the exception.
2. deleterious mutation: A deleterious mutation would be a mutation occurring in an exon, an error in the pre-mRNA splicing function, or a translocation that caused loss of function or (damage to) the final protein product. This type of error, if occuring in a somatic cell, will either kill the cell or initiate a tumor. In a germline cell, it will most likely have a negative fitness effect on the zygote.
3. beneficial mutation: A beneficial mutation is a mutation in an exon, or (rarely) a translocation, where the resultant product is either a) more efficient and hence able to perform its metabolic function at lower energy cost; b) create novel genes which allow new opportunties; or c) allow for a new cascade which permits either elimination of now-redundant genes or the co-option of existing cascades to new products.
It should be noted that my definitions are purely genetic. Whereas there are implications in each case for allelic frequency and organism fitness (and ultimately population fitness), the definitions deliberately do NOT discuss the ultimate expression of these changes in an organism. The reason I do it like this is because, once you start talking fitness, the concept
can only be empirically understood in the context of the complete environment (biotic and abiotic) in which the particular organism lives. In short, a beneficial mutation in terms of fitness is one that increases the reproductive success of an individual organism in their
current environment. A deleterious mutation does the opposite. Worse, any change in the environment may cause an allele that is beneficial to turn deleterious - without any further actual mutation.
This is the bit creationists don't seem to grasp. An individual organism's adaptation to its current environment, and by extension the population of which this individual is a part, is by definition the result of beneficial alleles. If it wasn't, NS would long ago have ruthlessly eliminated it.
For any discussion of mutation rates, fitness, fixation rates, mutational load, etc, etc, to be relevant, it must be done IN CONTEXT of the particular organism and its specific environment. Otherwise you're just playing hypothetical construct and semantic games rather than reality.
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