On the fixation of genes of large effects due to continued truncation selection in small populations of polygenic systems with linkage.
The proportion of fixed loci for desirable genes and the time required for fixation is studied in simulated diploid populations, which have initially aHARDY-WEINBERG structure. A symmetric ten-locus system of additive or dominant genes is simulated with linkages between adjacent loci varying as .005, .05, or .5. A constant degree of upper truncation selection within a population is considered over the generations. In different populations the intensity of truncation is varied asN/N,N/N+2,N/N+4, ..., whereN is the parental population size, specified as 2,4,8 or 16. The selection differential in initial generation, ī, thereby varies from zero to more than two standard deviations in some cases. The initial mean gene frequency,p, simulated in an initial population is .1 or .5.It is pointed out that when selective advantage of a gene is large and is changing with gene frequency, diffusion approximations assuming constant selective advantage, gives higher values for proportion of fixed genes in the case ofp equal to .1 and lower values forp equal to .5. With parental population size of 16 or less, a relation withN ī alone does not give the proportion of fixed genes. Higher order terms ofN ī appear to be involved in the relation. For the sameN ī, the proportion is much higher for lowN.The depressing effect of low recombinations between loci is of different magnitude for differentN andp for a givenN ī. The increase in the proportion of fixed genes due to increasingN is not as large when π is low. High intensity of selection offsets considerably the effects of population size and linkage when gene effects are large. It appears that with increased inbreeding and selection intensity, almost all the genes of large effects and at intermediate frequencies can be rapidly fixed regardless of linkage.Linkage has been shown to cause faster fixation of genes in the absence of selection. With selection, linkage tends to delay fixation. But in the case of very low recombinations, there appears to be a level of population size and selection intensity, below which there is more rapid fixation because of linkage. Selection for dominant genes in the case of very close linkage, delays fixation for a number of generations and this delay results in reducing the depressing effect of linkage.
Iowa State University, Ames, Iowa.
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Qureshi
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Kempthorne