Heritability and genetic constraints of life-history trait evolution in preindustrial humans.

An increasing number of studies have documented phenotypic selection on life-history traits in human populations, but less is known of the heritability and genetic constraints that mediate the response to selection on life-history traits in humans. We collected pedigree data for four generations of preindustrial (1745–1900) Finns who lived in premodern fertility and mortality conditions, and by using a restricted maximum-likelihood animal-model framework, we estimated the heritability of and genetic correlations between a suite of life-history traits and two alternative measures of fitness. First, we demonstrate high heritability of key life-history traits (fecundity, interbirth interval, age at last reproduction, and adult longevity) and measures of fitness (individual λ and lifetime reproductive success) for females but not for males. This sex difference may have arisen because most of the measured traits are under physiological control of the female, such that a male's fitness in monogamous societies may depend mainly on the reproductive quality of his spouse. We found strong positive genetic correlations between female age at first reproduction and longevity, and between interbirth intervals and longevity, suggesting reduced life spans in females who either started to breed relatively early or who then bred frequently. Our results suggest that key female life-history traits in this premodern human population had high heritability and may have responded to natural selection. However genetic constraints between longevity and reproductive life-history traits may have constrained the evolution of life history and facilitated the maintenance of additive genetic variance in key life-history traits.</p

Genetic Associations Between Personality Traits and Lifetime Reproductive Success in Humans

Personality has been associated with reproductive success in humans and other animals, suggesting potential evolutionary selection pressures. However, studies to date have only examined these associations on a phenotypic level, which may be inadequate in estimating evolutionary change. Using a large longitudinal twin dataset of contemporary Finns, we compared the phenotypic (breeder’s equation) and genetically informed (the Robertson–Price identity) associations between lifetime reproductive success (LRS) and two personality traits—neuroticism and extraversion. Neuroticism was not associated with LRS at the phenotypic nor genetic level, while extraversion was associated with higher LRS in men both phenotypically and genetically. Compared to the univariate phenotypic analysis, the genetic analysis suggested a larger selection response of extraversion, and a selection response of neuroticism due to indirect selection. We estimated that neuroticism decreases by .05 standard deviations and extraversion increases by .11 standard deviations by one generation. Our results highlight the importance of considering genetic associations between personality and fitness and investigating several inter-related personality traits and their covariance with each other to predict responses to selection more accurately

Changes in age‐structure over four decades were a key determinant of population growth rate in a long‐lived mammal

1. A changing environment directly influences birth and mortality rates, and thus population growth rates. However, population growth rates in the short term are also influenced by population age‐structure. Despite its importance, the contribution of age‐structure to population growth rates has rarely been explored empirically in wildlife populations with long‐term demographic data. 2. Here we assessed how changes in age‐structure influenced short‐term population dynamics in a semi‐captive population of Asian elephants Elephas maximus . 3. We addressed this question using a demographic dataset of female Asian elephants from timber camps in Myanmar spanning 45 years (1970–2014). First, we explored temporal variation in age‐structure. Then, using annual matrix population models, we used a retrospective approach to assess the contributions of age‐structure and vital rates to short‐term population growth rates with respect to the average environment. 4. Age‐structure was highly variable over the study period, with large proportions of juveniles in the years 1970 and 1985, and made a substantial contribution to annual population growth rate deviations. High adult birth rates between 1970 and 1980 would have resulted in large positive population growth rates, but these were prevented by a low proportion of reproductive‐aged females. 5. We highlight that an understanding of both age‐specific vital rates and age‐structure is needed to assess short‐term population dynamics. Furthermore, this example from a human‐managed system suggests that the importance of age‐structure may be accentuated in populations experiencing human disturbance where age‐structure is unstable, such as those in captivity or for endangered species. Ultimately, changes to the environment drive population dynamics by influencing birth and mortality rates, but understanding demographic structure is crucial for assessing population growth

Fitness benefits of prolonged post-reproductive lifespan in women

Most animals reproduce until they die, but in humans, females can survive long after ceasing reproduction. In theory, a prolonged post-reproductive lifespan will evolve when females can gain greater fitness by increasing the success of their offspring than by continuing to breed themselves. Although reproductive success is known to decline in old age, it is unknown whether women gain fitness by prolonging lifespan post-reproduction. Using complete multi-generational demographic records, we show that women with a prolonged post-reproductive lifespan have more grandchildren, and hence greater fitness, in pre-modern populations of both Finns and Canadians. This fitness benefit arises because post-reproductive mothers enhance the lifetime reproductive success of their offspring by allowing them to breed earlier, more frequently and more successfully. Finally, the fitness benefits of prolonged lifespan diminish as the reproductive output of offspring declines. This suggests that in female humans, selection for deferred ageing should wane when one's own offspring become post-reproductive and, correspondingly, we show that rates of female mortality accelerate as their offspring terminate reproduction

Socio-cultural similarity with host population rather than ecological similarity predicts success and failure of human migrations

Demographers argue that human migration patterns are shaped by people moving to better environments. More recently, however, evolutionary theorists have argued that people move to similar environments to which they are culturally adapted. While previous studies analysing which factors affect migration patterns have focused almost exclusively on successful migrations, here we take advantage of a natural experiment during World War II in which an entire population was forcibly displaced but were then allowed to return home to compare successful with unsuccessful migrations. We test two competing hypotheses: (1) individuals who relocate to environments that are superior to their place of origin will be more likely to remain-The Better Environment Hypothesis or (2) individuals who relocate to environments that are similar to their place of origin will be more likely to remain-The Similar Environment Hypothesis. Using detailed records recording the social, cultural, linguistic and ecological conditions of the origin and destination locations, we find that cultural similarity (e.g. linguistic similarity and marrying within one's own minority ethnic group)-rather than ecological differences-are the best predictors of successful migrations. These results suggest that social relationships, empowered by cultural similarity with the host population, play a critical role in successful migrations and provide limited support for the similar environment hypothesis. Overall, these results demonstrate the importance of comparing unsuccessful with successful migrations in efforts understand the engines of human dispersal and suggest that the primary obstacles to human migrations and successful range expansion are sociocultural rather than ecological.Peer reviewe

