When genes and environment disagree: Making sense of trends in recent human evolution

Abstract

In humans, for the first time, we are now able to observe ongoing natural selection at the molecular level. Natural selection operates when particular genetic variants render the individuals who bear them more likely to reproduce. As a consequence, those genetic variants increase in frequency in the next generation. In PNAS, Beauchamp (1) presents evidence of negative natural selection on genes implicated in higher educational attainment in a contemporary population in the United States. To understand his conclusion and avoid misinterpretation, we unpack the central concepts. The empirical study of natural selection in humans started with the examination of phenotypes: that is, individual traits, such as height or schizophrenia, measured without reference to genetics (Fig. 1). The simplest design establishes heritability of a phenotype by showing how much variation is attributed to genetic differences between relatives (using twins or other family members) (2). To claim evidence of natural selection, studies measure how much the number of children varies with the phenotype to produce a measure of the “magnitude” of natural selection. If the trait has some heritability and is associated with the number of children, researchers conclude that the traits are evolving as a result of natural selection (3). Height is highly heritable (4), so if taller individuals have more children, genes important for tall stature may become more frequent in future generations (5, 6). Fig. 1. The evolution of study designs used to document ongoing natural selection in contemporary human populations. The figure shows measurement and modeling approaches used to investigate ongoing natural selection. It differentiates between phenotypic and genotypic level and whether the level has been directly observed (gray box) or is inferred (red circles). The blue arrows mark the associations being used as evidence for natural selection. In classic study designs, the observed association between phenotype X … [↵][1]2To whom correspondence may be addressed. Email: courtiol{at}izw-berlin.de or melinda.mills{at}nuffield.ox.ac.uk. [1]: #xref-corresp-1-1