Department of Animal Biology (Anthropology)
A comparison between heritabilities of life history and morphological traits in human populations
Author(s): Esparza, M, Martínez-Abadías, N, Sjovold, T, González-José, R, Hernández, M
According to Fisher's fundamental theorem of natural selection, the amount of additive genetic variance in a trait, and hence its heritability, should decrease as the strength of selection on this trait increases. Supporting this idea, many authors have reported lower heritability values in life history traits than in morphological and physiological traits for different species. But there are no studies comparing the evolvability of different types of traits in human populations. The pedigree collection of decorated skulls from the historical population of Hallstatt (Austria) offers an exceptional opportunity to compare the heritabilities of life history and morphological traits in humans. In this study we first used church records to reconstruct the pedigrees and to obtain the values of individual life history traits, such as fertility, age at first and at last child, mean interbirth interval, adult lifespan and lifetime reproductive success). Second, we measured a sample of 353 complete adult skulls falling into the pedigreesusing a 3D Microscribe digitizer. A set of 50 landmarks were measured on each skull and from the 3D landmarks coordinates we estimated several size and shape variables reflecting the complex and modular structure of the human skull. We estimated the heritabilities of the life-history and morphological traits using a Restricted Maximum Likelihood method and statistically compared the resulting heritability values. On average, our results show lower heritabilities for life history traits than for morphological ones, confirming the initial hypothesis.
UMR 7204 Centre des sciences de la conservation
Actuarial senescence increases the risk of extinction of mammal populations
Author(s): Chantepie, S, Téplitsky, C, Sarrazin, F, Robert, A
Despite recent general acknowledgement that senescence has significant negative demographic manifestations in natural environments across a wide range of animal species, we still do not know if it can significantly impact the dynamics and viability of wild populations. Focusing on actuarial senescence (i.e., the decline of survival probabilities at old ages), we use specific demographic information to project the dynamics and extinction risk of wild populations of 58 species of mammals in the presence or absence of senescence. Our projections reveal major negative effects of ageing on population dynamics and viability, with a 27% decrease of the time to extinction on average and a potential deterioration of the species projected conservation status in 10% of the cases. The magnitude of the senescence cost is relatively homogenous among mammal orders at the exception of primates, which are disproportionally affected, due to their slow pace of life. Senescence is associated with particularly strong increases of the extinction risk in species with high annual survival probabilities of young adults and long intervals between litters, independently of their place in the phylogeny, indicating that the pace of life history can be used to forecast the detrimental effects of ageing on the viability of species.
Animal and Plant Sciences
Age-specific breeding success in a wild mammalian population: selection, constraint, restraint and senescence
Author(s): Dugdale, HL, Lisa, PC, Newman, C, Macdonald, DW, Burke, T
The Selection, Constraint, Restraint, and Senescence Hypotheses predict how breeding success should vary with age. The Selection Hypothesis predicts between-individual variation arising from quality differences; the other hypotheses predict within-individual variation due to differing skills or physiological condition (Constraint), residual reproductive lifespan (Restraint), or somatic and reproductive investment (Senescence). Studies tend to focus on either the initial increase in breeding success or later decrease; however, both require consideration when unravelling the underlying evolutionary processes. Additionally, few studies present genetic fitness measures, and rarely for both sexes. We therefore test these four hypotheses, which are not mutually exclusive, in a high-density population of European badgers Meles meles. Using an 18-year dataset (including 22 microsatellite loci) we show an initial improvement in breeding success with age, followed by a later and steeper rate of reproductive senescence in males than in females. Breeding success was skewed within age-classes indicating the influence of factors other than age-class. This was partly attributable to selective appearance and disappearance of badgers (Selection Hypothesis). Individuals with a late age of last breeding showed a concave down relationship between breeding success and experience (Constraint Hypothesis). There was no evidence of abrupt terminal effects; rather, individuals showed a concave down relationship between breeding success and residual reproductive lifespan (Restraint Hypothesis), with an interaction with age of first breeding only in females. Our results demonstrate the importance of investigating a comprehensive suite of factors in age-specific breeding success analyses, in both sexes, in order to fully understand evolutionary and population dynamics.
