The measurement of fitness and mutation rate in human populations

Evolutionary Ecology of Senescence and a Reassessment of Williams' 'Extrinsic Mortality' Hypothesis

The evolutionary theory of senescence underpins research in life history evolution and the biology of aging. In 1957 G.C. Williams predicted that higher adult death rates select for earlier senescence and shorter length of life, but preadult mortality does not matter to the evolution of senescence. This was subsequently interpreted as predicting that senescence should be caused by 'extrinsic' sources of mortality. This idea still motivates empirical studies, although formal, mathematical theory shows it is wrong. It has nonetheless prospered because it offers an intuitive explanation for patterns observed in nature. We review the flaws in Williams' model, explore alternative explanations for comparative patterns that are consistent with the evolutionary theory of senescence, and discuss how hypotheses based on it can be tested. We argue that focusing on how sources of mortality affect ages differently offers greater insight into evolutionary processes.

Evolution of maternal effect senescence

Increased maternal age at reproduction is often associated with decreased offspring performance in numerous species of plants and animals (including humans). Current evolutionary theory considers such maternal effect senescence as part of a unified process of reproductive senescence, which is under identical age-specific selective pressures to fertility. We offer a novel theoretical perspective by combining William Hamilton's evolutionary model for aging with a quantitative genetic model of indirect genetic effects. We demonstrate that fertility and maternal effect senescence are likely to experience different patterns of age-specific selection and thus can evolve to take divergent forms. Applied to neonatal survival, we find that selection for maternal effects is the product of age-specific fertility and Hamilton's age-specific force of selection for fertility. Population genetic models show that senescence for these maternal effects can evolve in the absence of reproductive or actuarial senescence; this implies that maternal effect aging is a fundamentally distinct demographic manifestation of the evolution of aging. However, brief periods of increasingly beneficial maternal effects can evolve when fertility increases with age faster than cumulative survival declines. This is most likely to occur early in life. Our integration of theory provides a general framework with which to model, measure, and compare the evolutionary determinants of the social manifestations of aging. Extension of our maternal effects model to other ecological and social contexts could provide important insights into the drivers of the astonishing diversity of lifespans and aging patterns observed among species.

Individual fitness and phenotypic selection in age-structured populations with constant growth rates

Powerful multiple regression-based approaches are commonly used to measure the strength of phenotypic selection, which is the statistical association between individual fitness and trait values. Age structure and overlapping generations complicate determinations of individual fitness, contributing to the popularity of alternative methods for measuring natural selection that do not depend upon such measures. The application of regression-based techniques for measuring selection in these situations requires a demographically appropriate, conceptually sound, and observable measure of individual fitness. It has been suggested that Fisher's reproductive value applied to an individual at its birth is such a definition. Here I offer support for this assertion by showing that multiple regression applied to this measure and vital rates (age-specific survival and fertility rates) yields the same selection gradients for vital rates as those inferred from Hamilton's classical results. I discuss how multiple regressions, applied to individual reproductive value at birth, can be used efficiently to estimate measures of phenotypic selection that are problematic for sensitivity analyses. These include nonlinear selection, components of the opportunity for selection, and multilevel selection.

Opportunity for selection in human health

Natural selection defined by differential survival and reproduction of individuals in populations is influenced by genetic, developmental, and environmental factors operating at every age and stage in human life history: generation of gametes, conception, birth, maturation, reproduction, senescence, and death. Biological systems are built upon a hierarchical organization nesting subcellular organelles, cells, tissues, and organs within individuals, individuals within families, and families within populations, and the latter among other populations. Natural selection often acts simultaneously at more than one level of biological organization and on specific traits, which we define as multilevel selection. Under this model, the individual is a fundamental unit of biological organization and also of selection, imbedded in a larger evolutionary context, just as it is a unit of medical intervention imbedded in larger biological, cultural, and environmental contexts. Here, we view human health and life span as necessary consequences of natural selection, operating at all levels and phases of biological hierarchy in human life history as well as in sociological and environmental milieu. An understanding of the spectrum of opportunities for natural selection will help us develop novel approaches to improving healthy life span through specific and global interventions that simultaneously focus on multiple levels of biological organization. Indeed, many opportunities exist to apply multilevel selection models employed in evolutionary biology and biodemography to improving human health at all hierarchical levels. Multilevel selection perspective provides a rational theoretical foundation for a synthesis of medicine and evolution that could lead to discovering effective predictive, preventive, palliative, potentially curative, and individualized approaches in medicine and in global health programs.

Natural selection and demographic transition in a Zapotec-speaking genetic isolate in the Valley of Oaxaca, southern Mexico

OBJECTIVE

The study analyzed the potential for natural selection and the demographic transition in an isolated Amerindian population in the process of secular change in body size.

