The evolution of different parts of an organism at different rates. For example, many aspects of the human phenotype have evolved relatively slowly or not at all since the hominins diverged from their primate ancestors, one notable exception being the nervous system, which has given humans their overwhelming selective advantage. Similarly, at a molecular level, some proteins evolve very rapidly, while others remain unchanged over millions of years. This high degree of evolutionary independence among different aspects of the phenotype permits flexibility; for example, when a population is faced with new selection pressures in a changing environment, only the most crucial components need to evolve, not the entire phenotype.