Are humans currently evolving? This question can be answered using data on lifetime reproductive success, multiple traits and genetic variation and covariation in those traits. Such data are available in existing long-term, multigeneration studies - both clinical and epidemiological - but they have not yet been widely used to address contemporary human evolution. Here we review methods to predict evolutionary change and attempts to measure selection and inheritance in humans. We also assemble examples of long-term studies in which additional measurements of evolution could be made. The evidence strongly suggests that we are evolving and that our nature is dynamic, not static.

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Measuring selection in contemporary human populations

Human height is a highly variable trait, both within and between populations, has a high heritability, and influences the manner in which people behave and are treated in society. Although we know much about human height, this information has rarely been brought together in a comprehensive, systematic fashion. Here, we present a synthetic review of the literature on human height from an explicit evolutionary perspective, addressing its phylogenetic history, development, and environmental and genetic influences on growth and stature. In addition to presenting evidence to suggest the past action of natural selection on human height, we also assess the evidence that natural and sexual selection continues to act on height in contemporary populations. Although there is clear evidence to suggest that selection acts on height, mainly through life-history processes but perhaps also directly, it is also apparent that methodological factors reduce the confidence with which such inferences can be drawn, and there remain surprising gaps in our knowledge. The inability to draw firm conclusions about the adaptiveness of such a highly visible and easily measured trait suggests we should show an appropriate degree of caution when dealing with other human traits in evolutionary perspective.

/pdf/evolutionary-perspectives-on-human-height-variation-3vdw2owadi.pdf

Evolutionary perspectives on human height variation

Modern molecular genetic datasets, primarily collected to study the biology of human health and disease, can be used to directly measure the action of natural selection and reveal important features of contemporary human evolution. Here we leverage the UK Biobank data to test for the presence of linear and nonlinear natural selection in a contemporary population of the United Kingdom. We obtain phenotypic and genetic evidence consistent with the action of linear/directional selection. Phenotypic evidence suggests that stabilizing selection, which acts to reduce variance in the population without necessarily modifying the population mean, is widespread and relatively weak in comparison with estimates from other species.

https://www.pnas.org/content/pnas/115/1/151.full.pdf

Evidence of directional and stabilizing selection in contemporary humans.

The Industrial Revolution and the accompanying nutritional, epidemiological and demographic transitions have profoundly changed human ecology and biology, leading to major shifts in life history traits, which include age and size at maturity, age-specific fertility and lifespan. Mismatch between past adaptations and the current environment means that gene variants linked to higher fitness in the past may now, through antagonistic pleiotropic effects, predispose post-transition populations to non-communicable diseases, such as Alzheimer disease, cancer and coronary artery disease. Increasing evidence suggests that the transition to modernity has also altered the direction and intensity of natural selection acting on many traits, with important implications for public and global health.

The transition to modernity and chronic disease: mismatch and natural selection

Previous research on the relationship between height and reproductive success in women has produced mixed results. One possible explanation for these is mediation by ecological factors, such as environmental stress. Here we investigate female height and reproductive success under conditions of environmental stress (poverty) using a large scale dataset from Guatemala (n = 2,571). Controlling for educational attainment, age and ethnicity, we examined relationships between height and childlessness, occurrence of a stillbirth, fertility and child survival. There was no significant relationship between height and never haven given birth. Extremely short women had a significantly raised likelihood of experiencing stillbirth. There were curvilinear relationships between height and age at first birth, fertility, and survival rates for children. Overall, though, the penalties for short stature, particularly in terms of child survival, were far greater than those associated with extreme tallness, and so female height is positively associated with overall fitness in this population.

/pdf/taller-women-do-better-in-a-stressed-environment-height-and-29yl6uxgcz.pdf

Taller women do better in a stressed environment: height and reproductive success in rural Guatemalan women.

The reproductive histories of 4829 white women who participated in the Ten State Nutrition Survey of 1968-1970 were analyzed in the attempt to study socioeconomic interactions with physique and fertility. Socioeconomic data included per-capita income. The range of per capita incomes in the sample was from $12 per capita for those on public assistance to $30000 per capita and distinctly skewed to the lower incomes. Most incomes were clustered between $500 and $5000. Associations between 5 measures of physique and fertility were explained by socioeconomic influences -- the per capita income. The exact relation between social variables and physique as part of this triangle did not yield gracefully to delineation. The rich were systematically leanest and for females at least decreasing socioeconomic status appeared to be associated with increasing fatness. Stature was less predictable appearing to ascend with social class or income. There was a systematic decrease in number of liveborn children for mothers of completed reproduction with increasing income. The lowest 3 income levels were each significantly different from each other and contrasted with all other levels. This socially-determined differential fertility of physique categories appeared to be linear rather than modal in character; income and fertility interacted in a curvilinear manner. The data also suggest the existence of a secular trend for certain physical characteristics particularly fatness which are subject to social transmission between generations.

Stephan M Bailey

Papers

Socioeconomic Interactions with Physique and Fertility