SETTING

A genetically isolated, Zapotec-speaking community located in the Valley of Oaxaca, southern Mexico, has been studied regularly from the mid-1960s to 2000. Children, adolescents and young adults have experienced a recent secular increase in body size since 1978 after a major period of no change.

METHODS

Potential for natural selection and the demographic transition were analyzed over a 100-year period, ca 1900-2000. National census data, results from anthropological surveys and community archives and reports were used.

RESULTS

Opportunity for natural selection changed markedly over the last century. Demographic transition to Stage II occurred ca 1955 and preceded a secular increase in body size. The crossover between curves for mortality (I(m)) and fertility (I(f)) occurred at approximately the time of onset of the secular trend among children, adolescents and young adults, i.e. those born since the early 1970s.

CONCLUSIONS

The 'classic' demographic transition occurred in the mid-1950s and preceded the secular increase in body size. A 'critical mass' of environmental improvement appears to be necessary to activate secular improvements in growth status, possibly turning on a gene complex that interacts with the improved environmental conditions. The lead time from the onset of demographic transition phase II to beginning of the secular trend is approximately 25 years (one generation) in this community.

Steady-state analysis of structured population models

Our systematic formulation of nonlinear population models is based on the notion of the environmental condition. The defining property of the environmental condition is that individuals are independent of one another (and hence equations are linear) when this condition is prescribed (in principle as an arbitrary function of time, but when focussing on steady states we shall restrict to constant functions). The steady-state problem has two components: (i). the environmental condition should be such that the existing populations do neither grow nor decline; (ii). a feedback consistency condition relating the environmental condition to the community/population size and composition should hold. In this paper we develop, justify and analyse basic formalism under the assumption that individuals can be born in only finitely many possible states and that the environmental condition is fully characterized by finitely many numbers. The theory is illustrated by many examples. In addition to various simple toy models introduced for explanation purposes, these include a detailed elaboration of a cannibalism model and a general treatment of how genetic and physiological structure should be combined in a single model.

Evolutionary mechanisms of senescence

This paper reviews theories of the evolution of senescence. The population genetic basis for the decline with age in sensitivity of fitness to changes in survival and fecundity is discussed. It is shown that this creates a pressure of selection that disproportionately favors performance early in life. The extent of this bias is greater when there is a high level of extrinsic mortality; this accounts for much the diversity in life-history patterns among different taxa. The implications of quantitative genetic theory for experimental tests of alternative population genetic models of senescence are discussed. In particular, the negative genetic correlations between traits predicted by the antagonistic pleiotropy model may be obscured by positive correlations that are inevitable in a multivariate system, or by the effects of variation due to deleterious mutations. The status of the genetic evidence relevant to these theories is discussed.

Age structure and the distribution of rare genetic diseases

Equilibrium frequencies of rare deleterious genes in an age-structured population have been studied by using mathematical models. In the models, variable age-of-onset and variable penetrance of a trait have been incorporated. Numerical analyses show that the equilibrium gene frequencies can be approximated extremely well to those in nonoverlapping generations, as Haldane [1927] claimed. This property of rare deleterious genes in an age-structured population is important because population dynamics of such genes in a finite population can be studied easily.

The mutation rate to Huntington's chorea

The problems of estimating the mutation rate to Huntington's chorea, or the proportion of new mutants among all sufferers, are discussed. The available survey data are reviewed. The prevalence of sporadic phenotypes, which include new mutations, is probably less than 2·5%. New mutants probably make up around 0·1% or less of all sufferers.

On the probability of demonstrating differential fertility in genetic studies

Computer simulation has been used to investigate the probability of detecting genetic differences in fertility in small cohorts. It was found that in cohorts of size 200 which have the demographic characteristics of females born in the United States in 1921--5, the probability is approximately 0.50 of detecting, by analysis of variance, either a 20% reduction in the monthly probability of conception or a 15--20% increase in the rate of foetal loss in the first month after conception. Consideration is given to the relative sensitivities of various observable measures of fertility to variation in underlying fertility components.

The effect of structural aberrations of the chromosomes on reproductive fitness in man. I. Methodology

Methods are presented to estimate relative fitness of carriers as a function of fertility, survival, are generation time in pedigrees under incomplete ascertainment. A large sample of diverse chromosomal aberrations reveals significant effects on all three parameters, giving a relative fitness of .769 +/- .039. There is no significant shift in segregation frequency. Implications of these results for population dynamics of structural rearrangements are discussed. The following paper applies these methods to specific classes of aberrations.