Theor. Appl. Genet. 59, 251-263 (1981) 9 by Springer-Verlag1981 Forging Links Between Population and Quantitative Genetics R. Frankham and R.K. Nurthen School of Biological Sciences,Macquarie University,North Ryde, New South Wales (Australia) Summary. An initially rare allele with a large effect on a quantitative character is expected to exhibit the follow- ing behaviour in artificial selection lines: 1. It should change in frequency, or be lost by chan- ce, at rates predictable from the effects of the allele on the quantitative character and the selection regime im- posed. 2. At the phenotypic level the behaviour of the allele should cause (a) asymmetrical responses to bidirectional selection, (b) variation among replicate lines in response to selection corresponding to the behaviour of the allele in individual lines, (c) changes in heritability in lines in which the allele increases in frequency and (d) selection response compatible with the effects of the allele. This paper reports an experimental evaluation of these predictions utilizing a rare allele of large effect (srn lab) de- tected in a sample of the Canberra outbred population of Drosophila melanogaster at a frequency of 1/120. Homo- sygosity for this allele reduced abdominal bristle number by more than 50%, altered the abdominal bristle pattern and reversed the sexual dimorphism for abdominal bristle number. Experiments were done to characterise srn lab and all evidence indicates that it is a single allele with a very large effect. Bidirectional selection for abdominal bristle number was carried out in three high and three low lines from this sample of the Canberra population. The sm lab allele rose in frequency and went to fixation in two of the low lines (in 10 generations) but was lost from the third. These times to fixation were slower than the expectations de- rived from computer simulations of the behaviour of such an allele but this can be attributed to the lower fitness of sm lab' homozygotes. The proportions of lines with the allele fixed or lost were compatible with expectations. At the phenotypic level, the behavior of sm lab had the ex- pected consequences, namely, (i) asymmetrical responses to bidirectional selection, (ii) variation in response among replicate low lines corresponding to the behaviour of sm lab , (iii) changes in heritabilities in the lines in which sm lab went to fixation, and (iv) selection responses com- patible with the effects of the allele. A test for rare alleles of large effect was proposed, bas- ed on the expected pattern of change in heritability under artificial selection. This test was applied to the high selec- tion lines but no evidence was found for important effects due to rare alleles of large effect increasing abdominal bristle number, a conclusion consistent with other inde- pendent evidence. This work provides experimental corroboration of the links between population genetics and quantitative gene- tics. Key words: Drosophila melanogaster - Rare allele - Dy- namic behaviour - Asymmetrical selection response - Changes in heritability Introduction In his classic paper Fisher (1918) interpreted 'the well estab- fished results of biometry in accordance with the Mende- lian scheme of inheritance'. This paper and the one of Wright (1921) provide the main theoretical cornerstones on which quantitative genetics rests, as they provide the rationale for predicting and interpreting phenotypic rela- tionships for quantitative characters on the basis of the multiple factor hypothesis. Around the same time these two authors (Fisher 1930; Wright 1931) along with Halda- ne (1932) were also laying the theoretical foundations of population genetics. However, Milkman (1978) lamented the tenuous nature of links between population and quan- titative genetics 'despite their common origin and common goal of defining and relating genotypic variation and phe- notypic variation' in his review of the 'Proceedings of the International Conference on Quantitative Genetics' (Pol- laket al. 1977). 0040-5752/81/0059/0251/$ 2.60