Natural selection associated with birth weight. III. Changes over the last twenty years

Further evidence for a significant effect of fetal genes on variation in birth weight

The contribution of fetal and maternal genes to the variation in birth weight was estimated in a sample of 5,625 grandchildren of monozygotic and dizygotic twins. Fetal and maternal genetic effects were separated by comparing the covariance structure for offspring of daughters of twins with that for offspring of sons of twins. Only insignificant amounts (3.0%) of the total variance in birth weight could be accounted for by maternal genes, while fetal genes seemed to account for the major part (69.4%) of the variation. Environmental factors common to sibs could explain 8.6% and random environmental factors 19.0% of the total variance. The findings are consistent with the results of an earlier study of birth weight in the same population but differ from findings in other populations.

Meditations on birth weight: is it better to reduce the variance or increase the mean?

A conceptual model is presented here in which the birth weight distribution is decomposed into a distribution of target weights and a distribution of perturbations from the target. The target weight is the adaptive goal of fetal development. In the simplest model, perinatal mortality is independent of variation in target weight and determined solely by the magnitude of the perturbation of birth weight from the target. In this model, mortality risk is concentrated in the tails of the birth weight distribution. A difference between populations in their distributions of target weights will be associated with a corresponding shift in their curves of weight-specific risk, without any difference between the populations in overall risk. In this model, risk would be reduced by decreasing the variance of the distribution of perturbations. The model is discussed in the context of the so-called "paradoxes of low birth weight."

Genetic disease in the 1960s: a structural revolution

From about 1955 to about 1975, an explosion of new institutions, disciplines, databases, interventions, practices, techniques, and ideas turned technically driven human genetics from a medical backwater to an exotic and appealing medical research frontier. In the early 1960s, health care professionals were attracted to the new insights of cytogenetics, including the chromosomal explanation of Down syndrome and of other congenital defects and abnormalities of sexual development. The discovery of a connection between myeloid leukemia and chromosomal abnormalities in leukemic cells made human cytogenetics suddenly relevant to cancer research and diagnosis. Successful dietary treatment of phenylketonuria brought genetic disease into the domain of public health and provoked legislative programs with sweeping long-term consequences. Meanwhile, those promoting the importance of genetic disease to medical education began to elaborate the idea that disease was literally becoming more genetic, as a consequence of techno-historical change. In this article, I present an overview of these remarkable events and a framework for understanding how and why they occurred. I emphasize the important roles of family members, religious isolates, legislators, pediatricians, and others who were not trained in genetic science, but who became advocates, at many levels, of genetic medicine. And I suggest that the idea, so important to the Human Genome Project, that "all disease is genetic disease" was structurally realized and institutionalized long before technologies for mapping the genome were available.

Fluctuating asymmetry and canalization: An appraisal based on a-b ridge counts among Indian populations with diverse backgrounds

The relationship between fluctuating asymmetry, measured as the absolute difference between the right and left a-b ridge counts, and total a-b (R + L) ridge count was studied in an assortment of Indian population samples representing a wide spectrum of socioeconomic and occupational backgrounds. They included marine fishermen, inland and estuarine fishermen as well as migrants and their parental counterparts, tribes, and castes of different hierarchy- lower, middle, and upper. The samples together numbered a total of 3,239 subjects, 2,240 males and 999 females. The results failed to support Jantz and Webb's (1980) hypothesis of a quadratic relationship between fluctuating asymmetry of a-b ridge count and its phenotypic value. Only 3 of 22 samples (about 13%) showed a significant fit with a reasonable degree of consistency over a set of independent random subsamples; even initially only 7 of 22 (about 30%) samples showed a significant fit, or nearly so. Supplementary evidence drawn from these populations with reference to the relationship between fluctuating asymmetry and heterozygosity levels and inbreeding coefficients was also consistent with the interference that fluctuating asymmetry of a-b as it is measured and examined at the population level does not reflect canalization. Am. J. Hum. Biol. 11:367-381, 1999. Copyright 1999 Wiley-Liss, Inc.

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.

Relationship between physical traits and rate of some common illnesses in newborn infants

The morbidity rates of newborn infants suffering from jaundice or urinary tract infections (UTI) as well as total morbidity were determined in morphologically different groups of 1088 newborn infants. On the basis of body weight, body length, head circumference and Quetelet's index; the surveyed neonates divided into a modal (MI = mean +/- 0.67 SD) and two extreme groups: small (SI less than MI) and large (LI greater than MI), for each variable. The results show that, in general, infants with high morbidity, including those suffering from jaundice and UTI, have lower values of all the mentioned morphological traits than do healthy infants. Discriminant analyses of sick and healthy infants (three pairs of comparisons) indicate that decreased weight at birth is typical for all studied categories of morbidity. An additional finding was that infants suffering from UTI or other types of morbidity (total morbidity) originate from small-sized families in which the parents are generally older and the mother tends to be short and overweight.

