The evolutionary advantage of recombination
TL;DR: Computer simulations of substitution of favorable mutants and of the long-term increase of deleterious mutants verified the essential correctness of the original Fisher-Muller argument and the reality of the Muller ratchet mechanism.
Abstract: The controversy over the evolutionary advantage of recombination initially discovered by Fisher and by Muller is reviewed. Those authors whose models had finite-population effects found an advantage of recombination, and those whose models had infinite populations found none. The advantage of recombination is that it breaks down random linkage disequilibrium generated by genetic drift. Hill and Robertson found that the average effect of this randomly-generated linkage disequilibrium was to cause linked loci to interfere with each other's response to selection, even where there was no gene interaction between the loci. This effect is shown to be identical to the original argument of Fisher and Muller. It also predicts the "ratchet mechanism" discovered by Muller, who pointed out that deleterious mutants would more readily increase in a population without recombination. Computer simulations of substitution of favorable mutants and of the long-term increase of deleterious mutants verified the essential correctness of the original Fisher-Muller argument and the reality of the Muller ratchet mechanism. It is argued that these constitute an intrinsic advantage of recombination capable of accounting for its persistence in the face of selection for tighter linkage between interacting polymorphisms, and possibly capable of accounting for its origin.
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TL;DR: The Economy of Nature and the Evolution of Sex*.
Abstract: The Economy of Nature and the Evolution of Sex *. By Michael T. Ghiselin. Pp. xii + 346. (University of California: Berkeley, Los Angeles and London, December 1974.) $12.05; £7.10.
2,034 citations
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...Будучи сформулиро ванным, это положение представляется достаточно оче видным, однако генетики-популяционисты не обращали на него внимания до тех пор, пока Фелсенстейн (Felsenstein, 1974) не вернулся к нему и не получил ряд следствий....
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...Фелсенстейн (Felsenstein, 1974) назвал такие селек тивные факторы «внешними» в противоположность факто рам, «внутренне присущим» генетическим процессам как таковым....
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...Интересное наблюдение сделал Фелсенстейн (Felsenstein, 1974): авторы, которые прямо или косвенно допускают существование случайных процессов в конечных популяциях, приходят к заключению, что ре комбинации ускоряют процесс эволюции, тогда как исследо ватели, изучающие модели бесконечных популяций…...
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...Фелсен стейн (Felsenstein, 1974) называют это явление «эффектом Хилла—Робертсона»....
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...Вторая, возможно более фундаментальная, причина была предложена в работе Фелсенстейна (Felsenstein, 1974)....
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TL;DR: Observed reductions in molecular variation in low recombination genomic regions of sufficiently large size, for instance in the centromere-proximal regions of Drosophila autosomes or in highly selfing plant populations, may be partly due to background selection against deleterious mutations.
Abstract: Selection against deleterious alleles maintained by mutation may cause a reduction in the amount of genetic variability at linked neutral sites. This is because a new neutral variant can only remain in a large population for a long period of time if it is maintained in gametes that are free of deleterious alleles, and hence are not destined for rapid elimination from the population by selection. Approximate formulas are derived for the reduction below classical neutral values resulting from such background selection against deleterious mutations, for the mean times to fixation and loss of new mutations, nucleotide site diversity, and number of segregating sites. These formulas apply to random-mating populations with no genetic recombination, and to populations reproducing exclusively asexually or by self-fertilization. For a given selection regime and mating system, the reduction is an exponential function of the total mutation rate to deleterious mutations for the section of the genome involved. Simulations show that the effect decreases rapidly with increasing recombination frequency or rate of outcrossing. The mean time to loss of new neutral mutations and the total number of segregating neutral sites are less sensitive to background selection than the other statistics, unless the population size is of the order of a hundred thousand or more. The stationary distribution of allele frequencies at the neutral sites is correspondingly skewed in favor of rare alleles, compared with the classical neutral result. Observed reductions in molecular variation in low recombination genomic regions of sufficiently large size, for instance in the centromere-proximal regions of Drosophila autosomes or in highly selfing plant populations, may be partly due to background selection against deleterious mutations.