University of Southern Denmark, Insitute of Biology
AgeGuess crowdsourcing human aging research
Author(s): Steiner, U, Misevic, D
Human lifespan increases by 2.5 years every decade. This unprecedented change in life histories poses fundamental challenges for evolutionary theories. Some evidence indicates that this change is due to a delay in aging rather than a change in the rate of aging. AgeGuess is a citizen science project and online game that investigates the differences between perceived age (how old you look to other people) and chronological age (how old you actually are) and their potential power as an aging biomarker. Is the increased life expectancy reflected in how old one looks, i.e. are the new 60’s the old 50’s? Are people who look older than they are more likely to die early? Does the rate of looking older differ among individuals or some individuals just looking older all their lives, i.e. does the difference between estimated and real age change over time? Is the difference between perceived and real age heritable? Are there periods in life when one ages faster? The project aims at such questions by a simple on-line game in which you can post your photos, have other people guess your age, as well as guess the age of other users. Curious? Please visit AgeGuess.org
Section of Ecology
Cooperation and conflict in humans in traditional large joint families
Author(s): Pettay, JE, Lahdenperä, M, Lummaa, V
Group living can be associated with cooperation and even cooperative breeding whereby non-reproductive individuals help to raise offspring that are not their own. However, it can also lead to evolutionary conflict, which is a less studied phenomenon. Humans are considered to be cooperative breeders, since mothers commonly gain help in raising offspring from other (usually related) group members, such as grandmothers and siblings. Nevertheless, simultaneous breeding in the same household among reproductive-aged females, such as mothers-in-law and daughters-in-law, has also been linked with reduced success. The importance of cooperation and conflict is likely to vary according to ecology and social structure of populations, leading to differential selection pressures on dispersal patterns. We used life-history data on humans collected from church book records from 19th century Eastern Finland where joint-families were traditionally common. In joint families several adult offspring, usually sons, stayed in their natal farm with their families. This creates a situation where reproductive-aged women are not related to other women in the family, leading to possible conflict over resources and lowered fitness, but cooperation between women is also possible. We analyse mother’s fecundity and survival of her offspring in relation to the presence and reproductive history of other reproductive-aged women in the family by event history analysis, whist controlling for potential confounders such as presence of other family members and temporal variation in mortality and fertility rates. Preliminary analysis suggest that living in larger joint families was beneficial for women’s fitness, both in terms of fecundity and offspring survival, compared to smaller nuclear families. These results suggest that in this population cooperation between family members was more important than conflict, potentially favouring reduced dispersal among adult siblings
Department of Integrative Biology
Demographic heterogeneity, selection, and population response
Author(s): Fox, GA, Kendall, BE
Demographic heterogeneity (unmodeled variation in traits underlying vital rates) has attracted much attention in recent years. Empirical studies show substantial heterogeneity in many populations. Theory shows that it can have strong impact on demographic variance (and by inference, extinction risk); some kinds of demographic heterogeneity can also have large effects on the mean population growth rate.
Our studies of heterogeneity point to a strong connection with natural selection, and provide new insights on how changes in phenotype distributions can occur in structured populations. The key is an understanding of the within-population phenotypic correlation structure. Correlations can occur within individuals over time, between individuals at a given time, and between individuals at different times.
In matrix models allowing persistent heterogeneity, survival heterogeneity increases the mean population growth rate, and the population becomes dominated by "good survivors" even if the parent-offspring correlation is zero or negative. By contrast, the effect of fertility heterogeneity depends on the sign of the parent-offspring correlation. This is because information transmission across time depends (for reproduction) entirely on the parent-offspring correlation, but for survival information is also transmitted by changes in the population's phenotype structure.
With overlapping generations, then, selection on survival and on reproduction can cause different responses. The demographic response (change in stable phenotype distribution) occurs because of the non-genetic correlation structure of the population. For survival, this response occurs regardless of the parent-offspring correlation, but for reproduction, the response depends strongly on the parent-offspring correlation. This suggests that selection on survival may often lead to larger responses and faster changes than is the case for selection on reproduction.
Ecole Normale Supérieure
Disposable springtails: highly plastic ageing patterns are explained by resource allocation trade-offs in Folsomia candida
Author(s): Mallard, F, Tully, T
Although often neglected, the evidences of senescence occurring in the wild are accumulating (1) and there is a growing interest towards a clarification of how the mortality trajectories have been shaped by the ecological conditions (2). The challenge now lies in understanding the diversity of ageing patterns in the light of evolutionary theory (3). We developed an experimental system in the laboratory to assess these questions on the small and long-living Collembola Folsomia candida (4,5). We performed long term microcosms experiments to question how mortality trajectories have been shaped on the short- and long-term by trade-offs between traits. We found that within species genetic differences in ageing patterns can be explained with differences in growth and reproductive strategies: comparison of different lineages showed that initial mortality rate and age at onset of senescence are negatively correlated - a result coherent with recent predictions based on the 'disposable soma' theory of ageing (6). We also demonstrate that plastic adjustments of major life history traits triggered by changes in resource availability even late in life lead cohorts to shift from constant mortality trajectories (negligible senescence) to accelerated senescence. Our results emphasises the need for a more integrated ecological comprehension of the effects of environment and its fluctuations to understand how natural selection shapes ageing patterns. 1. D. H. Nussey et al., Ageing Research Reviews 12, 214-25 (2013). 2. A. Baudisch, J. W. Vaupel, Science 338, 618-9 (2012). 3. M. Bronikowski et al., Science 331, 1325-8 (2011). 4. T. Tully, R. Ferrière, PLoS One 3, e3207 (2008). 5. T. Tully, A. Lambert, Evolution 65, 3013-20 (2011). 6. M. J. Wensink et al. Biogerontology 13, 197-201 (2012).
