Title: Measuring Selection in Contemporary Human Populations
Authors: Stephen C Stearns, Sean G Byars, Diddahally R Govindaraju and Douglas Ewbank.
Scope: 3 stars
Readability: 2 stars
My personal rating: 4 stars
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Topic of Article
The authors review many studies that find evidence for recent human genetic evolution.
Until very recently, it was assumed that humans stopped evolving genetically about 10,000 years ago. The assumption was the technology and culture were evolving so fast that there was no chance for slow biological forces to keep up.
This view is increasingly being called into question by recent genetic studies that show rapid and accelerating genetic evolution in recent history.
- There is ample evidence for rapid and accelerating genetic evolution in recent human history.
- Most research on topics in the social sciences and medicine neglects the possibility that recent changes may be due to genetic evolution. It is time for that to change. Genetic evolution must always be considered a cause instead of being immediately dismissed as impossible or unethical.
Important Quotes from Article
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.
Responses to contemporary selection can be estimated using data from multigeneration clinical, demographic and epidemiological studies that have been assembled at great expense over many years; however, much of this data has not yet been used to study natural selection. This missed opportunity is the result of historical and professional biases.
Therefore, the medical community has been unaware of phenotypic methods for measuring selection and evolutionary biologists have been unaware of large, multigeneration studies. Because of the recent surge in interest in evolutionary medicine, the two communities are just now starting to appreciate each other’s insights.
The emerging picture is that selection is acting in post-industrial societies to reduce age at first reproduction in both sexes, to increase age at menopause in females and to improve traits such as total blood cholesterol that are associated with the risk of disease and mortality.
Within a decade of the publication of On the Origin of Species, the misconception developed that modern hygiene and medicine have caused natural selection to stop working on human populations. This was fuelled by another misconception: that selection operates only through differences in survival. We now know that natural selection on traits occurs whenever there is variation among individuals in fitness and in traits and when the variation in traits is correlated with the variation in fitness. A response to selection will then follow if some portion of the variation in the traits is heritable. A good proxy for fitness is lifetime reproductive success (LRS) or number of children per parent per lifetime. LRS has both a survival component — one must survive to reproduce — and a reproductive component. Good hygiene and medical care that reduce prenatal, infant and child mortality rates reduce the variation among individuals in the survival component but that does not eliminate natural selection, as substantial variation among individuals in the reproductive component remains. For example, consider an extreme case in which medical and public health measures were so good that everyone who was born survived to age 80. This would not eliminate natural selection, as individuals would still differ in their LRS and that variation would drive natural selection. The potential for natural selection only vanishes when all individuals have exactly the same reproductive success or when no trait is correlated with the variation in reproductive success that still exists. These states are unlikely ever to occur in any population.
Connecting genotypes to phenotypes is a top priority but to do so we need both the genetic and phenotypic levels to be described with similar detail and precision: we need a phenome to match the genome.
More than 14 studies have reported significant selection in contemporary human populations.
These studies report three striking findings. First, both women and men are under selection for earlier age at first birth in both pre-industrial, natural fertility populations and in post-industrial populations. Second, women are under selection for later age at last birth in a pre-industrial population and later age at menopause in two post-industrial populations. Third, women are under selection for increased height in one pre-industrial population and for decreased height in three post-industrial populations.
Measuring selection intensities is necessary but not sufficient to predict the response to selection. We also need to know the heritabilities of the traits and the genetic correlations among them, which then require transformation to represent additive genetic variances and covariances (G).
This analysis makes the following points. First, most human traits have measurable heritability and will respond to selection if they are not constrained by phenotypic and genetic correlations with other traits.
The phenotypic data available in large data sets hold the key to how natural selection is currently acting on many human populations. This information will benefit basic science in the short term and medical science in the long term by changing our view of human populations from static to dynamic, genetically and culturally. The most interesting potential insights will be into how human interventions are changing human evolution. The studies reviewed here make clear that traits in many human populations are experiencing natural selection and have the genetic potential to respond to it.
- “The 10,000 Year Explosion: How Civilization Accelerated Human Evolution” by Cochran and Harpending
- “Who We Are and How We Got Here” by David Reich
- “The Story of the Human Body” by Daniel E. Lieberman
- “A Troublesome Inheritance: Genes, Race and Human History” by Nicholas Wade
- “The Gap: Science of What Separates Us from Animals” by Thomas Suddendorf
- “Is There Anything Good About Men?” by Roy F. Baumeister
- “A Farewell to Alms: A Brief Economic History of the World” by Gregory Clark
- “The Son Also Rises: Surnames and the History of Social Mobility” by Gregory Clark
- “Meta-analysis of … twin studies” by many