1,807 citations
Cites background from "The evolutionary advantage of recom..."
...Accumulation of deleterious mutations by Muller's ratchet (FELSENSTEIN 1974; HAIGH 1978) in these rather small populations may contribute to this discrepancy ....
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TL;DR: The complete genome sequence of the obligate intracellular parasite Rickettsia prowazekii, the causative agent of epidemic typhus, is described, which contains 834 protein-coding genes and is more closely related to mitochondria than is any other microbe studied so far.
Abstract: We describe here the complete genome sequence (1,111,523 base pairs) of the obligate intracellular parasite Rickettsia prowazekii, the causative agent of epidemic typhus. This genome contains 834 p ...
1,599 citations
Cites background from "The evolutionary advantage of recom..."
...The accumulation of such harmful but non-lethal mutations is referred to as ‘Muller's ratchet&rsquo...
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TL;DR: This review provides a nonmathematical description of the issues involved in detecting selection from DNA sequences and SNP data and is intended for readers who are not familiar with population genetic theory.
Abstract: There is an increasing interest in detecting genes, or genomic regions, that have been targeted by natural selection. The interest stems from a basic desire to learn more about evolutionary processes in humans and other organisms, and from the realization that inferences regarding selection may provide important functional information. This review provides a nonmathematical description of the issues involved in detecting selection from DNA sequences and SNP data and is intended for readers who are not familiar with population genetic theory. Particular attention is placed on issues relating to the analysis of large-scale genomic data sets.
1,509 citations
Cites background from "The evolutionary advantage of recom..."
...The mutations will tend to interfere with each other and reduce the local effective population size (8, 29 , 40, 57)....
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...Andolfatto et al. ( 4 ) developed a related test to determine whether any subset of consecutive variable sites contains fewer haplotypes than expected under a neutral model....
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01 Jan 1979
1,258 citations
References
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TL;DR: Although it is true that most text-books of genetics open with a chapter on biometry, closer inspection will reveal that this has little connexion with the body of the work, and that more often than not it is merely belated homage to a once fashionable study.
Abstract: PROBABLY most geneticists to-day are some-what sceptical as to the value of the mathematical treatment of their problems. With the deepest respect, and even awe, for that association of complex symbols and human genius that can bring a universe to heel, they are nevertheless content to let it stand at that, believing that in their own particular line it is, after all, plodding that does it. Although it is true that most text-books of genetics open with a chapter on biometry, closer inspection will reveal that this has little connexion with the body of the work, and that more often than not it is merely belated homage to a once fashionable study. The Genetical Theory of Natural Selection. Dr. R. A. Fisher. Pp. xiv + 272 + 2 plates. (Oxford: Clarendon Press; London: Oxford University Press, 1930.) 17s. 6d. net.
7,883 citations
"The evolutionary advantage of recom..." refers background or methods in this paper
...MULLER does not seem to have discussed this point, but FISHER ( 1930) wrote that he discounted the importance of " interspecific " selection " with the possible exception . . . of sexuality itself, which could be interpreted as evolved for the specific rather than for the individual advantage....
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...It turns out that those authors who assumed a finite population (FISHER 1930; MULLER 1932,1958,1964; CROW and KIMURA 1965,1969; BODMER 1970; MAYNARD SMITH 1971a; KIMURA and OHTA 1971; WILLIAMS and MITTON 1973) found an advantage associated with recombination ....
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...FISHER (1930) and MULLER (1932) stated essentially identical theories of the evolutionary advantage to a population of having recombination....
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...This same assumption is made by CROW and KIMURA (1965), MAYNARD SMITH ( 1968) , and BODMER ( 1970) , and is implicit in the work of FISHER ( 1930) and MULLER (1932)....
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TL;DR: It was shown that the selection process can be completely specified by Ni α, Ni βand Nc and the initial gene frequencies and linkage disequilibrium coefficient and it is easily possible to generalize from computer runs at only one population size.
