Fundamental concepts of convergent (parallel) evolution in human-pathogenic viruses and their implications for global health

Various environmental conditions force viruses to continuously evolve to survive. Evolving viruses with improved fitness are subject to positive selection and will pass on their genetic information to the next generation. If virus populations experience similar environmental pressure, they may undergo a dynamic process of molecular adaptation, which is known as convergent or parallel evolution (parallelism). Noteworthy, these phenomena are among the underlying mechanisms of cross-species transmission and emergence of novel viruses in the human population with a significant impact on global health. Therefore, it is essential to comprehend the fundamental concept of parallelism as well as its molecular identification. This will contribute to a better preparedness against future viral epidemics and pandemics. In this review, we first describe the basic concept of parallelisms and various selective pressures that drive this process. We highlight viruses that commonly infect humans, including hepatitis C virus (HCV), human immunodeficiency virus (HIV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as examples of rapidly evolving viruses undergoing this evolutionary process. Understanding these molecular mechanisms not only improves our knowledge of viral evolution but also informs surveillance strategies and public health responses. Continuous research in this area is crucial to anticipate and mitigate future viral threats.