Decreased developmental stability as assessed by fluctuating asymmetry of morphometric traits in preterm infants

Fluctuating asymmetry (FA) of 8 morphometric traits was studied in 113 preterm infants (26-36 wk of gestation), 103 term infants (37-41 wk), and their respective parents. With 3 different measures of FA, the highest values were obtained from extremely preterm infants (26-29 wk), and the lowest from the group of term infants. The estimates of FA values among parents, particularly mothers, showed a similar, albeit less pronounced, trend. Multiple regression analysis of individual mean FA values, calculated in infants for the 8 studied bilateral traits, documented a significant inverse correlation with gestational age and with the health status of the infants and their mothers, as well as a positive correlation with the mothers' mean FA values.

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.

Stabilizing selection in man: the effect of mother's age on birthweight variance

In order to study the effect of mother's age on birthweight variance, data were collected from Official Vital Statistics of the USA, Japan and Italy. Birthweight means and variances were calculated for all single births and for live births only. The results demonstrated that birthweight variance regularly increases in babies born to mothers of increasingly older age (with the exception of the youngest mothers). Moreover, the higher the variance in all births, the more the variance diminishes after late fetal selection. Therefore, a feedback phenomenon takes place in the stabilizing component of selection, i.e. the difference between the variance of all births and that of live births. As a consequence, at least part of the higher number of fetal losses experienced by older mothers can be attributed to the increased birthweight variance of their children.

Lerner's concept of developmental homeostasis and the problem of heterozygosity level in natural populations

In recent years adherents of the neutral mutation hypothesis have conducted a variety of statistical tests concerning the applicability of their theory to data of biochemical genetic polymorphism in natural populations. From the other side, the involvement of natural selection as a main evolutionary force responsible for the observed levels of polymorphism and heterozygosity have also been proposed in numerous field studies and theoretical considerations. However, none of these hypotheses completely and satisfactorily explains the collected data. We believe that one of the main causes of the discrepancy in theories is that the variability at each locus is considered independently in both of the above-mentioned approaches. Yet, it was suggested long ago, and now there is an increasing amount of evidence indicating cooperation between different loci which can influence the variability at each of them. Thus we think that the use of models considering the genome as a suit of independent genes is a priori expected to decrease the efficacy of the approximation. In the present review we attempt to draw attention to findings of interdependence of the variability of different characters and its possible limiting action on the growth of genetic diversity in natural populations. We do not try to give a universal explanation for the processes acting in populations and determining levels of heterozygosity. However, to our mind, the solution of the discussed question requires consideration of genes in their interactions.

Genotype-environment relationships: an analysis of stature distribution curves during the last century in Italy

The secular trend of stature in Italy from 1874 to 1960 has been examined using military records concerning nearly complete samples of males born in Italy in given years. The national mean value increased by nine cm (about one cm/decade) and the averages of the different Italian regions tend progressively to aggregate towards higher values (the north) and lower ones (the south). The rate of increase of stature was more or less constant up to subjects born in 1945, but in the period 1945-55 a very steep increase was observed. In more recent times, a clear slowing down has taken place both at national and regional level. The following moments around the mean were investigated: (1) variance--the national value has remained more or less constant and regional values pass from very diverse figures to very similar ones; (2) skewness--the initially negative national value has become slightly positive; in the regions, from very diverse negative values a close aggregation around zero is recently attained; (3) kurtosis--initial national hyperkurtosis has reduced to values typical of the Normal distribution; the same is true for regional figures which, moreover, have become closely aggregated. The increasing fit of stature distribution to the Normal distribution agrees with the hypothesis of an increasing expression of the genetic endowment in consequence of a progressive improvement in living conditions.

Birth weight, placental weight and gestation time in relation to natural selection in Thailand

Birth weight, placental weight and gestation time of 26258 single births at Ramathibodi Hospital during 1973-7 were studied in relation to natural selection by fitting quadratic function to natural log of survivors-to-nonsurvivors ratios. The survival criterion was 7 days after delivery. The estimated optimal values of birth weight and placental weight were 0.62 and 0.90 standard deviations, respectively, above the corresponding arithmetic means. Selection on birth weight and placental weight was asymmetrical about the optima, being more intense among the lower than the higher values. On the other hand, selection on gestation time seemed most stabilized with the optimum almost coinciding with the mean.