Ongoing natural selection drives the evolution of SARS-CoV-2 genomes

SARS-CoV-2 is a new RNA virus affecting humans and spreads extensively through world populations since its first outbreak in late December, 2019. Whether the transmissibility and pathogenicity of SARS-CoV-2 is actively evolving, and driven by adaptation to the new host and environments is still unknown. Understanding the evolutionary mechanism underlying epidemiological and pathological characteristics of COVID-19 is essential for predicting the epidemic trend, and providing guidance for disease control and treatments. Interrogating 22,078 SARS-CoV-2 genome sequences of 84 countries, we demonstrate with convincing evidence that (i) SARS-CoV-2 genomes are overall conserved under purifying selection. (ii) Ongoing positive selection is actively driving the evolution of specific genes. Notably, genes related to coronavirus infection and host immune system defense are under adaptive evolution while genes related to viral RNA replication, transcription and translation are under purifying selection. A spatial and temporal landscape of 54 critical mutants is constructed based on their divergence among viral haplotype clusters, of which multiple mutants potentially conferring viral transmissibility, infectivity and virulence of SARS-CoV-2 are highlighted.