What if fertility decline is not permanent? The need for an evolutionarily informed approach to understanding low fertility

‘Demographic transition theory’ assumes that fertility decline is irreversible. This commonly held assumption is based on observations of recent and historical reductions in fertility that accompany modernization and declining mortality. The irreversibility assumption, however, is highly suspect from an evolutionary point of view, because demographic traits are at least partially influenced by genetics and are responsive to social and ecological conditions. Nonetheless, an inevitable shift from high mortality and fertility to low mortality and fertility is used as a guiding framework for projecting human population sizes into the future. This paper reviews some theoretical and empirical evidence suggesting that the assumption of irreversibility is ill-founded, at least without considerable development in theory that incorporates evolutionary and ecological processes. We offer general propositions for how fertility could increase in the future, including natural selection on high fertility variants, the difficulty of maintaining universal norms and preferences in a large, diverse and economically differentiated population, and the escalating resource demands of modernization.     

Human population growth and the demographic transition

The world and most regions and countries are experiencing unprecedentedly rapid demographic change. The most obvious example of this change is the huge expansion of human numbers: four billion have been added since 1950. Projections for the next half century expect a highly divergent world, with stagnation or potential decline in parts of the developed world and continued rapid growth in the least developed regions. Other demographic processes are also undergoing extraordinary change: women''s fertility has dropped rapidly and life expectancy has risen to new highs. Past trends in fertility and mortality have led to very young populations in high fertility countries in the developing world and to increasingly older populations in the developed world. Contemporary societies are now at very different stages of their demographic transitions. This paper summarizes key trends in population size, fertility and mortality, and age structures during these transitions. The focus is on the century from 1950 to 2050, which covers the period of most rapid global demographic transformation.     

Unexpected Demography in the Recovery of an Endangered Primate Population

Assessments of the status of endangered species have focused on population sizes, often without knowledge of demographic and behavioral processes underlying population recovery. We analyzed demographic data from a 28-year study of a critically endangered primate, the northern muriqui, to investigate possible changes in demographic rates as this population recovered from near extirpation. As the population increased from 60 to nearly 300 individuals, its growth rate declined due to increased mortality and male-biased birth sex ratios; the increased mortality was not uniform across ages and sexes, and there has been a recent increase in mortality of prime-aged males. If not for a concurrent increase in fertility rates, the population would have stabilized at 200 individuals instead of continuing to grow. The unexpected increase in fertility rates and in adult male mortality can be attributed to the muriquis’ expansion of their habitat by spending more time on the ground. The demographic consequences of this behavioral shift must be incorporated into management tactics for this population and emphasize the importance of understanding demographic rates in the recovery of endangered species.     

Components and Public Health Impact of Population Growth in the Arab World

The Arab world, which consists of the 22 member states of the Arab League, is undergoing a rapid transition in demographics, including fertility, mortality, and migration. Comprising a distinctive geographic region spread across West Asia and North East Africa and unified by the Arabic language, these states share common values and characteristics despite having diverse economic and political conditions. The demographic lag (high fertility and low mortality) that characterizes the Arab world is unique, but the present trend of declining fertility, combined with the relatively low mortality, brings about significant changes in its population size. This research aimed to: (i) assess the population growth in the Arab world over 3 time periods, (ii) explore its components, and (iii) understand its public health impact. Data from the International Data Base (IDB) of the U.S. Census Bureau for 3 time periods (1992, 2002, and 2012) in 21 countries of the Arab world were analyzed by dividing them into four geographic sectors, namely, the Gulf Cooperation Council (GCC), West Asia, Maghreb, and the Nile Valley African Horn. The population of the Arab world has grown considerably due to both natural growth and migration. The immigration is pronounced, especially into resource-intensive GCC nations, not only from East Asian and Central African countries but also from resource-thrifty (limited-resource) Arab nations. The migrations within, as well as outside, the Arab world reveal an interesting demographic phenomenon that requires further research: migration flows and trends. However, the transformations in public health statistics related to mortality—the impact of demographic changes—depict a new era in the Arab world.     

