P. M. Sheppard

Selection in the polymorphic land snail Cepæa nemoralis

THE structure of natural populations of animals and plants and of variation both within and between colonies needs more study as Diver (1940) has pointed out. Research along these lines is of particular importance because of the evidence he has obtained that many, apparently continuous, natural populations are broken up into small breeding-communities, giving the conditions in which genetic drift, as suggested by Sewall Wright (e.g. 1940), might be expected to occur if it were operative. Diver has stated that from his observations on populations of Cepeea nemoralis (L.) and C. hortensis (MUll.) he obtained no indications that the variations in shell colour and banding have any selective value. He considers that these small breeding-communities show definite divergences from each other because of non-random mating, but expresses surprise (1940, p. 312) that such divergences have not proceeded further by means of genetical drift. The purpose of this paper is to describe an investigation into this problem of the relative importance of selection and drift in determining the distribution of different colour and banding patterns in C. nemoralis. We find, on the contrary, that they have definite selective values, related to the environment, determining the general aspect of different populations and therefore of their gene ratios. Although small inbreeding communities within each population may diverge from one another in colour and banding patterns, this random divergence cannot proceed far, because of the effects of natural selection.

Experimental Studies on the Prevention of Rh Haemolytic Disease

In the context of haemolytic disease of the newborn, ABO incompatibility means that the father’s blood is unsuitable for transfusion into the mother, and our interest in this stems from the work of Levine (1943), who noted a deficiency of such mating types in the parents of affected children. He deduced, therefore, that ABO incompatibility affords a degree of protection against Rh haemolytic disease, and this has been confirmed by many workers. Of particular interest are the experiments of Stern et al. (1956), who showed that male volunteers could be much more easily sensitized to Rh if the injected blood were ABO compatible.

Further Experimental Studies on the Prevention of Rh Haemolytic Disease

In a previous report (Finn et al., 1961) we gave reasons for thinking that the rapid removal of Rh-positive foetal erythrocytes from the circulation of a mother who was Rh-negative would prevent her from becoming immunized and producing Rh antibodies. We have now investigated the matter further, and the present paper describes the completed results of the earlier work (Experiment I) and then gives details of some subsequent observations (Experiments II and III). The reasoning involved and the scope of the investigations are first discussed.

Quaternary Refugia in Tropical America: Evidence from Race Formation in Heliconius Butterflies

The hypothesis of Haffer, Turner, and others, that patterns of race and species formation in the tropical forests of South America are the result of the isolation of populations in forest refugia during widespread climatic changes in the geologically recent past, is supported by the distribution of races in the butterfly genus Heliconius: the location of the refuges for these butterflies shows an excellent accord with the refuges deduced by Haffer in his studies of forest birds. The strict parallel variation through most of South America of the various races of H. melpomene, H. erato and of ten similarly-patterned species shows the result of selection for Müllerian mimicry; as the patterns must be subject to strong stabilizing selection, and as the low vagility of the butterflies normally produces isolation by distance even in a continuous population, it is suggested that the extreme divergence of pattern that some (but not all) Heliconius underwent in the forest refugia results from selection pressure in favour of mimicking the most abundant or distasteful local species, which would vary from refuge to refuge, rather than from geographical isolation per se.

P.T.C. taste response and thyroid disease.

The deaths from leukaemia in England and Wales for the period 1945 to 1957 have been classified under three headings-acute leukaemias of all types, chronic myeloid leukaeniia, and chronic lymphatic leukaemia. The age-specific mortality rates have been calculated for each type of leukaemia, for each sex, and for three periods of time-1945 to 1949, 1950 to 1954, and 1955 to 1 957. A study of these data suggests that the principal feature in the real increase in leukaemia is a change in the incidence of acute leukaemia. This is compatible with the concept that an increased exposure to ionizing radiations plays some part in the changed incidence of the disease.

A Local Survey of the Distribution of Industrial Melanic Forms in the Moth Biston betularia and Estimates of the Selective Values of these in an Industrial Environment

