A Mendelian population without artificial external constraints does in general not increase at a constant rate. Formulas neglecting the changes in population size introduce an error which is negligible under ordinary circumstances but whose cumulative effect over long periods may be disastrous. Questions relating to the cost of natural selection, the nature of an unstable equilibrium, the survival of genes, etc. cannot be treated without regard to absolute population sizes. The limitation of the notion of relative fitnesses is illustrated by the fact that in some typical situations the survival of the a-gene depends only on the absolute fitness of the Aa-heterozygote, but not on the fitnesses of the homozygotes. Furthermore, a decrease of the (absolute or relative) fitness of one genotype may actually increase the viability of the population and its ultimate size.

Even when the relative frequency qn of the a-gene tends to zero the absolute number of such genes may increase from generation to generation at a geometric rate. Therefore the circumstance that qn → 0 may be insignificant as compared to the fact that the earth cannot sustain an infinitely increasing population. Ultimately the population size is bound to influence the environment and so the fitnesses will change. Thus we must consider density-dependent fitnesses and then observed fitnesses cannot be used to predict the ultimate fate of a population. It is now known (Dobzhansky, 1965) that relative fitnesses are sometimes very sensitive to small changes in environment and that the same species may occupy a great variety of environmental niches. It is therefore quite likely that at least part of a population will find itself in a modified environment before too many generations have passed. For the evolution of a species and the development of new forms it is then not important that under fixed conditions the relative frequency qn of the a-gene would tend to zero. The problem is whether the actual number of such genes will increase for a period sufficiently long to encounter changed conditions or to establish itself in new combinations. This question is significant because the convergence of the frequencies qn to zero may be extremely slow. Thus even in a population of fixed size a disappearing gene could exist long enough to contribute to evolutionary processes.

Speaking generally, the thinking in terms of an assumed steady state and relative fitnesses seems to aggravate the problem of applying the wonderful results of modern genetics to the theory of evolution. For example, various mechanisms which are often considered as eliminating genetic variability may sometimes produce the opposite effect. The theory of evolution should distinguish between what the physicist would call macroscopic and microscopic equilibrium. Even if the world as we see it were in a perfect equilibrium this would not imply an approximate steady state for individual species, not to speak of genes. It is clear that an evolution to higher forms depends on a frequent decrease in fertility rates. If one considers slow changes rather than an unattainable steady state then a loss of fitness may be beneficial in the long run and contribute to genetic variety.

Oocyte pachytene analysis of embryos heterozygous for the fd translocation confirmed the facts of this translocation as described by Ohno and Cattanach. Besides that it revealed that a small piece of the X-chromosome has been transferred to the autosome where in about 25% of cases it shows less stainable expansion.

Evolutionary factors which tend to decrease the mutation rate through natural selection and those which tend to increase the mutation rate are discussed from the standpoint of population genetics. The author's theory of optimum mutation rate based on the principle of minimum genetic load is re-examined, assuming that mutation rate is adjusted in the course of evolution in such a way that the sum of mutational and substitutional load is minimized. Another hypothesis is also examined that only selection toward lowering the mutation rate is effective and the present mutation rate in each organism represents the physical or physiological limit that may be attained by natural selection.

The possibility cannot be excluded that the spontaneous mutation rate is near the minimum that may be attained under the present mode of organization of the genetic material, and at the same time is not very far from the optimum in the sense of minimizing the genetic load.

The isolation and characterization of an isogenic series of twelve Hfr strains is described. From their points of origin it was concluded that the sex factor integrates at a limited number of sites on the bacterial chromosome. Although the nutritional requirements of these Hfr strains were similar to those of the parent F+ strain, it was observed that one group had a markedly slower growth rate. The relevance of these observations to theories on the nature of F+ fertility and on the formation of Hfr strains is discussed.

Maps have been made showing the order of am mutant sites using (a) frequencies of am+ recombinants from crosses between am mutants and (b) the modes of distribution of the am+ recombinants among the two parental and two recombinant classes with respect to the flanking markers inos and sp.

It is possible to arrange the sites in an order such that, in almost all the crosses yielding useful numbers of am+ recombinants, the sp allele originally in coupling with the distal am mutant site occurs in the majority of the recombinants. No such consistent pattern was found with respect to the inos marker. The order obtained by reference to sp agreed with the best order deducible from recombination frequencies.

The data are consistent with the hypothesis that am+ recombinants arise by a process of gene conversion, that there is a gradient of conversion frequencies from the right (inos or distal) end of the gene to the left (sp or proximal) end, and that conversion tends to be associated (though less than 50% of the time) with crossing-over, especially on the distal side.

There is no obvious relationship between the map position of a given am mutant and the properties of the protein product of the mutant gene.

(a) Coprinus lagopus has in nature both a monokaryon with one haploid nucleus per cell and a dikaryon with two haploid nuclei per cell. It also has in experiment both a heterokaryon with two types of nuclei distributed irregularly in the mycelium and a diploid monokaryon with one diploid nucleus per cell.

(b) Two pairs of linked recessive auxotrophic mutants, me-5 ad-8 and adhis-1 chol-1 have been combined in two different sets of multiple heterogenotes so that in the dikaryon and heterokaryon the + (active) alleles of the four genes are distributed equally in the two component nuclei. In one combination the linked genes are in trans and in the other they are in cis arrangement in diploid, dikaryon and heterokaryon.

(c) Complementation between the + genes, as measured by growth on minimal and complete media, was complete in the diploid and dikaryon but was only partial in the heterokaryon.

