Fig. 1

Modes in which selection or admixture can remove, maintain, or increase genetic diversity. a Schematic representation of the different types of natural selection. Purifying selection removes deleterious alleles (in black) from the population, and genes evolving under strong purifying selection are usually associated with rare, severe disorders. Conversely, mutations conferring a selective advantage (e.g., increased resistance to complex infectious disease) can increase in frequency in the population, or be maintained, through different forms of positive and balancing selection. Positive selection is represented here by the classic hard-sweep model where, following an environmental change, a newly arisen advantageous mutation or a mutation at very low frequency (in red) will be immediately targeted by positive selection and will ultimately reach fixation. Balancing selection is illustrated here by the case of heterozygote advantage (or overdominance), where the presence of heterozygotes (in blue) is favored in the population. b Long-term balancing selection. Advantageous genetic diversity can be maintained over long periods of time and survive speciation, resulting in “trans-species polymorphism” (represented by black and red arrows). In this example, a trans-species polymorphism that is present in the modern European population (where it has survived the known bottleneck out of Africa) is shared with other primates, such as chimpanzees and gorillas. c Modern humans can also acquire genetic diversity (whether advantageous or not) through admixture with other hominins, such as Neanderthals or Denisovans (Box 2). The green and blue arrows represent the direction and estimated magnitude of admixture between modern humans and Neanderthals and Denisovans, respectively (see [17])