The survey shows that there is a rapid decline in the frequency of the industrial melanic carbonaria of the moth Biston betularia from a value of about 97 % in Liverpool to less than 10 % 50 miles to the west in North Wales. The decline in the frequency of the intermediate phenotype insularia in this area, controlled by an allelomorph at the same locus, is from about 14% on the Wirral (no reliable frequency is available for Liverpool) to about 4 % 30 miles to the west. The frequency of carbonaria has declined in one locality (Caldy) since 1959, perhaps as the result of the introduction of smokeless zones to the east of it. It is argued on the basis of experimental data that the typical form is not at an advantage with respect to visual predation by birds at this locality, but it is at less of a disadvantage than formerly. The increase in its frequency is explained by postulating a com pensating non-visual disadvantage of the carbonaria homozygote, for which there is independent evidence. This disadvantage may be of the order of 15 %. Experiments using dead moths placed in life-like positions on tree trunks at Caldy and in Liverpool confirmed that carbonaria is better camouflaged on the blackened tree trunks of industrial areas. Estimates of the selective disadvantage of the typical form in Liverpool, using data from the survey and these experiments, together with a variety of assumptions, indicate values of the order of 60 %, which is somewhat higher than previous estimates. At Caldy the typical form appears to have been at a disadvantage of about 50 % prior to the introduction of the smokeless zones and is now at about a 20 % disadvantage, using assumptions similar to those in the Liverpool estimates. Although these estimates are subject to considerable error, there is little doubt that they reflect the correct order of magnitude of the relative selective values.

Prevention of Rh-Haemolytic Disease: A Third Report

In two papers (Finn et d., 1961; Clarke it al., 1963) we described experiments which were successful in preventing Rh immunization in Rh-negative male volunteers. The basis of the procedure was to remove rapidly from the circulation previously injected chromium-tagged Rh-positive red cells by giving hightitre incomplete anti-D either as an infusion of plasma or as gamma3-globulin. In our second paper we stated that the next steps should be to find out whether foetal red cells could be cleared equally well as adult and whether female volunteers could be protected in the same way as men. The results of experiments to test these points form the first part (I) of the present paper. The second part (II) concerns two factors of great importance in the application of the technique to preventing Rh immunization due to pregnancy. These are the frequency with which transplacental haemorrhage from foetus to mother occurs during pregnancy as distinct from at delivery, and the relation of the production of immune antibodies to the size of transplacental haemorrhage assessed after delivery. In the third part of the paper (III) we discuss some of the details of the clinical trial, recently started in Liverpool, of anti-D gamma2-globulin injection given to Rh-negative Women after delivery.

Absence of crossing-over in female butterflies (Heliconius).

SummaryAbsence of recombination between linked markers in female Heliconius is suggested by coupling backcross broods in H. erato, by a repulsion F2 in H. melpomene, and by other crosses with this species. No recombinants have been found in the offspring of doubly heterozygous females in either species. This supports the contention that the absence of chiasmata at oogenesis in these butterflies prevents genetic crossing-over. Chiasmata are absent in all the female Lepidoptera examined by Suomalainen and others, but Ephestia seems to show the absence of chiasmata but the presence of genetic recombination in the female, and therefore would repay further study.

The Theory of Adaptive Polymorphism

The hypothesis of adaptive polymorphism as proposed by Dobzhansky cannot as yet be considered well established. It is generally agreed that genetical variability enables a species to evolve in response to changes. Dobzhansky has made the important suggestion that variability also allows a population to exploit a constant, or a changing, environment more efficiently than a single genotype could do. Consequently he believes that arrangements which maintain variability, notably balanced polymorphism, give the populations in which they occur an adaptive superiority over others. However, it appears that by adaptive polymorphism is usually meant only polymorphism maintained by selection. Such polymorphism can occur in a population without affecting its adaptation. The relative adaptive value of populations is a very difficult concept which has not been clearly defined. Observations on relative coefficients of selection of different polymorphs within a population give no direct evidence on the ability of any one polymorph to survive as a pure stock. And further, the evidence which has been adduced to support the hypothesis is capable of different interpretations. In no case has direct evidence been obtained that different polymorphs in a population are in any sense exploiting the environment in different ways, and thereby affecting the adaptedness of the population significantly.

Further studies on the industrial melanic moth Biston betularia (L.) in the northwest of the British Isles

The frequencies of the three phenotypes carbonaria, insularia and typical of the moth Biston betularia are given for a number of new localities and new samples from old localities in northwest England and the Isle of Man. In one locality (Caldy) the frequency of carbonaria has been decreasing at the rate of 1% per three years since 1959, probably due to the introduction of smokeless zones. Two methods have been used to measure the relative degree of camouflage at Caldy, where the melanic is common, and in the Isle of Man where it is much rarer. In Caldy carbonaria was the better camouflaged, but there was no evidence for this on the Isle of Man. One possible selective agent not previously reported is the bat, P. pipistrellus The rôle of moth movement over land and water is discussed in relation to the maintenance of the polymorphism and the steepness of the clines between North Wales and Liverpool.