Bevölkerungswachstum: Bildung ist die Lösung

Population growth – education is the answer Over the last decades world population has been constantly growing by some 80 million per year. Whereas the growth rate as well as the fertility rate have been cut by half since the 1970th, population growth will continue well over mid‐century. As the developed countries have completed the demographic transition from high mortality and fertility rates to low ones, population growth is fading out there or has already been reversed into decline. In the least developed countries mortality has fallen as well, whereas fertility decline has stalled. Therefor population growth is very high making the solution of the widespread problems in this part of the world more and more difficult. One obvious way out of this trap would be a better education that could open new development perspectives. A positive side effect is that educated women have much less offspring than their counterparts who never went to school.     

Demographic analysis of the washington regional primate research center pigtailed macaque colony, 1967-1996

This work presents the results of a demographic analysis of 30 years of breeding records from the University of Washington's recently closed Primate Field Station at Medical Lake, Washington. Summaries of population growth, age-specific fertility and mortality rates, first-year survival, and seasonality of reproduction are presented, as well as an analysis of survival by decade. In addition, we present data on interbirth intervals in this population. In general, pigtailed macaques represent a typical Old World monkey pattern of age-specific fertility and mortality, with a few minor exceptions. We suggest that pigtailed macaques are most similar to rhesus and Barbary macaques, and that Japanese and bonnet macaques differ somewhat in their demographics.     

Population and prehistory I: Food-dependent population growth in constant environments

We present a demographic model that describes the feedbacks between food supply, human mortality and fertility rates, and labor availability in expanding populations, where arable land area is not limiting. This model provides a quantitative framework to describe how environment, technology, and culture interact to influence the fates of preindustrial agricultural populations. We present equilibrium conditions and derive approximations for the equilibrium population growth rate, food availability, and other food-dependent measures of population well-being. We examine how the approximations respond to environmental changes and to human choices, and find that the impact of environmental quality depends upon whether it manifests through agricultural yield or maximum (food-independent) survival rates. Human choices can complement or offset environmental effects: greater labor investments increase both population growth and well-being, and therefore can counteract lower agricultural yield, while fertility control decreases the growth rate but can increase or decrease well-being. Finally we establish equilibrium stability criteria, and argue that the potential for loss of local stability at low population growth rates could have important consequences for populations that suffer significant environmental or demographic shocks.     

Human biology in Tlaxcala, Mexico: demography

A demographic survey was conducted as part of an ongoing population study of large Mestizo and Indian communities in Tlaxcala, Mexico. Comparative data on population structure and movement, mate selection, age at marriage, differential fertility, and mortality were collected through the administration of a standardized demographic proforma, and then cross-tabulated by computer analysis. The resulting differences between the Indian and Mestizo populations are interpreted in terms of the relative importance of hybridization, natural selection, and genetic drift. Sizeable variance in achieved reproduction and the high neonatal mortality suggest the operation of natural selection in these two populations. Observed patterns of population movement and mate selection indicate that the Mestizo population of the city of Tlaxcala is highly hybridized, in contrast to the endogamous Indian community of San Pablo del Monte.     

REBUILDING SEAL STOCKS IN THE KATTEGAT-SKAGERRAK

The harbor seal ( Phoca vitulina ) population in the Kattegat-Skagerrak area has been dwindling for several centuries due to excessive hunting pressure. Corrected hunting statistics during 1890–1976 are used to estimate changes in population size over the past century. After protection was introduced in the 1960s and 1970s the harbor seal population in the area increased at an exponential rate of 0.12 and exceeded 5,000 animals in 1986. The present rate of population growth is used for modelling the influence of fertility and age-specific mortality. It is found that the observed high rate of increase is only realistic if female fertility rate is very high, the range of juvenile mortality rate is 0.33–0.52 and adult mortality is less than 0.15. Commonly cited higher mortality rates are not realistic in the Kattegat-Skagerrak area.     

