L. Ulizzi

The effects of demographic transition on the opportunity for selection: changes during the last century in Italy.

The index of opportunity for selection proposed by Crow has been calculated for the Italian population during the last century. The evolution of its two components, the pre-reproductive mortality and the variance in fertility, has been also analysed and compared with similar data for the United States. The results clearly show the relevance of socio-economic changes to the evolution of selective patterns in our species; in particular the relative incidence of mortality and fertility to selection intensity: the total index has in fact been reduced by 75% during the last century, the relative amount due to fertility increasing from 57 to 89%. The probable different relevance of genetic factors in the two components has also been discussed.

Changes of selection opportunities with a changing environment: regional heterogeneity in Italy

The mortailty and fertility components of the index of opportunity for selection (the Crow index) have been studied in the various Italian regions during the last century. To estimate the relevance of environmental factors on both components, their trends and those of their interregional variances have been compared with illiteracy. The results show that the selection opportunities due to mortality have decreased in a smiliar way in all the regions with a trend parallel to that of the national percentage of illiteracy, suggesting that this component is directly affected by the general socio-cultural level. As to the component due to fertility differentials, the regions, though scattered in the time scale, show a characteristic cycle of rise and fall. Moreover, the interregional variances of this component and of illiteracy are quite parallel, thus supporting the hypothesis that the reproductive pattern is strongly influenced by the individual cultural level.

Genetic fitness in Huntington's Disease and Spinocerebellar Ataxia 1: a population genetics model for CAG repeat expansions

An analysis of genetic fitness was performed in Huntingtons Disease (HD) and Spinocerebellar Ataxia 1 (SCA1) families. Two partially overlapping samples were used: clinically defined HD and SCA1 patients from families ascertained in definite geographical areas, and molecularly typed carriers of HD and SCA1 mutations (CAG trinucleotide expansions). In both cases, a control group of normal relatives was used. HD and SCA1 patients born before 1915–20 had more children than normal controls. Carriers of HD and SCA1 mutations, all in the low/medium expansion range (37–49 and 47–54 CAG repeats respectively), had a higher number of children than controls up to more recent times (1935–1950). The reproduction of heterozygotes for large expansions could be analysed only in subjects born after 1950 and provided indirect evidence of a lower than normal number of children. The above results fit a model based on a differential fitness according to the degree of expansion. Such a model predicts that 1) up to relatively recently the frequency of alleles in the low/medium range has been maintained or even increased by the increased fitness of their carriers, as well as by new mutations, and 2) the frequency of large expansions, part of which are lost at each generation, is maintained through further expansions of alleles in the low/medium expansion range. The implications of such a model on linkage disequilibrium and the possible spread of these diseases in future generations are discussed.

Natural selection in industrialized countries: a study of three generations of Italian newborns

We have studied the impact of natural selection on the Italian population, analysing the relationship between stillbirth and three related variables: birth weight, birth order and maternal age. A progressive relaxation of selection with time has been demonstrated by the reduction of the Haldane index calculated with respect to the three variables.

Sex ratio and natural selection in humans: a comparative analysis of two Caucasian populations

In the course of the last century industrialized countries have experienced significant changes in mortality rates. Since the sex ratio of a population may be considered a function of, among other factors, differential mortality in the two sexes, it is expected to correlate with changes in sex‐specific mortality rates.

Heterogeneous effects of natural selection on the Italian newborns

We have studied the impact of natural selection through stillbirth on the Italian population, taking into account the socio‐economic heterogeneity of the country. The results suggest that older age at delivery and lower cultural level of the mothers, indicators of critical biological and socio‐economic conditions, even at present increase stillbirth risk. Moreover, in the less favourable environment of the southern regions, selection is still sex‐specific.

Natural selection associated with birth weight V. The secular relaxation of the stabilizing component

The secular trend of stabilizing selection on birth weight has been studied during the last decades in 17 countries, representative of America, Asia and Europe. A relaxation of the intensity of selection is regularly observed, which can probably be attributed to a progressive reduction of the environmental component of birth weight variance.

Natural selection associated with birth weight. VI. Towards the end of the stabilizing component

The secular trend for stabilizing selection on birth weight has been analysed in Italy from 1954 to 1985 in order to study changes in the forces of natural selection which have occurred as a consequence of progress in health care. In previous papers we demonstrated a very rapid relaxation of stabilizing selection on birth weight. In this paper we show that in the last few years this kind of selection has been coming to an end for the vast majority of Italian newborns.

Effects of environmental changes on the phenotypic expression of human polygenic traits

The very large increase in adult stature during the last century is well documented for many countries and this phenomenon has been associated with the rapid improvement of environmental conditions. In the case of Italy, data relative to its regions and drawn from the military records of the last century showed a rapid normalization (i.e. elimination of both negative skewness and hyperkurtosis) of the distribution (1874–1938), followed by acceleration of the average increase (1938–57), which is now approaching a plateau (1957–64). Two different interactions between genotype and environment are hypothesized: the curve normalization is interpreted as being due to the elimination of extremely unfavourable conditions which inhibit growth, while the rapid stature increase is possibly due to the achievement of an environmental threshold, such as to permit a complete expression of the genetic potential. In fact, under the multifactorial model, a perfectly normal distribution is expected, unless there is differential selection before age 18.

SECULAR CHANGES OF THE SEX-RATIO OF STILLBIRTHS AND EARLY DEATHS IN ITALY: EVIDENCE FOR POSTPONEMENT OF MALE SPECIFIC RISK

SummaryAnalyses of Vital Statistics from various industrialized countries have shown recent reduction of both the rate and sex-ratio of stillbirths. Two alternate explanations are conceivable for the finding: 1) the improvement of environmental conditions has actually reduced the male relative risk, 2) such an improvement has merely postponed late fetal mortality into early periods of extrauterine life.In order to discriminate between these two possibilities, both the secular trends of the sex-ratio of stillbirths and deaths in early infancy were analysed using Italian Vital Statistics. The results can be summarized as follows: 1) the sex-ratio of stillbirths continuously decreased during the examined period, from 1863 to 1979; 2) the sex-ratio of early deaths decreased until about 1920 and thereafter sharply increased. Therefore, Italian data seem to support the hypothesis of a postponement of late fetal deaths into early infant period.