(d) A similar comparison of a recessive suppressor of the mutant me-1 revealed that the recessive gene was not expressed and therefore was fully recessive in the diploid and dikaryon but was only partially recessive in the heterokaryon.

(e) The exact similarity of complementation of the structural genes of the auxotrophic mutants and the recessiveness of the me-1 suppressor in diploids and dikaryons exclude a localization of gene interaction in the nucleus. In a heterokaryon cytoplasmic dilution of the product of a dominant gene can occur and has the effect of producing a phenotypic difference between heterokaryons and diploids.

1. Four lines selected for large size were crossed to form a base population for further selection for high 6-week weight; three small lines were crossed similarly, and the crossbred population was selected for low 6-week weight.

2. In every case, a cross between two selected lines resulted in heterosis increasing body weight. This shows that all of the selected lines were differentiated with respect to genes affecting body weight.

3. Further selection for large size produced a stock whose mean weight was 25% higher than the largest of the original lines at its limit. But the response to selection for small size was slow, and after twenty-four generations of selection, the low weights of two of the original lines had not been recovered.

4. The evidence points to linkage of genes affecting body weight in the mouse. It is suggested that this is a particular feature of crosses between previously selected lines, rather than a general feature of mouse populations.

1. Two methods are examined of introducing new genetic variance into a line of mice selected for high 6-week weight which, at its limit, displayed no additive genetic variance.

2. The first method—irradiation—gave largely negative results. Any further gain under selection that was achieved could not be clearly distinguished from a possible environmental trend.

3. The second method—outcrossing to an unselected strain and then selecting from the cross—resulted in a clear gain over the original limit, but nine generations were required even to recover the original limit.

4. Various methods of transcending selection limits are evaluated in terms of their application to livestock improvement.

1. A new semi-dominant gene called shaven (Sha) causes hairlessness in the mouse and it is closely linked to naked. Homozygous shaven mice never grow a first coat and adults grow a few short hairs only. Shaven heterozygotes grow a full coat which is greasy.

2. The percentage recombination between shaven and naked was found to be 0·8%. Three-point tests with naked and caracul showed that the order of the three loci is either N–Sha–Ca or N–Ca–Sha.

3. The failure of homozygous shaven mice to grow a coat is due to abnormal keratinization of the hairs in the follicles. The follicles were found to be deficient in sulphydryl groups.

4. The greasiness of the Sha+ coat is due to the presence of a sudanophilic fluid in the hair medullae.

5. The close linkage between the two genes with similar phenotypic effects is discussed.

Six lines have been isolated in which individual chromosomes of Ae. umbellulata have been added to the normal complement of T. aestivum. The phenotype and cytology of these lines and of the material employed in their production is briefly described. The use of some of these lines in investigations on the evolution of the diploid Aegilops species and in the introduction of useful alien variation into T. aestivum is mentioned.

It is generally accepted that reproductive isolation leading to the irreversible division of a Mendelian population into two species must be initiated during a period of geographical isolation (see Mayr, 1963). Thoday & Gibson (1962, Gibson & Thoday, 1963) obtained partial isolation within a population of Drosophila melanogaster by artificial disruptive selection for the number of sternopleural chaetae. This occurred in two experiments from the same base population after seven and twelve generations respectively. On the strength of these results they concluded that speciation does not require geographical isolation and that sympatric speciation by disruptive selection in a heterogeneous habitat is at least a theoretical possibility.

In their recent paper, Waddington & Robertson (1966) discuss the general importance of disruptive selection and make various points that call for comment.

First, their statement that ‘It is most desirable that Thoday's results … should be repeated, with special attention to the strict virginity of the flies used, …’ appears to some readers as a scarcely veiled imputation that my experiments, especially that of Thoday & Gibson (1962), were executed without technical competence, though Waddington has informed me such imputation was not intended.

A new mutant, Small eyes (symbol Sey), that reduces the size of the eye in the mouse, is described. It is dominant, and lethal when homozygous.

The influence on allelic recombination of non-lethal doses of gamma radiation delivered at different stages during meiosis has been determined and the data compared with previous results concerning gene recombination. Both kinds of recombination are influenced at only two, probably the same two, meiotic stages. During the first sensitive stage, probably in preleptotene, the frequency of allelic recombination is increased, but that of gene recombination decreased. The frequency of both is increased during the second sensitive stage, which is probably in pachytene.

Escherichia coli strain W adsorbs phage λ very efficiently but the phage does not form plaques on this strain because the DNA of λ is broken down in a majority of the infected cells shortly after adsorption. In a 10−3 to 10−4 fraction of the infected cells λ grows and small bursts of phage are produced. This phage does not carry the W-specific host modification and is unable to complete a second round of infection in W (Kerszman, Glover & Aronovitch, 1967). λω mutants have been isolated which are able to escape this restriction process and which plate on W with an efficiency of 1·0 and when grown in W these mutants carry a W-specific host modification.

Escherichia coli strain W was found to be lysogenic for a temperate phage Wφ. This phage, which plates on E. coli C, forms λ-like plaques 2–3 mm. diameter with turbid centres. It is serologically unrelated to λ but is closely related to P2 which it resembles in the electron microscope. Its buoyant density in CsCl has been measured and it is different from λ but similar to P2. E. coli C made lysogenic for Wφ restricts the growth of λ, and elsewhere (Kerszman, Glover & Aronovitch, 1967) it has been shown that the DNA of phage λ is degraded shortly after infection of bacteria lysogenic for Wφ. A mutant of Wφ has been isolated which has lost the property of restricting the growth of λ.