Demography, life history, and social structure in Propithecus diadema edwardsi from 1986-2000 in Ranomafana National Park, Madagascar

Prosimian lemurs differ fundamentally from anthropoid primates in many traits related to social structure. By exploring the demography of Milne-Edwards' sifakas (Propithecus diadema edwardsi), and comparing it to other well-studied primates, we explore the effect of demographic and life-history factors on social structure. Specifically, we compare lemur survivorship and fertility patterns to two published composite models: one created for New World and another created for Old World monkeys. Using longitudinal data collected on individual Propithecus diadema edwardsi from four study groups from 1986-2000 in Ranomafana National Park, Madagascar, we quantify 1) group composition, 2) birth seasonality, 3) interbirth interval, 4) life-table values, and 5) population growth estimates. The mortality, survivorship, and life-expectancy schedules indicate high infant and juvenile mortality. Fertility remains high until death. The intrinsic rate of increase and net reproductive rate indicate a shrinking population. We suggest that high mortality rather than low fertility causes the observed population decline. While sifaka survivorship closely resembles New World patterns, fertility resembles Old World patterns, i.e., like New World monkeys, few sifakas survive to reproductive age, and those that do, reproduce at a slow rate resembling the Old World pattern. This necessarily impacts social structure. An adult sifaka at the end of her lifespan will have one only daughter who survives to reproductive age, compared to 3.4 for New World or 2.7 for Old World monkeys. Demography limits the formation of large kin-based groups for sifakas, and survivorship and fertility patterns do not easily permit sifakas to form large same-sex family groups.     

Significance of indices of potential selection for residents of the USSR

Crow's indices of the opportunity for selection and their components connected with differential mortality (Im) and differential fertility (If) were estimated for populations of Soviet Union republics and for a number of USSR ethnic groups on the basis of demographic statistics. More than 10-fold decrease in the Im value was revealed in the total population of the USSR during 1926-1987. At present, the Im values in republics vary from 0.020 to 0.094 for urban population and from 0.030 to 0.121 in rural population, the ratio of perinatal mortality in the whole structure of prereproductive mortality being higher in the republics with lower values of the Im. The range of the If values for different peoples (0.148-0.643) is wider than for the populations of the republics (0.326-0.578). Interethnic differences contribute 47% of the variance in fertility. The structure of Crow's indices is given for urban and rural populations of the republics. Genetic implications of the data presented are discussed with respect to possible manifestation of the effects of inter-group selection.     

Demography of the Juang tribal population of Orissa

Demographic analysis of genealogical data collected in 1954 for 23 Juang villages was undertaken employing indirect estimation techniques and computer projection methodology. Results indicated that this group did not feature the historically high fertility levels associated with Indian tribal groups, although fertility was higher than previously reported for the Juang. The population did feature a mortality differential, with worse mortality conditions than the Indian national population at this time. Reversed sexual mortality differentials, common in South Asian populations, were also present for the Juang. Computer projection investigation revealed a steadily growing population, in contrast to some Indian tribal groups faced with extinction.     

Natural selection among Kinnaura of the Himalayan highland: A comparative analysis with other Indian and Himalayan populations

The present investigation on fertility and mortality differential among Kinnaura of the Himalayan highland is based on data collected from 160 post-menopausal women belonging to the middle and high altitude region of Kinnaur district of Himachal Pradesh (Indian Himalayas). Selection potential based on differential fertility and mortality was computed for middle-and high-altitude women. Irrespective of the methodology, the total index of selection was found to be highest among middle-altitude women (0.386) as compared with high-altitude (0.370) women, whereas for the total population it is estimated to be 0.384. It was found that the Kinnaura of the Himalayan highland showing moderate index of total selection and relative contribution of the mortality component (Im) to the index of total selection is higher than the corresponding fertility component (If). The analysis of embryonic and post-natal mortality components shows that the post-natal mortality components are higher in comparison with the embryonic mortality components among highlanders and needs special intervention and health care. The present findings are compared with other Indian tribes as well as non-tribes of the Himalayan region and other parts of the country. It reveals that this index among Kinnaura is moderate than the other population groups; among the Himalayan population, the highest was reported for Galong (It = 1.07) of Arunachal, whereas the lowest was reported from Ahom (It = 0.218) of Manipur. The correlation and regression analysis between total index of selection (It) and fertility (If) and mortality (Im) components for pooled data of populations of the Indian Himalayan states show that If and Im account for 21.6 and 29.1% variability, respectively. In Crow's total index of selection (It) along with strong association, which is significant at the 1% level, this indicates that mortality plays a greater role in natural selection in comparison with fertility among populations of the Indian Himalayas.     

