Which statement best describes evolution?

The concept of evolution is fundamentally about changes in heritable characteristics within populations over generations. Option B accurately encapsulates this idea by stating that evolution occurs when heritable characteristics of a species change. This emphasizes that the traits being passed down from one generation to the next can lead to variations within a species, which may be influenced by environmental pressures, natural selection, and genetic changes over time.

The gradual change mentioned in option A is a part of the evolutionary process, but it doesn't encompass the broad scope of evolution as a mechanism. Evolution can happen through both gradual and rapid changes, depending on the circumstances. While extinction is a consequence of evolutionary processes, as noted in option C, it is not an intrinsic aspect of evolution itself. Lastly, option D is overly narrow, as genetic mutations are only one mechanism driving evolutionary change; evolution also involves genetic recombination, gene flow, and other factors that influence heritable traits in a population.