Correction to: Female-biased sex ratios in urban centers create a “fertility trap” in post-war Finland

This is a correction to: Behavioral Ecology, Volume 32, Issue 4, July/August 2021, Pages 590–598, https://doi.org/10.1093/beheco/arab007 </div

Food and fitness: associations between crop yields and life-history traits in a longitudinally monitored pre-industrial human population

Severe food shortage is associated with increased mortality and reduced reproductive success in contemporary and historical human populations. Studies of wild animal populations have shown that subtle variation in environmental conditions can influence patterns of mortality, fecundity and natural selection, but the fitness implications of such subtle variation on human populations are unclear. Here, we use longitudinal data on local grain production, births, marriages and mortality so as to assess the impact of crop yield variation on individual age-specific mortality and fecundity in two pre-industrial Finnish populations. Although crop yields and fitness traits showed profound year-to-year variation across the 70-year study period, associations between crop yields and mortality or fecundity were generally weak. However, post-reproductive individuals of both sexes, and individuals of lower socio-economic status experienced higher mortality when crop yields were low. This is the first longitudinal, individual-based study of the associations between environmental variation and fitness traits in pre-industrial humans, which emphasizes the importance of a portfolio of mechanisms for coping with low food availability in such populations. The results are consistent with evolutionary ecological predictions that natural selection for resilience to food shortage is likely to weaken with age and be most severe on those with the fewest resources

Limits to Fitness Benefits of Prolonged Post-reproductive Lifespan in Women

Recent advances in medicine and life-expectancy gains have fueled multidisciplinary research into the limits of human lifespan [1-3]. Ultimately, how long humans can live for may depend on selection favoring extended longevity in our evolutionary past [4]. Human females have an unusually extended post-reproductive lifespan, which has been explained by the fitness benefits provided from helping to raise grandchildren following menopause [5, 6]. However, formal tests of whether such grandmothering benefits wane with grandmother age and explain the observed length of post-reproductive lifespan are missing. This is critical for understanding prevailing selection pressures on longevity but to date has been overlooked as a possible mechanism driving the evolution of lifespan. Here, we use extensive data from pre-industrial humans to show that fitness gains from grandmothering are dependent on grandmother age, affecting selection on the length of post-reproductive lifespan. We find both opportunities and ability to help grandchildren declined with age, while the hazard of death of women increased greatly in their late 60s and 70s compared to menopausal ages, together implying waning selection on subsequent longevity. The presence of maternal grand-mothers aged 50-75 increased grandchild survival after weaning, confirming the fitness advantage of post-reproductive lifespan. However, co-residence with paternal grand-mothers aged 75+ was detrimental to grandchild survival, with those grandmothers close to death and presumably in poorer health particularly associated with lower grandchild survival. The age limitations of gaining inclusive fitness from grand-mothering suggests that grandmothering can select for post-reproductive longevity only up to a certain point

Testing storage methods of faecal samples for subsequent measurement of helminth egg numbers in the domestic horse

Parasite infection status, intensity and resistance have traditionally been quantified via flotation techniques, but the need for immediate analysis following defecation imposes limitations and has led to the use of several faecal storage techniques. However, their effect on nematode egg counts has not been systematically validated and is often generalised across taxa despite evidence of differences between species. Here, we take the domestic horse Equus ferus caballus as a model to examine the impact of commonly used storage techniques on egg recovery: 1) high and low concentrations of ethanol and formalin fixative solutions for up to four weeks and 2) refrigeration (3–5 °C) over a two-week period. We found a significant decline in faecal egg counts (FEC) following storage in high and low concentrations of both fixative solutions after two weeks, which stabilised after four weeks, and this pattern was uniform across replicates. FECs remained relatively stable over a week of refrigeration, but declined when refrigeration exceeded 8 days. Prior to FEC analysis, we recommend sample refrigeration for no more than one week. Storage in either fixative solution is sub-optimal for the preservation of nematode eggs, although the uniformity of the decline across samples could hold potential for projective calculation of parasite egg shedding when storage time is effectively controlled for

Early-life environment and differences in costs of reproduction in a preindustrial human population

Reproduction is predicted to trade-off with long-term maternal survival, but the survival costs often vary between individuals, cohorts and populations, limiting our understanding of this trade-off, which is central to life-history theory. One potential factor generating variation in reproductive costs is variation in developmental conditions, but the role of early-life environment in modifying the reproduction-survival trade-off has rarely been investigated. We quantified the effect of early-life environment on the trade-off between female reproduction and survival in pre-industrial humans by analysing individual-based life-history data for >80 birth cohorts collected from Finnish church records, and between-year variation in local crop yields, annual spring temperature, and infant mortality as proxies of early-life environment. We predicted that women born during poor environmental conditions would show higher costs of reproduction in terms of survival compared to women born in better conditions. We found profound variation between the studied cohorts in the correlation between reproduction and longevity and in the early-life environment these cohorts were exposed to, but no evidence that differences in early-life environment or access to wealth affected the trade-off between reproduction and survival. Our results therefore do not support the hypothesis that differences in developmental conditions underlie the observed heterogeneity in reproduction-survival trade-off between individuals