The stage-structured epidemic: linking disease and demography with a multi-state matrix approach model

Stage-structured epidemic models provide a way to connect the interacting processes of infection and demography. Reproduction and development can replenish the pool of susceptible hosts, and demographic structure leads to heterogeneous transmission and disease risk. Epidemics, in turn, can increase mortality or reduce fertility of the host population. Here we present a framework that integrates both demography and epidemiology in models for stage-structured epidemics. We use the vec-permutation matrix approach to classify individuals jointly by their demographic stage and infection status. We describe demographic and epidemic processes as alternating in time with a periodic matrix model. The application of matrix calculus to this framework allows for the calculation of R₀{\mathcal{R}_0} and sensitivity analysis.     

Disease diversity and human fertility

The existence of parasitic constraints on the evolution of life-history traits in free-living organisms has been demonstrated in several plant and animal species. However, the association between different diseases and human traits is virtually unknown. We conducted a comparative analysis on a global scale to test whether the diversity of human diseases, some of them responsible for high incidences of morbidity and mortality, were associated with host life-history characteristics. After controlling for direct confounding effects exerted by historical, spatial, economic, and population patterns and their interactions, our findings show that human fertility increases with the diversity and structure of disease types. Thus, disease control may not only lower the costs associated with morbidity, but could also contribute directly or indirectly to reductions in human population growth.     

Demography and childcare in preindustrial societies

The preliminary comparison of hunter gatherers, horticulturalists, and pastoralists is based on 57 preindustrial populations with demographic and child care data out of a potential of 1264 documented cultures from the Ethnographic Atlas. The purpose of this effort is to demonstrate that the demographic characteristics of a population influence its child care practices and provides clues to understanding child care patterns. Traditional practices and provides clues to understanding child care patterns. Traditional practices including multiple caregiving, multistage play groups, and parents or siblings as cultural transmitters are reviewed in a demographic context. Other emerging practices are also discussed: the role of stepparents and differential parental investment in sons and daughters. Anthropological data published and unpublished included only those using standardized methods on total fertility, infant or child mortality, and/or sex/age distribution. Problems with the data set include limited cultural representation, small study sizes, limited time trends, and reliability. There is a concentration on the ]Kung San, Efe, Aka, Gidjingali, Yanomamo, Dusan, Semai, and Kipsigis. Only 7 of the 57 are outside the tropics. Foragers are farmers are primarily represented, because the pastoralists are primarily East African and smaller samples. Tables provide cultural specific data on total fertility rates (TRF), infant and child mortality, and sex ratios at birth and among the juvenile and adult population. Sections are devoted to methods, general patterns, traditional characteristics of childcare based on 5 hypotheses, and emergent trends with 2 more hypotheses on stepparenting and male preference. 2 patterns prevail: 1) hunter gatherers and horticulturalists/pastoralists show great intercultural variability in fertility and mortality rates, and 2) the ranges and means of both groups are very similiar. In the discussion of specific cultures, the hypothesis is proposed and then examples are drawn from the 57 studies to provide support or rejection of the hypothesis. The 1st postulated that the level of multiple care increases with the number of adult women without children increasing. The 2nd hypothesis is that the greater the density or compactness of the settlement, the greater the level of multiple care. It is reasoned in the 3rd that fertility and mortality patterns influence the nature of indulgent care of infants. The 4th hypothesis is that sex and age distributions and compactness of the camp influence the nature of the play ground and type of supervision. The 5th is that father involvement will be greater in societies with low population densities or isolated. The 6th is that a child rarely stays with natural parents throughout the dependency period. The 7th is that male biased juvenile sex ratios will exist in societies where the cost of raising males is or = that of raising families, or where males contribute more calories to the diet, or where male mortality is high.     

Demographic and genetic interrelationships among the Gavlis of Dharwad

Lot of work has been done in recent years on the genetics of isolated and small population groups. But J. Sutter (1963) notes that these studies have not yielded satisfactory results, because these investigators have applied the formulae and models constructed by the mathematicians which are based on the assumption of panmixia, whereas panmixia cannot occur in human populations especially if the population is very small. Sometimes we speak of genetic drift and selection without taking into account the fact that the population at the same time is controlled by two most important demographic parameters of fertility and mortality which can alter genetic drift and selection. The geneticists are primarily interested in fertility. They want to determine, for any given couple, the number of offspring reaching the age of reproduction. One might therefore assume that the measurement of fertility should play a major role in population genetics. Thus, there is an urgent need for the establishment of meaningful relationship between demography and population genetics. In view of the above facts, an attempt is made in the present study to analyse the “Demographic and Genetic Interrelationships among the Gavlis of Dharwad” so as to throw light on some of the complex genetic issues like endogamy, inbreeding and selection potential.     

