Selective Neutrality of 6pgd Allozymes in E. COLI and the Effects of Genetic Background
TLDR
The observed selection is due to the allozymes themselves (or to associated regulatory elements), as the selection disappears when the chemostats are limited by a different carbon source (ribose plus succinate).Abstract:
We have used gluconate-limited chemostats to study selective differences between isogenic strains of Escherichia coli K12 into which four naturally occurring alleles coding for allozymes of 6-phosphogluconate dehydrogenase (6PGD) had been transferred. The limit of detectability of selection with our procedures is a selection coefficient of 0.5%. In the normal E. coli K12 genetic background, all alleles are selectively neutral or nearly neutral. The absence of detectable selection does, however, depend on genetic background and on such environmental factors as cell density. In a genetic background containing a mutation that cuts off the alternative metabolic route for 6-phosphogluconate, selection between allozymes can be detected, and the selection is in the direction expected from the measured apparent K m values of the allozymes. Even when the alternative metabolic route is not blocked by mutation, one of the 6PGD allozymes has a detrimental, but density-dependent, interaction with a mutation conferring resistance to bacteriophage T5. In all cases, the observed selection is due to the allozymes themselves (or to associated regulatory elements), as the selection disappears when the chemostats are limited by a different carbon source (ribose plus succinate). Nevertheless, the four alleles do seem to be selectively neutral or nearly neutral in the normal E. coli K12 genetic background. Moreover, the distribution of allele frequencies in natural populations of E. coli is in accord with the expectations of selective neutrality. I am inclined to suspect that we see, at least in some [cases], variations which are of no service to the species, and which consequently have not been seized on and rendered definite by natural selection…. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed…. We may easily err in attributing importance to characters, and in believing that they have been developed through natural selection;… many structures are now of no direct use to their possessors, and may never have been of any use to their progenitors…. [On the other hand,] we are much too ignorant in regard to the whole economy of any organic being to say what slight modifications would be of importance or not.read more
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Positive Darwinian selection after gene duplication in primate ribonuclease genes
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References
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Book
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TL;DR: Molecular Genetics (Biology): An Overview | Sciencing Experimental in Molecular Genetics Experiments in molecular genetics (1972 edition) | Open ...
The neutral theory of molecular evolution.
TL;DR: The neutral theory as mentioned in this paper states that the great majority of evolutionary changes at the molecular level are caused not by Darwinian selection but by random drift of selectively neutral mutants, which has caused controversy ever since.
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Mathematical Population Genetics
TL;DR: In this paper, the authors present a model based on the Wright-Fisher In.nitely Many Alleles Model and the Cannings (Exchangeable) Model: Two-Alleles 3.4.
Journal ArticleDOI
A molecular approach to the study of genic heterozygosity in natural populations. ii. amount of variation and degree of heterozygosity in natural populations of drosophila pseudoobscura
Richard C Lewontin,J. L. Hubby +1 more
TL;DR: This study shows that there is a considerable amount of genic variation segregating in all of the populations studied and that the real variation in these populations must be greater than the authors are able to demonstrate.