Abstract: (i) A computer simulation study has been made of selection on two linked loci in small populations, where both loci were assumed to have additive effects on the character under selection with no interaction between loci. If N is the effective population size, i the intensity of selection in standard units, α and β measure the effects of the two loci on the character under selection as a proportion of the pheno-typic standard deviation and c is the crossover distance between them, it was shown that the selection process can be completely specified by Ni α, Ni βand Nc and the initial gene frequencies and linkage disequilibrium coefficient. It is then easily possible to generalize from computer runs at only one population size. All computer runs assumed an initial population at linkage equilibrium between the two loci. Analysis of the results was greatly simplified by considering the influence of segregation at the second locus on the chance of fixation at the first (defined as the proportion of replicate lines in which the favoured allele was eventually fixed). (ii) The effects of linkage are sufficiently described by Nc. The relationship between chance of fixation at the limit and linkage distance (expressed as 2Nc /( 2Nc + 1)) was linear in the majority of computer runs. (iii) When gene frequency changes under independent segregation were small, linkage had no effect on the advance under selection. In general, segregation at the second locus had no detectable influence on the chance of fixation at the first if the gene effects at the second were less than one-half those at the first. With larger gene effects at the second locus, the chance of fixation passed through a minimum and then rose again. For two loci to have a mutual influence on one another, their effects on the character under selection should not differ by a factor of more than two. (iv) Under conditions of suitable relative gene effects, the influence of segregation at the second locus was very dependent on the initial frequency of the desirable allele. The chance of fixation at the first, plotted against initial frequency of the desirable allele at the second, passed through a minimum when the chance of fixation at the second locus was about 0·8. (v) A transformation was found which made the influence of segregation at the second locus on the chance of fixation at the first almost independent of initial gene frequency at the first and of gene effects at the first locus when these are small. (vi) In the population of gametes at final fixation, linkage was not at equilibrium and there was an excess of repulsion gametes. (vii) The results were extended to a consideration of the effect of linkage on the limits under artificial selection. Linkage proved only to be of importance when the two loci had roughly equal effects on the character under selection. The maximum effect on the advance under selection occurred when the chance of fixation at both of the loci was between 0·7 and 0·8. When the advance under selection is most sensitive to changes in recombination value, a doubling of the latter in no case increased the advance under selection by more than about 6%. The proportion selected to give maximum advance under individual selection (0·5 under independent segregation) was increased, but only very slightly, when linkage is important. (viii) These phenomena could be satisfactorily accounted for in terms of the time scale of the selection process and the effective size of the population within which changes of gene frequency at the locus with smaller effect must take place.
1,776 citations
TL;DR: The evolution of diploidy from haploidy confers an immediate reduction in the mutation load by concealment of deleterious recessives, but this advantage is lost once a new equilibrium is reached and the development of diPloidy may be because of an immediate advantage rather than because of any permanent benefit.
Abstract: In an asexual population two favorable mutants can be incorporated into the population only if one occurs in a descendant of the individual in which the other occurred. In a sexual population both mutants can be incorporated through recombination. A mathematical formulation is given of the relative rates of incorporation of the new mutations with and without recombination. Recombination is of the greatest advantage when the double mutant is more advantageous than either single mutant, when the mutant effects are small, when mutations occur with high frequency, and when the population is large. On the other hand, for the incorporation of individually deleterious but collectively beneficial mutations, recombination can be disadvantageous. Close linkage has effects similar to those of asexual reproduction. Experimental data on DDT resistance in Drosophila and chloramphenicol resistance in bacteria are cited showing greater development of coadaptation in an asexual system. The evolution of diploidy from haplo...
588 citations
TL;DR: The completely deterministic model is treated, that is, an effectively infinite population so that chromosome frequencies suffice to describe the population and it is shown that if the mutations are favorable and the double mutant is fitter than expected under simple multiplicity, then the frequency of the double is the same with recombination as without it.
189 citations
"The evolutionary advantage of recom..." refers background in this paper
...Some papers (MAYNARD SMITH 1968; ESHEL and FELDMAN 1970) are wholly deterministic....
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...Those who assumed an infinite population (MAYNARD SMITH 1968; ESHEL and FELDMAN 1970) found no such advantage if there was no epistasis and no initial linkage disequilibrium....
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