Which process describes the gradual change in species over generations?

The process that describes the gradual change in species over generations is natural selection. Natural selection is a key mechanism of evolution, where individuals with traits that are better suited to their environment tend to survive and reproduce, passing those advantageous traits on to their offspring. Over time, this leads to a gradual shift in the characteristics of a species, as advantageous traits become more common within the population and less favorable traits diminish.

In this context, it's important to differentiate natural selection from other processes. Genetic mutation refers to random changes in an organism's DNA, which can introduce new traits but doesn't inherently lead to gradual changes in species without the influence of selection pressures. Ecosystem succession is the process of change in the species structure of an ecological community over time, but it focuses more on community dynamics than on changes within a single species. Speciation, meanwhile, is the process through which new species arise; while it may result from natural selection over time, it specifically describes the formation of distinct species rather than the gradual change within a single species.