Department of Animal and Plant Sciences
Effects of climate on survival of Asian elephants
Author(s): Mumby, HS, Courtiol, A, Mar, KU, Lummaa, V
Climate change has intensified interest in understanding how climatic variability affects animal life histories. Despite this, little is known of their effect on survival in those species. Asian elephants (Elephas maximus) are endangered across their natural distribution, and inhabit regions often characterised by high seasonality of both temperature and rainfall. We investigated the effects of monthly climatic variation on survival and causes of death in Asian elephants of all ages and both sexes, using a unique demographic dataset of 839 semi-captive longitudinally monitored elephants from four sites in Myanmar between 1965 and 2000. Temperature had a pronounced effect on survival, with the lowest predicted survival during the hottest and coldest months in both sexes across all ages. Because during a year the elephants spent twice as long in temperatures higher than their optimum (24C) rather than temperatures below it, most deaths occurred during the “too hot” rather than the “too cold” period. Decreased survival at higher temperatures resulted partially from increased deaths from heat stroke and infectious disease, whilst the lower survival in the coldest months is associated with an increase in non-infectious diseases or poor health in general. Variation in survival was also related to rainfall with the highest survival rates during the wettest months. Our results show that even the normal-range monsoon variation in climate can exert large impact on elephant survival in Myanmar leading to large absolute differences in mortality, particularly among the youngest age classes. The persistence of a long-term trend towards higher global temperatures combined with the possibility of higher variation in temperature between seasons may pose a growing challenge to the survival of species such as the endangered Asian elephants.
Department of Biology
Estimating fluctuating selection in age-structured populations
Author(s): Kvalnes, T, Engen, S, Sæther, B, Jensen, H
In age-structured populations, viability and fecundity selection of varying strength may occur in different age classes. On the basis of an original idea by Fisher of weighting individuals by their reproductive value, we show that the combined effect of selection on traits at different ages acts through the individual reproductive value defined as the stochastic contribution of an individual to the total reproductive value of the population the following year. The selection differential is a weighted sum of age-specific differentials that are the covariances between the phenotype and the age-specific relative fitness defined by the individual reproductive value. This enables estimation of weak selection on a multivariate quantitative character in populations with no density regulation by combinations of age-specific linear regressions of individual reproductive values on the traits. Demographic stochasticity produces random variation in fitness components in finite samples of individuals and affects the statistical inference of the temporal average directional selection as well as the magnitude of fluctuating selection. Uncertainties in parameter estimates and test power depend strongly on the demographic stochasticity. Large demographic variance results in large uncertainties in yearly estimates of selection that complicates detection of significant fluctuating selection. The method is illustrated by an analysis of age-specific selection in house sparrows on a fitness-related two-dimensional morphological trait, tarsus length and body mass of fledglings.
Institut des sciences de l'evolution
Evolution of senescence in heterogeneous landscapes
Author(s): Cotto, O, Ronce, O
The current theory of senescence is developed in a very simple ecological and demographic context, with a unique population at equilibrium in a homogeneous habitat. In the wild, species live in a variable environment in space and time, where the assumption of equilibrium is often transgressed. In this study, we use models of quantitative genetics in structured populations in order to investigate the evolution of senescence in a variable environment. Adaptation to local environment depends on phenotypic traits which expression varies with age. We study different scenarios where the environment changes abruptly, gradually or cyclically with time and where the environment is heterogeneous in space with different populations connected by migration. The strength of selection decreases with age, which predicts slower adaptation of traits expressed late in the life cycle, potentially generating stronger senescence in habitats where selection changes in space or in time. This prediction is however complicated by the fact that the genetic variance also increases with age. With numerical calculations, we found that in most cases the rate of senescence is enhanced when the environment varies. Especially, migration between different habitats is a durable source of senescence in heterogeneous landscapes. We also show that the rate of senescence can sometime decrease transiently, when the population is not at equilibrium, with possible implications in experimental evolution and in the study of invasive species. Our results highlight the need to study age-specific adaptation, as a changing environment can impact differently each age-class with different consequences on demography.