Genetic mechanism of adaptive evolution: the example of adaptation to high altitudes

Since Darwin's time, elucidating the mechanism of adaptive evolution has been one of the most important scientific issues in evolutionary biology and ecology. Adaptive evolution usually means that species evolve special phenotypic traits to increase fitness under selective pressures. Phenotypic adaptation can be observed at different hierarchical levels of morphology, physiology, biochemistry, histology, and behavior. With the breakthroughs of molecular biology and next-generation sequencing technologies, mounting evidence has uncovered the genetic architecture driving adaptive complex phenotypes. Studying the molecular genetic mechanisms of evolutionary adaption will enable us to understand the forces shaping biodiversity and set up genotype-phenotype-environment interactions. Genetic bases of adaptive evolution have been explained by multiple hypotheses, including major-effect genes, supergenes, polygenicity, noncoding regions, repeated regions, and introgression. The strong selection pressure exerted by high-altitude extreme environments greatly promotes the occurrence of phenotypic and genetic adaptation in species. Studies on multi-omics data provide new insights into adaptive evolution. In this review, we systematically summarize the genetic mechanism of adaptive evolution, research progress in adaptation to high-altitude environmental conditions, and existing challenges and discuss the future perspectives, thereby providing guidance for researchers in this field.