Lifetime reproductive output: individual stochasticity,variance, and sensitivity analysis

Lifetime reproductive output (LRO) determines per-generation growth rates, establishes criteria for population growth or decline, and is an important component of fitness. Empirical measurements of LRO reveal high variance among individuals. This variance may result from genuine heterogeneity in individual properties, or from individual stochasticity, the outcome of probabilistic demographic events during the life cycle. To evaluate the extent of individual stochasticity requires the calculation of the statistics of LRO from a demographic model. Mean LRO is routinely calculated (as the net reproductive rate), but the calculation of variances has only recently received attention. Here, we present a complete, exact, analytical, closed-form solution for all the moments of LRO, for age- and stage-classified populations. Previous studies have relied on simulation, iterative solutions, or closed-form analytical solutions that capture only part of the sources of variance. We also present the sensitivity and elasticity of all of the statistics of LRO to parameters defining survival, stage transitions, and (st)age-specific fertility. Selection can operate on variance in LRO only if the variance results from genetic heterogeneity. The potential opportunity for selection is quantified by Crow’s index \(\mathcal {I}\), the ratio of the variance to the square of the mean. But variance due to individual stochasticity is only an apparent opportunity for selection. In a comparison of a range of age-classified models for human populations, we find that proportional increases in mortality have very small effects on the mean and variance of LRO, but large positive effects on \(\mathcal {I}\). Proportional increases in fertility increase both the mean and variance of LRO, but reduce \(\mathcal {I}\). For a size-classified tree population, the elasticity of both mean and variance of LRO to stage-specific mortality are negative; the elasticities to stage-specific fertility are positive.     

High Altitude Hypoxia, Culture, and Human Fecundity/Fertility: A Comparative Study

This paper presents new demographic findings for a high altitude Himalayan population residing in Ladakh, India, and reviews problematic issues regarding the hypothesized relationship between fertility/fecundity and altitude in the Himalayas in light of these findings. It concludes that the low completed fertility ratio reported for the Sherpas of Khumbu, Nepal, is not caused by hypoxia-induced low fecundity, but is the product of cultural factors affecting the exposure of females to the risk of intercourse, a critical confounding factor that has not received adequate consideration in previous studies. Contrary to earlier reports, the present study demonstrates that all high altitude Himalayan populations for which published data exist exhibit moderately high fertility and fecundity, and do not differ significantly in their fertility levels. Furthermore, it argues that the claims for a statistically significant difference in fertility between high, moderate, and low altitude Himalayan populations are groundless, and suggests that a parallel reevaluation of Andean findings is required. [fertility, fecundity, hypoxia, Himalayas, Andes]     

Opportunity for natural selection in a Basque population and its secular trend: evolutionary implications of epidemic mortality

Analysis of the interaction between mortality patterns and opportunity for natural selection could help to elucidate potential evolutionary implications of epidemic mortality. In this paper secular trends are studied in relation to Crow's index (It) and its components of mortality (Im) and fertility (If), using parish records for family reconstitution in a Basque population. A principal components analysis (91% of the variance accounted for) showed marked quantitative and qualitative variations of Im and If depending on the stage of demographic transition of the population analyzed: In pretransitional societies the opportunity for natural selection is determined mainly by differential prereproductive mortality, whereas in posttransitional societies selection resulting from differential fertility plays a key role. The highest values for the mortality component (range 0.81-1.26) and for the relative contribution of Im, to It (range 47.1-57.2%) were observed in periods with a high incidence of infectious diseases and when the most severe mortality crises were detected (1830-1859, 1860-1889, and 1890-1919). A differential incidence of epidemic mortality was also found at prereproductive ages (before 16 years) and at reproductive ages (16-45 years), which provides strong support for the idea of the long-term genetic consequences of mortality crises.