The response to long-term selection for increased abdominal bristle number was studied in six replicate lines of Drosophila melanogaster derived from the sc Canberra outbred strain. Each line was continued for 86–89 generations with 50 pairs of parents selected at an intensity of 20%, and subsequently for 32–35 generations without selection. Response continued for at least 75 generations and average total response was in excess of 36 additive genetic standard deviations of the base population (σA) or 51 times the response in the first generation. The pattern of longterm response was diverse and unpredictable typically with one or more accelerated responses in later generations. At termination of the selection, most of the replicate lines were extremely unstable with high phenotypic variability, and lost much of their genetic gains rapidly upon relaxation of selection.

The variation in response among replicates rose in the early phase of selection to level off at approximately 7·6 around generation 25. As some lines plateaued, it increased further to a level higher than would be accommodated by most genetic models. The replicate variation was even higher after many generations of relaxed selection. The genetic diversity among replicates, as revealed in total response, the individuality of response patterns and variation of the sex-dimorphism ratio, suggests that abdominal bristle number is influenced potentially by a large number of genes, but a smaller subset of them was responsible for selection response in any one line.

Lethal frequencies on the second and third chromosomes were estimated three times in six replicate lines of Drosophila melanogaster selected for increased abdominal bristle number, at G 14–16, G 37–44 and G 79. Ten lethals were detected at a frequency of about 5% or higher at G 14–16, of which only one recurred in subsequent tests. Another ten lethals which had not been detected previously were found at G 37–44, and the 5 most frequent ones recurred at G 79. In the last test, 15 presumably new lethals were detected, of which at least 4 appeared well established. In addition, six reversions (from sc to sc+), a new mutant at the scute locus and sca were discovered. The effects on the selected character of some lethals and visible mutants were large and variable, but not always sufficient to explain the observed frequencies. The major lethals detected at G 37–44 and G 79 for the first time were most probably ‘mutations’ (in the broad sense) which occurred during selection. The likely origins of such ‘mutations’ were discussed, with a suggestion that the known mutation rate for recessive lethals would not be incompatible with the observed frequency of occurrence of the ‘mutations’.

When the technique of saturation mapping is employed to estimate the number of loci in a distinct chromosomal region, there is always the possibility that some loci will not be detected. If the number of mutants per locus follows a Poisson distribution, the number of mutationally silent loci can be estimated. This paper describes a method for fitting such data to a Poisson distribution truncated at the zero class and a method for estimating the number of mutationally silent loci. The use of these methods is demonstrated by their application to some published data.

This paper shows that a number of models of the maintenance of polymorphism in a heterogeneous environment, including those of Levene and Dempster, can be derived from a simple assumption about the way in which the numbers and kinds of individuals emerging from a niche depend on the number of eggs laid in it. It is shown that for such models, unless selective advantages per locus are large, protected polymorphism requires that the relative niche sizes lie in a narrow range. This lack of robustness applies also to models of stable polymorphism proposed by Clarke and by Stewart & Levin. Excluding models relaying on habitat selection or restricted migration, the only models which may escape this criticism are diploid models with partial dominance with respect to fitness, such as one proposed by Gillespie, in which in all niches the fitness of heterozygotes is higher than the arithmetic mean of the homozygotes.

Experiments were conducted to look for minority male mating advantages among several Drosophila melanogaster genotypes. The strains of flies used were Canton-S, Oregon-R, a strain carrying the recessive mutation vermilion, and highly geo-positive and geo-negative populations which had been selected for over 200 generations in Hirsch-geotaxis mazes. Two series of experiments were carried out, one series using ten pairs of flies, and another series using 20 pairs. Regression analysis gave one significant slope out of seven, suggesting that in the present study, frequency-dependent advantages are not as common or as strong as reported for D. pseudoobscura.

With the aim of knowing the probable magnitude of non-random association between inversion chromosomes and electromorphs, both deterministic and stochastic studies are conducted on the evolutionary change of non-random association, which is defined as the difference in the frequency of a given allele between inversion and non-inversion chromosomes. In these studies inversion chromosomes are assumed to be subject to selection but electromorphs are selectively neutral, and recombination is allowed to occur between inversion and non-inversion chromosomes with a low frequency. The deterministic study has shown that in a variety of selective schemes for inversion chromosomes the non-random association decays at a rate equal to the recombination value in every generation. Thus, if the recombination value is of the order of 10−5 ˜ 10−4, it would take a long time for the non-random association to disappear. Furthermore, the stochastic study has indicated that random genetic drift generates non-random association of inversions and electromorphs in finite populations and the standard error of non-random association often becomes larger than the mean. In addition to these problems the time required for the electromorph frequencies in the inversion and noninversion chromosomes to become equal in a finite population and the probability that the population of inversion chromosomes remains monomorphic for the allele which existed in the initial inversion introduced are studied. Considering all these quantities, it is concluded that data on the non-random association between electromorphs and inversions are not very informative for the study of the maintenance of protein polymorphism. It is also indicated that in the study of association between electromorphs and inversion chromosomes non-random association or Yule's coefficient of association has a better property than the usual linkage disequilibrium measure or correlation coefficient. Implications of this study on some experimental observations are discussed.

It is widely acknowledged that genetic drift is an important source of variation in response to artificial directional selection. How large should a selection line be in order to reduce the effect of genetic drift to an acceptably low level?

This paper investigates two criteria that can be used to answer this question in relation to short-term response to selection. The first criterion is coefficient of variation of response, and the second criterion is chance of success, where a successful selection programme is one in which the observed response is greater than a certain proportion, β, of expected response.

For a simple mass selection programme with intensity i and heritability h2, the size of population required in order for the coefficient of variation of response to be γ after t generations, is approximately 2/(γih)2t, and the size required for the chance of success to be α after t generations is approximately 2{zα/(β−l)ih}2/t, where zα is the standard normal deviate corresponding to the probability α.

As an example, suppose it is required that after t generations the coefficient of variation of response be 10% or that there be a 90% chance of achieving at least 9/10 of expected response. Since ih ≤ 2 in most selection programmes, the size of population required is at least 50/t or 82/t respectively. If ih ≤ 1, the corresponding sizes are 200/t and 328/t.

Results are extended to enable the calculation of size of population required for any type of artificial directional selection programme, including those in which generations overlap.

Cross-fertilizing stocks of Euplotes crassus, differing as to the ability to undergo autogamy, were subjected to breeding analysis. The resulting data are consistent with the interpretation that a single locus (a) with a pair of alleles (a+; a−) controls the ability to undergo autogamy. The two alleles express a simple dominance relationship in which the dominant allele (a+) permits expression of the autogamy trait. Cross-breeding experiments also suggest that the a locus is not linked to the mt locus which determines the mating-type trait.