The neutral theory of molecular evolution, proposed by Motoo Kimura in the 1960s, suggests that most genetic variations within a population are due to random mutations that do not significantly affect an organism's fitness. This theory contrasts with the idea that natural selection is the primary driver of evolutionary change. Instead, it emphasizes that many changes at the molecular level are neutral and can spread through populations by genetic drift. According to this theory, the rate of molecular evolution is largely constant over time and can be measured by the number of neutral mutations. This perspective has important implications for understanding genetic diversity and the evolutionary history of species, as it highlights the role of chance in shaping genetic variation.